Two pairs of atoms are drawn, P and Q.

Which pair is joined by a covalent bond?

1. A. ✓ P only
2. B. Q only
   In Q the electron has moved across from K to F completely, so nothing is shared and there is no covalent bond.
3. C. Both P and Q
   Only one of the two drawings shows a pair of electrons sitting between two atoms and held by both.
4. D. Neither P nor Q
   One of the two drawings does show a shared pair.

Why: In P a pair of electrons sits between the two atoms and both attract it: a covalent bond.
In Q the electron moved across completely, so nothing is shared.

A lithium atom hands one electron to a fluorine atom. The electron moves across completely.

What are the two atoms now?

1. A. Lithium is a negative ion; fluorine is a positive ion.
   Electrons are negative, so the atom that loses one becomes positive and the atom that gains one becomes negative.
2. B. ✓ Lithium is a positive ion; fluorine is a negative ion.
3. C. Lithium and fluorine are both uncharged atoms joined by a covalent bond.
   A covalent bond needs a pair of electrons held by both atoms, and here the electron moved across completely.
4. D. Lithium and fluorine are both positive ions, since both took part in the transfer.
   Electrons are negative, so the atom that loses one becomes positive and the atom that gains one becomes negative.

Why: Electrons are negative.
Lithium lost one, so it is now a positive ion, Li⁺.
Fluorine gained one, so it is now a negative ion, F⁻.

Nitrogen pulls shared electrons harder than hydrogen does.

What kind of bond is an N–H bond?

1. A. A nonpolar covalent bond
   Nitrogen pulls the shared pair harder than hydrogen, so the sharing is uneven.
2. B. Two ions, with nothing shared
   The pair is still shared, only pulled closer to the nitrogen; neither atom has taken an electron completely, so neither is an ion.
3. C. ✓ A polar covalent bond
4. D. No bond: the atoms are not joined
   The pair of electrons still sits between the two atoms and holds them together.

Why: Nitrogen pulls the shared pair harder than hydrogen does.
Uneven sharing makes the bond a polar covalent bond.

Carbon and hydrogen pull on shared electrons about equally.

What kind of bond is a C–H bond?

1. A. A polar covalent bond
   Carbon and hydrogen pull on the shared pair about equally, so the pair sits in the middle and the two ends of the bond are alike.
2. B. Two ions, with nothing shared
   Carbon and hydrogen share the pair; neither has taken an electron completely, so neither is an ion.
3. C. No bond: the atoms are not joined
   A pair of electrons shared by two atoms is exactly what holds them together.
4. D. ✓ A nonpolar covalent bond

Why: The two atoms pull about equally, so the shared pair sits in the middle: a nonpolar covalent bond.

Oxygen pulls shared electrons harder than carbon does.

Which labels fit a C–O bond?

1. A. C is δ−, O is δ+
   The atom that pulls harder gathers the shared pair and becomes slightly negative, and here that is the oxygen.
2. B. ✓ C is δ+, O is δ−
3. C. C is a positive ion, O is a negative ion
   The pair is still shared, so each atom carries only a small partial charge; neither is an ion.
4. D. Neither atom carries any charge
   When one atom pulls the shared pair harder, that end becomes slightly negative and the other slightly positive.

Why: Oxygen pulls the shared pair harder, so it is slightly negative, δ−.
The carbon is left slightly positive, δ+.

A student looks at an O–H bond and says: “Oxygen pulls harder, so the oxygen atom is a negative ion.”

What is wrong with this statement?

1. A. Nothing: polar means the atoms are ions.
   A polar bond carries partial charges, far smaller than the full charge on an ion; the pair of electrons is still shared.
2. B. Oxygen should be slightly positive, because the atom that pulls harder gives up its electrons.
   The atom that pulls harder draws the shared pair toward itself and becomes slightly negative, not positive.
3. C. Oxygen has no charge at all; only the hydrogen carries a charge.
   Drawing the shared pair closer makes the oxygen slightly negative and the hydrogen slightly positive; both carry a partial charge.
4. D. ✓ Oxygen is only slightly negative; the pair is still shared, so it is not an ion.

Why: The pair of electrons is still shared, only pulled closer to the oxygen.
So the oxygen carries a partial charge, δ−, and is not an ion.

Quick recall: in an O–H bond, one of the two atoms pulls the shared electrons harder.

Which atom pulls harder, and so is δ−?

1. A. ✓ The oxygen
2. B. The hydrogen
   Oxygen pulls the shared pair harder, so the pair sits closer to the oxygen, and the hydrogen is left δ+.

Why: Oxygen pulls the shared pair harder, so the pair sits closer to it; electrons are negative, so the oxygen is δ− and the hydrogen δ+.

Why is the oxygen in a water molecule slightly negative?

1. A. Oxygen has taken an electron completely from each hydrogen.
   Oxygen has not taken any electron completely; the pairs are still shared, so the oxygen carries only a partial charge, not the full charge of an ion.
2. B. Oxygen has more electrons than hydrogen, and electrons are negative.
   A neutral oxygen atom has as many protons as electrons, so having more electrons than hydrogen gives it no charge; the partial charge comes from where a shared pair sits.
3. C. ✓ Oxygen pulls the shared electrons in each O–H bond harder than hydrogen.
4. D. Hydrogen pulls the shared electrons harder and pushes them onto the oxygen.
   It is the oxygen, not the hydrogen, that pulls harder; nothing is pushed.

Why: In each O–H bond the oxygen pulls the shared pair closer to itself.
Electrons are negative, so the oxygen end becomes slightly negative.

In one water molecule, how many atoms are slightly negative?

1. A. None
   Each O–H bond is polar, so its atoms do carry partial charges.
2. B. ✓ One: the oxygen
3. C. Two: the hydrogens
   The atom that pulls the shared pair harder becomes δ−, and that is the oxygen, not the hydrogens.
4. D. All three
   A polar bond has one δ− end and one δ+ end, so not every atom can be negative.

Why: The oxygen pulls the shared pair of each O–H bond harder, so it alone is δ−.
Both hydrogens are δ+.

Four students marked the partial charges on a water molecule.

Which drawing is marked correctly?

1. A. W
   W has δ+ on the oxygen and δ− on the hydrogens, the reverse of the real charges; oxygen pulls the shared pairs harder, so it is the δ− atom.
2. B. X
   X marks only one hydrogen; both O–H bonds are polar, so both hydrogens are δ+.
3. C. Y
   Y uses full charges, + and −; the pairs are still shared, so the charges are partial.
4. D. ✓ Z

Why: Z has δ− on the oxygen and δ+ on each hydrogen: one mark for each atom, all of them partial charges.

Nitrogen pulls shared electrons harder than hydrogen does. An N–H group sits on the end of a larger molecule, with its atoms numbered 1 (the N) and 2 (the H).

Which marks belong at 1 and 2?

1. A. ✓ 1: δ−, 2: δ+
2. B. 1: δ+, 2: δ−
   The atom that pulls the shared pair harder becomes δ−, and here that is the nitrogen.
3. C. 1: −, 2: +
   The pair is still shared, so both charges are partial, not the full charges of ions.
4. D. 1: δ−, 2: δ−
   A polar bond has one δ− end and one δ+ end, never two δ− ends.

Why: Nitrogen pulls the shared pair harder, so it is δ−.
The hydrogen, with the pair pulled away from it, is δ+.

Here is a molecule of ethanol, the alcohol in wine. Solid lines are covalent bonds. Carbon and hydrogen pull on shared electrons about equally; oxygen pulls harder than either. Three of the atoms are numbered 1, 2 and 3.

Which marks belong at 1, 2 and 3?

1. A. 1: δ−, 2: δ+, 3: δ+
   The hydrogen at 3 is bonded to a carbon, and carbon and hydrogen pull about equally, so that bond is nonpolar and the hydrogen carries no partial charge.
2. B. ✓ 1: δ−, 2: δ+, 3: no partial charge
3. C. 1: δ+, 2: δ−, 3: no partial charge
   The atom that pulls the shared pair harder becomes δ−, and in the O–H bond that is the oxygen.
4. D. 1: −, 2: +, 3: no charge
   The pair in the O–H bond is still shared, so the charges are partial, not the full charges of ions.

Why: Only the O–H bond in ethanol is polar.
Oxygen pulls that shared pair harder, so the oxygen at 1 is δ− and the hydrogen at 2 is δ+.
The hydrogen at 3 is bonded to a carbon; carbon and hydrogen pull about equally, so it carries no partial charge.

Two water molecules are drawn, with two of the lines marked X and Y.

What are X and Y?

1. A. X and Y are both covalent bonds.
   The dashed line Y runs between two molecules, and no pair of electrons is shared between molecules.
2. B. X and Y are both hydrogen bonds.
   The solid line X sits inside one molecule, where the oxygen and hydrogen share a pair of electrons: a covalent bond.
3. C. ✓ X is a covalent bond; Y is a hydrogen bond.
4. D. X is a hydrogen bond; Y is a covalent bond.
   A solid line inside one molecule is a covalent bond and a dashed line between molecules is a hydrogen bond, not the other way around.

Why: X sits inside one molecule, where the atoms share a pair of electrons: a covalent bond.
Y runs from a δ+ hydrogen to the δ− oxygen of the neighboring molecule: a hydrogen bond.

A long molecule has folded back on itself. A hydrogen bonded to a nitrogen on one arm sits close to an oxygen on the other arm. The dashed line between them is marked with a question mark.

Is the dashed line a hydrogen bond?

1. A. No: hydrogen bonds form only between separate molecules.
   A hydrogen bond can form between two distant parts of one large molecule once it folds and brings them close.
2. B. No: it must be a covalent bond, because both atoms sit inside the same molecule.
   A covalent bond is a shared pair of electrons between neighboring atoms, and the dashed line is an attraction between partial charges on two distant parts of the chain.
3. C. Yes, but only if the molecule is water.
   Hydrogen bonds are not limited to water: any δ+ hydrogen bonded to an oxygen or nitrogen can be attracted to a δ− oxygen or nitrogen.
4. D. ✓ Yes: a δ+ hydrogen attracted to a δ− oxygen, even within one molecule.

Why: Nitrogen pulls the shared pair harder than hydrogen, so that hydrogen is δ+.
It is attracted to the δ− oxygen on the other arm.
That attraction is a hydrogen bond, here within one molecule.

In liquid water, nearly every molecule is hydrogen-bonded to neighbors at any instant, yet each hydrogen bond lasts only a tiny fraction of a second.

What does this tell you about the hydrogen bonds?

1. A. ✓ Hydrogen bonds are weak, so they break as molecules move, and new ones form at once.
2. B. Hydrogen bonds are as strong as covalent bonds, which is why every molecule has partners.
   A bond that lasts only a fraction of a second is weak, far weaker than a covalent bond.
3. C. Once one breaks, that molecule stays free of hydrogen bonds for good.
   If broken bonds never re-formed, most molecules would soon have no partners, yet nearly all of them are bonded at every instant.
4. D. The covalent O–H bonds inside each molecule are breaking and re-forming.
   The O–H covalent bonds inside each molecule stay intact; it is the hydrogen bonds between molecules that break and re-form.

Why: Each hydrogen bond is weak, so it breaks as the molecules move, and a new one forms at once with the next neighbor.
That is why every molecule is bonded at every instant even though no single bond lasts.

Water boils at 100 °C. Another liquid, whose molecules are about the same size but are nonpolar, boils at −60 °C.

Why does water need so much more heating to boil?

1. A. Water’s O–H covalent bonds must be broken before the liquid can boil, and they are strong.
   The O–H covalent bonds inside each molecule stay intact when water boils; steam is still whole water molecules.
2. B. ✓ Hydrogen bonds make water’s molecules attract one another more than the nonpolar liquid’s do.
3. C. Water’s molecules are lighter than the other liquid’s, so they attract one another more.
   How much molecules attract one another is not set by their weight.
4. D. Electrons move between water molecules, forming ions that hold the liquid together.
   No electrons move between water molecules and no ions form; the δ+ hydrogens are simply attracted to δ− oxygens.

Why: Boiling means pulling the molecules apart from one another.
In water that means breaking a very large number of hydrogen bonds, which takes far more heating than separating molecules held only by the weaker attractions in a nonpolar liquid.

Quick recall: water molecules are attracted to one another.

What is the attraction between one water molecule and the next called?

1. A. ✓ A hydrogen bond
2. B. A covalent bond
   A covalent bond is a shared pair of electrons inside one molecule; between two water molecules no electrons are shared.
3. C. An ionic bond
   No electrons have moved between the water molecules, so there are no ions to attract one another.

Why: The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of the next: a hydrogen bond.

A thin thread of water is pulled up a narrow tube. The pull at the top carries all the way down the thread, yet each molecule’s nearest neighbors keep changing.

What holds the thread together?

1. A. Covalent bonds between neighbors that never break
   Covalent bonds hold the atoms inside one molecule; they do not join one water molecule to the next.
2. B. ✓ Hydrogen bonds between neighbors that keep swapping partners
3. C. Hydrogen bonds between neighbors that never break
   Hydrogen bonds in liquid water break and re-form constantly as the molecules move, which is why the neighbors keep changing.
4. D. Covalent bonds between neighbors that keep swapping partners
   Covalent bonds hold the atoms inside one molecule; they do not join one water molecule to the next.

Why: Neighboring water molecules are held together by hydrogen bonds.
Each hydrogen bond is weak and keeps swapping partners, but together they pass the pull down the whole thread: cohesion.

A glass is filled past the brim, and the water stands in a bulge above the rim.

Why does the water at the top stay in place?

1. A. ✓ Hydrogen bonds hold each surface molecule to the molecules around it.
2. B. The glass pulls the water inward toward its center.
   The bulge stands above the rim, where there is no glass beside it.
3. C. The water molecules push against the air above them.
   Nothing pushes the water up; the molecules at the top are held by hydrogen bonds to the molecules beside and beneath them.
4. D. The covalent bonds inside each molecule hold the surface up.
   Covalent bonds hold an oxygen and its two hydrogens together inside one molecule; holding one molecule to the next is done by hydrogen bonds.

Why: Each molecule at the top is held by hydrogen bonds to its neighbors beside and beneath it.
Water sticking to water: cohesion.

Two identical narrow tubes stand in a dish of water. One is glass. The inside of the other is coated with wax, which is nonpolar. Water climbs high in the glass tube and barely rises in the waxed one.

Why?

1. A. Wax weakens the hydrogen bonds between the water molecules.
   The water in both tubes is the same; what differs is the tube wall.
2. B. The waxed tube holds less water because the wax takes up space.
   A thin coat of wax leaves almost the same space inside the tube.
3. C. Wax pushes the water back down toward the dish.
   Wax does nothing to the water; it simply offers no partial charges.
4. D. ✓ Water can hydrogen-bond to glass but has nothing to hold on to on wax.

Why: Climbing a tube needs adhesion to the tube wall.
Glass carries partial charges that water hydrogen-bonds to; wax carries none, so the water cannot hold on and barely rises.

After a shower, drops of water cling to the glass of the mirror and stay there.

What is the attraction between the water and the glass called?

1. A. Cohesion
   Cohesion is water sticking to water, and here the water is sticking to glass.
2. B. A covalent bond
   No electrons are shared between the water and the glass.
3. C. ✓ Adhesion
4. D. A full charge
   A full charge is a kind of charge, not an attraction.

Why: Water holding on to something other than water is adhesion: the molecules hydrogen-bond to the partial charges on the glass.

Why is a water molecule at the surface pulled inward, toward the rest of the liquid?

1. A. A surface molecule has more neighbors than a molecule deep inside.
   A surface molecule has fewer neighbors, not more: none above it.
2. B. A surface molecule’s covalent bonds are stronger than those deep inside.
   The covalent bonds are the same everywhere in the liquid; the difference at the surface is in the hydrogen bonds to neighbors.
3. C. The air above a surface molecule presses it down into the liquid.
   The inward pull comes from the water’s own hydrogen bonds, not from the air.
4. D. ✓ A surface molecule has neighbors beside and beneath it, but none above.

Why: With no neighbors above, a surface molecule’s hydrogen bonds pull it only sideways and inward.
That draws the surface molecules tightly together: surface tension.

Which of these depends on surface tension?

1. A. ✓ A water strider standing on a pond without breaking through
2. B. Water soaking upward through a dry paper towel
   Water climbing a towel is water holding on to the fibers, adhesion, and dragging more water up behind it, cohesion.
3. C. Drops of water clinging to a bathroom mirror
   Drops clinging to a mirror are water holding on to glass, something other than water: adhesion.
4. D. A thread of water in a narrow tube moving as one column
   Each molecule in the thread holds the next, water to water: cohesion.

Why: The strider stands on the surface, where molecules with no neighbors above are pulled sideways and inward into a tightly held layer.
That property is surface tension.
The other three are adhesion or cohesion at work.

The bottom of a dry paper towel is dipped in water. Ten minutes later the water has climbed well above the waterline.

Which properties does the climb depend on?

1. A. Adhesion only
   Adhesion alone would pull only the leading water along the fibers; the rest of the water follows only because water also holds on to water.
2. B. Cohesion only
   Water holding on to itself does not climb on its own; it needs adhesion to the fibers to move along them.
3. C. Surface tension only
   Surface tension is the surface holding together, and climbing a towel is not a surface resisting being broken.
4. D. ✓ Adhesion and cohesion

Why: Water hydrogen-bonds to the fibers (adhesion) and to the water behind it (cohesion).
Adhesion and cohesion are both needed for the climb.

In a narrow tube inside a plant stem, water is being pulled up from the top. An air bubble forms and splits the water thread in two. The water beneath the bubble stops rising.

Which property has the bubble broken?

1. A. Adhesion
   On both sides of the bubble the water still touches and holds the tube wall; adhesion is untouched.
2. B. ✓ Cohesion
3. C. Surface tension
   No surface has given way; the thread failed because the water-to-water chain is broken.
4. D. Adhesion and cohesion together
   Adhesion to the tube wall is untouched on both sides of the bubble; only the water-to-water hold across the gap is gone.

Why: The thread rises as one column because each molecule holds the next: cohesion.
An air gap leaves no water to hold on to, so the pull from above can no longer reach the water beneath the gap, and that water stops rising.

Two small drops of water on a glass plate touch and merge into one drop.

Which property joins them?

1. A. ✓ Cohesion
2. B. Adhesion
   Adhesion is water holding to something other than water; here water joins water.
3. C. Surface tension
   Surface tension is the surface resisting being broken; here two bodies of water simply join.

Why: Water sticking to water is cohesion.
The molecules of the two drops hydrogen-bond to one another, so the drops become one.

A steel paper clip laid gently on still water stays resting on the surface.

Which property holds it up?

1. A. Adhesion
   Adhesion is water holding to another surface; the clip is not being pulled along by the water.
2. B. Cohesion
   Cohesion is water holding to water; the property at work is the surface itself resisting being broken.
3. C. ✓ Surface tension

Why: The surface molecules have no neighbors above, so their hydrogen bonds pull them sideways and inward.
The tightly held surface resists being broken and holds the clip up: surface tension.

A raindrop clings to a window pane and slides down slowly instead of falling.

Which property holds the drop to the glass?

1. A. Cohesion
   Cohesion is water holding to water; the drop is holding to glass.
2. B. ✓ Adhesion
3. C. Surface tension
   Surface tension is the surface resisting being broken; here the drop is held to a surface other than water.

Why: Glass carries partial charges, so the water molecules hydrogen-bond to it.
Water sticking to something other than water is adhesion.

A sugar cube dipped in tea draws the tea up through itself.

Which properties does the rise depend on?

1. A. Adhesion only
   Adhesion pulls only the leading water along the sugar; the rest follows because water also holds to water.
2. B. Cohesion only
   Water holding to itself does not climb on its own; it needs adhesion to the sugar to move along it.
3. C. ✓ Adhesion and cohesion

Why: The water hydrogen-bonds to the sugar (adhesion) and to the water behind it (cohesion).
Adhesion and cohesion are both needed for the rise.

Water barely rises in a tube coated inside with wax.

Why?

1. A. ✓ Wax carries no partial charges, so water cannot hydrogen-bond to it.
2. B. Wax breaks the hydrogen bonds between the water molecules.
   Wax does nothing to the water’s own hydrogen bonds; it simply offers no partial charges.
3. C. Wax is heavier than glass, so it pushes the water down.
   The mass of the tube does not act on the water; what matters is whether the surface carries partial charges.

Why: Climbing a tube needs adhesion to the tube wall.
Wax carries no partial charges, so water cannot hydrogen-bond to it, and the water barely rises.

A water strider’s foot presses on the pond surface, and the surface holds.

Why does the surface hold?

1. A. Air trapped under the foot holds the foot up, like a cushion of air under a boat.
   No air is trapped; the foot rests on the water’s own tightly held surface.
2. B. ✓ Surface molecules have no neighbors above, so their hydrogen bonds pull them sideways and inward into a tight layer.
3. C. The water under the foot is colder and stiffer than the water around it, so it holds the foot.
   The temperature of the water under the foot is the same as the rest of the pond.

Why: A surface molecule has neighbors beside and beneath it but none above.
So its hydrogen bonds pull it sideways and inward, and the surface molecules are drawn tightly together.
That tight surface holds the foot up: surface tension.

Equal masses of water and of rubbing alcohol, in identical sealed flasks, sit on identical hot plates for ten minutes, so each takes in the same energy. The alcohol’s temperature ends up much higher.

Which statement about the two liquids is correct?

1. A. Alcohol has the larger specific heat capacity, because its temperature rose more.
   A large specific heat capacity means a lot of energy is needed for each degree, so the temperature rises less, not more.
2. B. The two have the same specific heat capacity, because both are liquids.
   Being a liquid does not fix how much energy each degree of warming needs; the two liquids warmed by very different amounts from the same energy.
3. C. ✓ Water has the larger specific heat capacity; it needs more energy per degree.
4. D. The water’s temperature rose less because it took in less energy from its hot plate.
   Identical hot plates for the same time deliver the same energy to both flasks.

Why: The same energy raised the alcohol’s temperature a lot and the water’s a little.
Water needs more energy for each degree of temperature: it has the larger specific heat capacity.

Two beakers hold water: one 50 g, the other 200 g. Each beaker is warmed by exactly one degree.

How do the energies needed compare?

1. A. ✓ The 200 g needs four times as much; the specific heat capacity is the same for both.
2. B. The 200 g needs four times as much, so its specific heat capacity is four times greater.
   Specific heat capacity is the energy per gram per degree, a property of water itself, the same for any amount of it.
3. C. Both beakers need the same energy, because both hold water.
   Specific heat capacity is per gram, so more grams need more energy for the same one degree.
4. D. The 50 g needs more, because a small sample warms more easily.
   Warming more easily means needing less energy, and the smaller sample has fewer grams to warm.

Why: Specific heat capacity is the energy per gram per degree, the same for any amount of water.
Four times the mass needs four times the energy.

Two sealed flasks sit on identical hot plates: 100 g of water and 100 g of cooking oil, whose molecules do not hydrogen-bond to one another. Both take in the same energy. After five minutes the oil’s temperature is much higher than the water’s.

Why?

1. A. Oil molecules are heavier, so each one heats faster.
   What matters is where the energy goes once it is inside the liquid, not the weight of a molecule.
2. B. ✓ In the water, much of the energy went into pulling hydrogen bonds apart.
3. C. Water passes heat to the air more quickly than oil does.
   The flasks are sealed and identical, so heat leaving to the air cannot be the difference.
4. D. Oil molecules always move faster than water molecules at the same temperature.
   The speed of the molecules at a given temperature is not what sets the difference; the oil warmed more because none of its energy went into pulling molecules apart.

Why: In the oil, the energy goes straight into making the molecules move faster.
In the water, part of the energy goes into pulling hydrogen bonds apart, so the molecules speed up less and the temperature climbs slowly.

A liquid needs only about a quarter as much energy as water to warm one gram by one degree.

What is most likely true of this liquid?

1. A. Its molecules attract one another far more strongly than water molecules do.
   Stronger attractions would mean more energy going into pulling molecules apart, so more energy per degree, not less.
2. B. Its molecules have no attraction to one another at all.
   With no attraction at all between its molecules the substance could not hold together as a liquid.
3. C. It takes in less energy from the same hot plate than water does.
   The same hot plate delivers the same energy; the difference is what the energy does inside the liquid.
4. D. ✓ Its molecules attract one another far less strongly than water molecules do.

Why: A low specific heat capacity means little of the added energy goes into pulling molecules away from one another, so the attractions between its molecules must be weak.
Water’s hydrogen bonds are what make its value high.

A grower stands several large barrels of water inside one greenhouse and leaves an identical greenhouse empty. Overnight the outside temperature drops sharply.

What should the grower expect?

1. A. Both greenhouses cool by the same amount, because the barrels are sealed.
   Heat still flows out of sealed barrels into the cooling air, and the water gives up a great deal of heat for each degree it drops.
2. B. ✓ The greenhouse with barrels cools less: the water gives up its heat slowly.
3. C. The greenhouse with barrels cools faster: water pulls heat out of the air.
   The barrels are warmer than the night air, so heat flows from the water into the air, not the other way.
4. D. The greenhouse with barrels cools less: water blocks heat from escaping through the glass.
   The barrels do not block anything; heat still escapes through the glass.

Why: Water has a high specific heat capacity, so the barrels hold a great deal of heat and give it up slowly as they cool.
The night air in that greenhouse stays warmer, for the same reason a lake keeps a coast mild.

A person rests in a warm room for an hour. Heat flows into their body from the room, yet their body temperature rises only a fraction of a degree.

Which statement explains this best?

1. A. The body’s water stops any heat from entering it.
   Heat does enter the body; water simply needs a lot of energy for each degree, so the temperature changes slowly.
2. B. The body is mostly water, and water warms up quickly, so the heat spreads evenly through the body.
   Water warms slowly, not quickly: part of the energy added to it goes into pulling hydrogen bonds apart, so each degree needs a lot of energy.
3. C. Body temperature can never change, whatever the surroundings are.
   Body temperature does change; water’s high specific heat capacity slows the change, it does not prevent it.
4. D. ✓ Most of the body is water, which takes a lot of energy to warm, so its temperature changes slowly.

Why: Most of the body is water, and water needs a lot of energy for each degree.
So the heat that enters changes the body’s temperature only slowly, which helps it hold steady: homeostasis.

Quick recall: in liquid water, each molecule is held to its neighbors by hydrogen bonds.

What must a water molecule do before it can leave the liquid as vapor?

1. A. Break its O–H covalent bonds
   The O–H covalent bonds stay intact; water vapor is still whole water molecules.
2. B. Nothing: it leaves as soon as it reaches the surface
   A molecule at the surface is still held by hydrogen bonds to the molecules beside and beneath it.
3. C. ✓ Break free of its hydrogen bonds

Why: Hydrogen bonds hold each molecule to its neighbors, so a molecule must break them before it can leave.

Rubbing alcohol evaporates from skin faster than water does, yet each gram of alcohol that evaporates carries away less energy than a gram of water.

What explains the smaller energy per gram?

1. A. Alcohol has a smaller specific heat capacity than water, so it warms up faster.
   Specific heat capacity is the energy to raise a liquid’s temperature, not the energy for its molecules to leave as gas.
2. B. Alcohol molecules are lighter, so each one carries less energy away.
   What sets the energy each gram carries away is how strongly the molecules attract one another, not their mass.
3. C. Alcohol molecules attract one another more strongly, so fewer of them escape.
   Stronger attractions would mean more energy per gram and slower evaporation, the opposite of what is seen.
4. D. ✓ Alcohol molecules attract one another far less strongly than water molecules.

Why: Heat of vaporization is set by how much energy it takes to pull a molecule free of its neighbors.
Alcohol molecules attract one another weakly, so each gram leaves with less energy; water’s hydrogen bonds make each gram need more energy to leave.

A pan of water sits on a stove. First the water warms from 20 °C to 100 °C. Then it boils away.

Which quantity tells you the energy needed for the second stage, boiling away?

1. A. Specific heat capacity
   Specific heat capacity is the energy needed to raise the temperature of the liquid, which is the first stage.
2. B. ✓ Heat of vaporization
3. C. Cohesion
   Cohesion is a property, water holding on to water, not a quantity of energy.
4. D. Surface tension
   Surface tension is a property of the surface, not a quantity of energy.

Why: Turning one gram of liquid into gas is what heat of vaporization measures.
Warming the liquid first is a different quantity, specific heat capacity.

On a very humid day, sweat runs off your skin in drops instead of evaporating, and you feel much hotter than usual.

Why does this leave you hotter?

1. A. Liquid sweat traps heat against the skin by hydrogen-bonding to the skin’s surface.
   Liquid sweat does nothing to heat the skin; it simply fails to cool it, because it never leaves as vapor.
2. B. ✓ Only evaporating sweat carries heat away; a drop that runs off takes almost none.
3. C. Humid air is warmer than dry air, so it heats your skin more.
   Humid air carries more water vapor, not more heat.
4. D. Producing sweat releases heat into the skin, and more sweat means more heat.
   Making sweat does not warm the skin; sweat cools only when it evaporates.

Why: Cooling comes from the fastest molecules escaping as vapor and carrying their energy away.
Sweat that runs off as liquid never does that, so little heat leaves your skin.

After some of the water in a dish evaporates, why is the water left behind cooler?

1. A. ✓ The fastest molecules escaped, taking their energy with them.
2. B. Evaporation broke covalent bonds, which used up the heat.
   Molecules leave whole; what they break are the hydrogen bonds to their neighbors.
3. C. The vapor above the dish pushed heat out of the liquid.
   Nothing pushes heat out; the cooling comes from what left.
4. D. The slowest molecules escaped, leaving the fast ones behind.
   The slowest molecules do not have the energy to break their hydrogen bonds and leave.

Why: Only the fastest molecules can break their hydrogen bonds and leave.
They carry their energy away, so the molecules left behind are slower on average, and the water is cooler.

A student explains why water climbs a glass tube and stays low in a wax-coated one. Step 1: Oxygen pulls the shared electrons of each O–H bond harder than hydrogen, so the oxygen is δ− and each hydrogen δ+. Step 2: Water molecules hydrogen-bond to the partial charges on glass, but wax has none. Step 3: This water-to-glass attraction is cohesion. Step 4: With nothing to hold on to on the wax, the water stays low in the waxed tube.

Which step contains an error?

1. A. Step 1
   Step 1 is correct: oxygen pulls the shared electrons harder, so the oxygen is δ− and each hydrogen δ+.
2. B. Step 2
   Step 2 is correct: glass carries partial charges that water hydrogen-bonds to, and nonpolar wax carries none.
3. C. ✓ Step 3
4. D. Step 4
   Step 4 is correct: with no attraction to the wax, the water cannot cling to the tube wall and climb.

Why: Step 3 names the wrong property.
Water holding on to glass, something other than water, is adhesion; cohesion is water holding on to water.

Which list puts the chain of reasons for surface tension in order?

1. A. ✓ polar O–H bond, partial charges, hydrogen bonds, taut surface layer
2. B. hydrogen bonds, partial charges, polar O–H bond, taut surface layer
   Hydrogen bonds cannot come first: they exist only because the partial charges exist, and the partial charges exist only because the O–H bond is polar.
3. C. partial charges, polar O–H bond, taut surface layer, hydrogen bonds
   The taut surface layer is the result of hydrogen bonds pulling surface molecules sideways and inward, so it cannot come before them.
4. D. polar O–H bond, hydrogen bonds, partial charges, taut surface layer
   A hydrogen bond is an attraction between partial charges, so the charges must come before the hydrogen bonds.

Why: The polar O–H bond creates the partial charges; the partial charges attract to make hydrogen bonds; at the surface those bonds pull molecules sideways and inward, making the taut layer.

Quick recall: liquid water.

What are the molecules in liquid water doing?

1. A. ✓ Sliding past one another as hydrogen bonds break and new ones form
2. B. Sitting still, each held in place by its hydrogen bonds
   In a liquid the molecules keep moving; each hydrogen bond breaks as they move and a new one forms with the next neighbor.

Why: In liquid water the molecules keep moving; a hydrogen bond breaks as they move and a new one forms with the next neighbor.

Two drawings, P and Q, show water molecules.

Which drawing shows ice?

1. A. P: the molecules are closer together, so they must be locked in place
   In ice the molecules are farther apart, not closer; being close together is what liquid water looks like.
2. B. P: the molecules keep sliding past one another
   Molecules sliding past one another is liquid water, and P shows the liquid.
3. C. ✓ Q: hydrogen bonds hold the molecules farther apart in a fixed, open arrangement
4. D. Q: the molecules have no hydrogen bonds between them, so they spread out
   It is the hydrogen bonds, all held in place, that keep the molecules of ice apart; without them the molecules would not hold their open arrangement.

Why: In Q the molecules sit at the corners of open rings, each held in place by hydrogen bonds and farther from its neighbors than in P. That is ice.
In P the molecules are close together, sliding past one another: liquid water.

A block of ice and some liquid water have exactly the same mass.

Which takes up more room?

1. A. ✓ The ice, because its molecules are held farther apart
2. B. The liquid water, because its molecules move about more
   The molecules of liquid water, though moving, sit closer together on average than the molecules of ice.
3. C. Neither: the same mass always takes up the same room
   How much room a mass takes up depends on how far apart its molecules sit, and that differs between ice and liquid water.
4. D. The ice, because it has more molecules
   The same mass of water is the same number of molecules, whether ice or liquid.

Why: The same mass is the same number of molecules.
In ice the hydrogen bonds hold them farther apart, so they take up more room: ice is less dense than liquid water.

A pond freezes over in winter.

Where does the ice form, and why?

1. A. Throughout the pond at once, because all the water reaches 0 °C together
   The surface water, touching the cold air, reaches freezing first; the rest of the pond is still above 0 °C.
2. B. ✓ At the surface, and it stays there because ice is less dense than liquid water
3. C. At the bottom, because the ice that forms sinks through the liquid water
   Ice does not sink through liquid water; it is less dense than the liquid.
4. D. At the surface, because ice is warmer than the liquid water beneath it
   Ice is not warmer than the liquid water beneath it; it is at 0 °C or colder, while the water below is a few degrees above freezing.

Why: The surface water, touching the cold air, loses heat first and freezes first.
The ice stays at the surface because it is less dense than liquid water.
The pond freezes from the top down, and the water beneath stays liquid.

A student says: “Ice is solid because the hydrogen bonds in ice are much stronger than the hydrogen bonds in liquid water.”

What is wrong with this statement?

1. A. Nothing: the bonds in any solid are stronger than the bonds in its liquid
   The hydrogen bonds in ice are the same kind as in liquid water, not stronger.
2. B. Ice has no hydrogen bonds at all; covalent bonds hold its molecules to one another
   Covalent bonds hold the atoms inside one water molecule; between molecules, in ice as in the liquid, the attraction is the hydrogen bond.
3. C. The hydrogen bonds in ice are weaker than in the liquid, which is why ice is less dense
   The bonds are not weaker; ice is less dense because the four hydrogen bonds each molecule holds keep the molecules in an open arrangement.
4. D. ✓ They are the same kind of bond; in ice each molecule just holds four of them in place

Why: The hydrogen bonds in ice are the same kind as in liquid water.
What changes at freezing is that the molecules slow down and each one holds all four of its hydrogen bonds in place, in an open arrangement, instead of breaking and re-forming them.

Here is a question: ‘Sweat on a runner’s skin dries, and the skin is cooler. Which property of water is at work?’ Its options are adhesion, cohesion, evaporative cooling and surface tension. Before reading the options, a student decides what answer to expect.

What should the student expect the answer to be about?

1. A. ✓ Water leaving as vapor and carrying energy away
2. B. Water attracted to the skin, a surface other than water
   Water attracted to a surface is adhesion; nothing in the question is about the sweat sticking to the skin.
3. C. Water molecules attracting one another in the drops
   Water attracted to water is cohesion; nothing in the question is about the sweat holding together.
4. D. The taut surface layer on each drop of sweat on the skin
   The taut surface layer is surface tension; nothing in the question rests on the sweat.

Why: The question describes sweat drying and the skin cooling.
Water leaving as vapor carries energy away, so that is the property to expect before the options are read.

A question reads: ‘On a hot afternoon a pond warms by only 2 °C while the rocks beside it get too hot to touch. Which property of water is at work?’ Surface tension is one of its options.

Which fact rules surface tension out?

1. A. Surface tension is water holding to water, and the rocks beside the pond are stone.
   Water holding to water is cohesion, not surface tension.
2. B. ✓ Surface tension is the taut surface layer, and nothing in the question rests on the pond’s surface.
3. C. Surface tension is water holding to another surface, and the pond touches the rocks only at its edge.
   Water holding to another surface is adhesion, not surface tension.
4. D. Surface tension is seen only in small drops, and a pond is far too large to have any.
   A pond has a surface layer like any body of water.
   A false fact rules nothing out.

Why: Surface tension is the surface layer pulled tight so it holds together.
Nothing rests on or presses the pond’s surface, and the question is about the pond warming slowly, so that fact rules surface tension out.

A water strider is an insect that stands on the surface of a pond. A student answering ‘Which property of water lets the water strider stand on the pond?’ first picked cohesion, then changed to surface tension. Four students give their reasons for changing.

Which reason for changing to surface tension is a fact from the biology?

1. A. It had not been the answer to any of the last several questions.
   How long since an option was last the answer has nothing to do with the biology: it is a hunch.
2. B. It was the longest and the most detailed of the four options.
   The length and detail of an option have nothing to do with the biology: it is a hunch.
3. C. It sounds more scientific and more exact than cohesion does.
   How scientific an option sounds has nothing to do with the biology: it is a hunch.
4. D. ✓ It is the taut surface layer, and the strider stands on it.

Why: Surface tension is the surface layer pulled tight so it holds together, and the strider stands on that layer.
That fact rules surface tension in; the other three reasons have nothing to do with the biology.

Four students each explain why they kept their first answer, adhesion, to ‘Which property of water is at work when a drop clings to a mirror?’

Which student is going on a hunch?

1. A. Adhesion is water holding to another surface, and the glass is another surface.
   Adhesion is water holding to a surface other than water, and glass is such a surface: that is a fact, not a hunch.
2. B. Cohesion is water holding to water, but the drop is holding to the glass, so cohesion is out.
   Cohesion is water holding to water, and the drop is holding to glass: that is a fact, not a hunch.
3. C. ✓ Adhesion has been the answer to every question about glass and mirrors so far.
4. D. Surface tension is the taut surface layer, and nothing here presses on the drop’s surface.
   Surface tension is the taut surface layer, and nothing presses on the drop’s surface: that is a fact, not a hunch.

Why: ‘Adhesion has been the answer every time so far’ has nothing to do with the biology of the drop and the glass.
It is a hunch.
The other three each name a fact that rules an option in or out.

Here is one part of a free-response question: ‘(c) The glass tube is replaced by one coated inside with wax. Predict how high the water climbs in it.’

What is the task verb of this part?

1. A. Climbs
   ‘Climbs’ is a verb from inside the instruction: it says what the part is about, not what kind of answer it wants.
2. B. Coated
   ‘Coated’ is a verb from the setup sentence, ‘The glass tube is replaced by one coated inside with wax’.
3. C. ✓ Predict
4. D. Replaced
   ‘Replaced’ is the first verb of the part, but the first sentence is setup.

Why: The first sentence is setup.
The instruction that follows opens with Predict, so Predict is the task verb, and it names the kind of answer the part wants.

Here is one part of a free-response question: ‘(b) Ice floats on a pond in winter. Explain why the water at the bottom of the pond stays liquid.’

What is the task verb of this part?

1. A. ✓ Explain
2. B. Floats
   ‘Floats’ is a verb from the setup sentence, ‘Ice floats on a pond in winter’.
3. C. Stays
   ‘Stays’ is a verb from inside the instruction: it says what the part is about, not what kind of answer it wants.
4. D. Why
   ‘Why’ is not a verb; it points at what the part is about.

Why: The first sentence is setup.
The instruction that follows opens with Explain, so Explain is the task verb, and it names the kind of answer the part wants.

Here is one part of a free-response question: ‘(a) Two glasses hold the same volume of water, and one is heated on a stove. Identify the glass in which the water molecules move faster.’

What is the task verb of this part?

1. A. Heated
   ‘Heated’ is a verb from the setup sentence, ‘one is heated on a stove’.
2. B. Hold
   ‘Hold’ is the first verb of the part, but the first sentence is setup.
3. C. Move
   ‘Move’ is a verb from inside the instruction: it says what the part is about, not what kind of answer it wants.
4. D. ✓ Identify

Why: The first sentence is setup.
The instruction that follows opens with Identify, so Identify is the task verb.

Here is one part of a free-response question: ‘(c) A sewing needle is laid gently on the surface of a bowl of water and does not sink. Describe what the surface of the water does under the needle.’

What is the task verb of this part?

1. A. Laid
   ‘Laid’ is the first verb of the part, but ‘A sewing needle is laid gently on the surface’ is setup.
2. B. ✓ Describe
3. C. Sink
   ‘Sink’ belongs to the setup sentence, ‘and does not sink’.
4. D. Does
   ‘Does’ is a verb from inside the instruction: it belongs to what the part asks about.

Why: The first sentence is setup.
The instruction that follows opens with Describe, so Describe is the task verb, and it names the kind of answer the part wants.

Part (a): Describe what happens when a narrow glass tube is stood upright in a dish of water. Four students answer.

Which answer describes what happens?

1. A. ✓ The water inside the tube rises above the level of the water in the dish.
2. B. Adhesion.
   ‘Adhesion’ is a name, not a description of what is seen.
3. C. Glass carries partial charges that water molecules hydrogen-bond to.
   Partial charges and hydrogen bonds are the cause, which answers why; this part asked what happens.
4. D. In a tube lined with wax, the water inside would stand no higher than the water in the dish.
   A tube lined with wax is a different tube, and what would happen in it is a prediction, not a description of this one.

Why: Describe what happens wants what is seen: the water inside the tube stands higher than the water in the dish.
A name, a cause on its own, or a prediction about a changed tube answers a different verb.

Part (a): Describe how sweat cools the skin. Four students answer.

Which answer describes how it happens?

1. A. Sweat is mostly water, and water is an unusual liquid with many special properties that help living things stay alive.
   Water in general has many properties, and the sentence never says how sweat cools the skin.
2. B. Sweat cools the skin because water has a high specific heat capacity and warms only slowly under the sun.
   Sweat cools by evaporating, not by warming slowly.
3. C. ✓ The fastest molecules leave the sweat as vapor, carrying energy away; the sweat left behind, and the skin, are cooler.
4. D. Evaporative cooling: sweat is a liquid because its water molecules attract one another by hydrogen bonds.
   A name and a fact about why sweat is a liquid are not the steps by which it cools the skin.

Why: ‘Describe how’ wants the steps in order: the fastest molecules leave the sweat as vapor, they carry energy away with them, and the sweat left behind and the skin under it are cooler.

Part (b): Explain why a drop of water on a waxed car hood stays as a rounded bead. Four students answer.

Which answer explains why?

1. A. The drop sits on the wax as a rounded bead and stays there, keeping its rounded shape until the hood is wiped dry.
   The bead sitting on the wax is what happens, said in more words; no cause is given.
2. B. ✓ Water is not attracted to the wax, which carries no partial charges, so its molecules attract one another instead and pull it into a bead.
3. C. Wax is nonpolar, so its surface carries no charges or partial charges at all to attract water molecules.
   Nothing on the wax to attract water is the cause, but the sentence never reaches the bead: it stops one step short.
4. D. Cohesion, the attraction of water to water, is one of the many properties that hydrogen bonds give to liquid water.
   A name, cohesion, and a fact about it, with neither joined to the bead.

Why: The cause: wax carries no partial charges, so nothing attracts the water.
The link and the result: the water molecules attract one another instead, so the water pulls into a bead.
The sentence reaches the thing the part named.

Part (b): Explain why a pond warms far less than the rocks beside it over a hot day. Four students answer.

Which answer explains why?

1. A. The pond warms far less than the rocks because a pond is made of water, and water is a liquid that is always slow to warm up.
   ‘Water is slow to warm up’ restates what happens and gives no cause.
2. B. The water molecules in the pond attract one another by hydrogen bonds, and there are a very great many of them.
   Hydrogen bonds are the start of the cause, but the sentence never says where the added energy goes or that the water warms less.
3. C. Water has a high specific heat capacity, which is one of the several properties that hydrogen bonds give to water.
   High specific heat capacity is the name of the property; the sentence never says where the added energy goes or that the pond warms less.
4. D. ✓ Energy added to the water partly goes into pulling hydrogen bonds apart, not only speeding molecules up, so it warms less.

Why: The cause: energy added to the water partly goes into pulling hydrogen bonds apart rather than only making the molecules move faster.
The result the part named: so the water warms less.

Part (c): A runner sweating in dry air moves into air already full of water vapor. Predict what happens to the cooling of the runner’s skin. Four students answer.

Which answer is a prediction?

1. A. Water vapor in the air is made of the same molecules as the sweat on the runner’s skin.
   A fact about the air, not a statement of what will happen to the cooling.
2. B. The cooling of the runner’s skin changes once the air is full of water vapor.
   ‘Changes’ has no direction: it could mean more cooling or less.
3. C. Sweat cools the skin only when its fastest molecules break free of it as vapor.
   The fastest molecules breaking free is the reason sweat cools, not what will happen to the cooling in humid air.
4. D. ✓ Far less sweat evaporates, so the skin is cooled far less than it was in dry air.

Why: A prediction says what will happen to the thing named, with its direction: far less sweat evaporates, so the skin is cooled far less.

Part (c) says only: ‘The water in the kettle is heated until it boils. Predict what happens to the hydrogen bonds between its molecules.’ A student writes: ‘One hydrogen bond is far weaker than a covalent bond.’

What does the answer still need to earn the point?

1. A. ✓ The outcome and its direction: the hydrogen bonds break and the molecules leave as steam.
2. B. The name of the kind of bond that holds the atoms together inside each water molecule.
   Another name is not a prediction.
3. C. A second fact: that hydrogen bonds keep breaking and re-forming all the time in liquid water.
   Two facts about hydrogen bonds are still no prediction.
4. D. The same fact in more exact words, giving the energy needed to break one hydrogen bond.
   A more exact reason is still a reason, not a prediction.

Why: The student gave a fact, not a prediction.
A part that says only Predict awards its point for the outcome and its direction: the hydrogen bonds break and the molecules leave as steam.

Part (d): A student claims that water will stand higher inside a narrow glass tube than in the dish around it. Justify this claim. Four students answer.

Which answer justifies the claim?

1. A. Water always stands higher inside a narrow glass tube than in the dish around it; anyone who has tried it has seen this.
   Repeating the claim more firmly adds nothing that supports it.
2. B. Water is polar: its oxygen pulls the shared electrons harder, so the oxygen is δ− and each hydrogen is δ+ in every molecule.
   ‘Water is polar’ is a true fact, but nothing says how it makes the water stand higher in the tube.
3. C. ✓ Glass carries partial charges, so water hydrogen-bonds to it; the pull draws the edge of the water up, so it stands higher.
4. D. Glass is smooth and hard and does not soak up water, so the water in the tube has a firm, solid wall to stand against.
   Smoothness and hardness pull nothing, so the fact does not support the claim.

Why: The fact: glass carries partial charges, so water hydrogen-bonds to it.
How it supports the claim: that pull draws the edge of the water up the glass, so the water stands higher inside.

Part (d): Justify your prediction that in a tube lined with wax the water will stand no higher inside than outside. A student writes: ‘The water will stay exactly level in the waxed tube, just as I said, and I am sure of it.’

What does this answer need to earn the point?

1. A. ✓ The fact that wax carries no partial charges, and how that leaves nothing to pull the water up.
2. B. The prediction stated once more, in clearer and more confident words, so the scorer sees it.
   Repeating the claim, however confidently, is what the student has already done.
3. C. A measurement of exactly how high the water stands inside the clean glass tube, in millimeters.
   A measurement of the clean glass tube does not test the waxed tube the claim is about.
4. D. The name of the property at work in the clean glass tube, adhesion, added to the end of the sentence.
   ‘Adhesion’ on its own supports nothing about the waxed tube.

Why: The student has only repeated the claim.
A justification gives the fact, wax carries no partial charges, and says how it supports the claim: nothing for water to hydrogen-bond to, so nothing pulls the water up.

Part (e): Identify the kind of attraction drawn as the dashed line between molecule 1 and molecule 2.

Which answer earns the point?

1. A. A covalent bond
   A covalent bond is a bond inside a molecule, drawn as a solid line; the dashed line runs between two molecules.
2. B. ✓ A hydrogen bond
3. C. A partial charge
   A partial charge is a kind of charge, not an attraction; the partial charges are what attract.
4. D. A polar covalent bond
   A polar covalent bond is the O–H bond inside each molecule.

Why: The dashed line runs from a δ+ hydrogen of molecule 1 to the δ− oxygen of molecule 2.
That attraction between molecules is a hydrogen bond.
Identify wants exactly that name.

Part (e) is worth 1 point: Identify the property that let the water climb the plain towel. Student P writes ‘Adhesion.’ Student Q writes a paragraph that names adhesion and then explains how water hydrogen-bonds to the fibers.

How many points does each student earn?

1. A. P 0, Q 0
   Both students name adhesion, which is exactly what Identify asks for.
2. B. P 0, Q 1
   One word is enough: ‘Adhesion.’ earns the point on its own.
3. C. ✓ P 1, Q 1
4. D. P 1, Q 2
   The part is worth one point, for the name; the paragraph’s explanation earns no more.

Why: Identify awards its point for the name and nothing more.
P gave it in one word and Q gave it inside a paragraph; each earns the one point, and Q’s extra sentences earn no more.

Part (f): Represent the attraction between molecule 5 and molecule 6. Four students mark the drawing.

Which drawing represents it correctly?

1. A. W
   W joins the two oxygens, both δ−, and two δ− atoms do not attract each other.
2. B. X
   X joins two hydrogens, both δ+, and two δ+ atoms do not attract each other.
3. C. ✓ Y
4. D. Z
   Z uses a solid line, the mark for a covalent bond, not for an attraction between molecules.

Why: In Y the dashed line runs from a hydrogen of molecule 5 to the oxygen of molecule 6.
That line is the attraction, drawn between the right two atoms.
Adding δ+ at the hydrogen end and δ− at the oxygen end completes it.

Part (f): Represent the partial charges on the two atoms of an O–H bond by stating which label belongs on each atom.

Which statement answers the part?

1. A. ✓ δ− on the oxygen, δ+ on the hydrogen
2. B. δ+ on the oxygen, δ− on the hydrogen
   Oxygen pulls the shared electrons harder, so the oxygen is δ−, not δ+.
3. C. − on the oxygen, + on the hydrogen
   The pair of electrons is still shared, so the charges are partial, not full.
4. D. The two atoms carry opposite partial charges, one each
   Opposite partial charges, one each, never says which atom gets which label.

Why: The part asks for the placement in words.
Oxygen pulls the shared electrons harder, so δ− goes on the oxygen and δ+ on the hydrogen, and the statement says which is which.

An insect called a water strider stands on the surface of a pond. Here is a student’s answer to one part: ‘A molecule at the surface has neighbors beside and beneath it but nothing above it, so its hydrogen bonds pull it sideways and inward, and the surface holds together like a taut skin that holds the strider up.’

Which task verb does this answer satisfy?

1. A. Describe
   The answer does not say what is seen; it gives the cause and follows it to the result.
2. B. ✓ Explain
3. C. Identify
   The answer names nothing: it never says ‘surface tension’.
4. D. Predict
   The student changes nothing and gives no outcome under a change.

Why: The answer names the cause, a surface molecule pulled only sideways and inward, joins it with ‘so’, and reaches the result, the strider held up.
That is what Explain asks for.

Part (a) says: ‘Describe what happens when sweat on the skin dries.’ A student writes: ‘If the air is already full of water vapor, far less of the sweat will evaporate.’

Which task verb has the student answered?

1. A. Describe
   The answer never says what is seen when sweat dries; it says what would happen in humid air.
2. B. Explain
   The answer gives no cause.
3. C. Identify
   The answer names nothing.
4. D. ✓ Predict

Why: The student changed the situation to humid air and said what would follow, with a direction: far less evaporation.
That answers Predict.
The part asked Describe, so what is seen when sweat dries is still missing.

A scoring guide for one part reads: ‘Award 1 point for: only the fastest water molecules break free of their hydrogen bonds and leave as vapor, carrying their energy away, so the water left behind is cooler. Accept: the escaping molecules take heat with them, cooling what remains.’

How many points can an answer to this part earn?

1. A. ✓ 1 point
2. B. 2 points
   The Accept line is not a second point.
3. C. 3 points
   Two lines cannot make three points, and the Accept line adds none.
4. D. Half a point for each line
   Both lines describe the same one idea, so there is nothing to split; either wording earns the whole point.

Why: The Award line names one idea worth one point.
The Accept line is another wording of that same idea, so the most an answer can earn is 1 point.

The scoring guide again, in full: ‘Award 1 point for: only the fastest water molecules break free of their hydrogen bonds and leave as vapor, carrying their energy away, so the water left behind is cooler. Accept: the escaping molecules take heat with them, cooling what remains.’

What must an answer contain to earn the point?

1. A. The guide’s own words, ‘hydrogen bonds’ and ‘carrying their energy away’, used somewhere in the answer.
   The guide’s own phrases are not required; the idea is.
2. B. The full first line of the guide, copied out word for word, with nothing at all changed.
   The guide is a list of ideas, not a line to be copied.
3. C. Any true statement about sweat, or about water evaporating from the surface of the skin.
   The point is for one particular idea, not for any true statement about sweat.
4. D. ✓ The idea that the fastest molecules leave as vapor with their energy, so what is left is cooler.

Why: The point is for the idea the Award line names: the fastest molecules leave as vapor with their energy, so the water left behind is cooler.
Any wording that carries that idea earns it.

Part: Explain why the water climbs the paper towel. Guide: ‘Award 1 point for: water molecules hydrogen-bond to the partial charges on the paper fibers, so the water is pulled up the towel. Accept: adhesion to the fibers pulls the water up.’ Four more students answer.

Which answer earns the point?

1. A. ✓ Water is attracted to the fibers because both carry partial charges, so the water is pulled up.
2. B. Hydrogen bonds. Partial charges. Adhesion.
   Three names on their own say neither what attracts the water nor what that does.
3. C. Water is very important to life, has many special properties, and moves up the towel and through it.
   True sentences with no cause: nothing says what pulls the water up.
4. D. The towel is made of fibers, and water moves up between the fibers, filling the gaps between them.
   Water moving up between the fibers is the observation, not the attraction that pulls it up.

Why: The answer names the thing, water attracted to the charged fibers, and what it does, pulls the water up.
That is the Award idea in the student’s own words, so it earns the point.

Part: Explain why a drop of water on a waxed leaf stays as a rounded bead. Guide: ‘Award 1 point for: wax carries no charges or partial charges, so water is not attracted to it and its molecules attract one another instead, drawing the drop into a bead. Accept: no adhesion to the wax, so cohesion pulls the drop together.’ Four students answer.

Which answer earns the point?

1. A. Cohesion and surface tension, two of the properties that hydrogen bonds give to water and to the drop.
   Two right names say neither what attracts the water, or fails to, nor what that does to the drop.
2. B. The drop stays a bead because wax is waterproof, so the water cannot soak into the leaf beneath it.
   ‘Waterproof’ only restates that the water stays out; it gives no cause for the bead.
3. C. ✓ Nothing on the wax attracts the water, so its molecules attract one another and pull the drop into a bead.
4. D. The water molecules attract one another by hydrogen bonds, which are weak one at a time but strong together.
   Hydrogen bonds between the molecules are part of the idea, but nothing mentions the wax or reaches the bead.

Why: The answer says what does not attract the water, the wax, what attracts it instead, other water molecules, and what that does, pulls the drop into a bead.
That is the Award idea in other words.

A two-point part: Explain why water climbs a narrow glass tube. ‘Award 1 point for: water hydrogen-bonds to the partial charges on the glass. Award 1 point for: that attraction pulls the edge of the water up the glass.’ Four answers of different lengths.

Which answer earns the most points?

1. A. Water climbs glass tubes. It rises inside them. The level inside is higher than outside. Narrow tubes show this best.
   All four sentences say only that the water climbs; none names the attraction or what it does: 0 points.
2. B. Glass is a smooth solid, and water is a liquid that flows, so the water flows up into the tube.
   Smoothness and flowing are no attraction: 0 points.
3. C. Water molecules hydrogen-bond to the partial charges on the surface of glass. This is adhesion. Adhesion is water sticking to something other than water. Glass carries partial charges, and so does paper.
   The paragraph names the attraction, which earns 1, but never says what the attraction does to the water.
4. D. ✓ Water is attracted to the partial charges on the glass, and that attraction draws the edge of the water upward.

Why: One sentence carries both ideas: the attraction, water drawn to the charged glass, and what it does, pulling the edge of the water up.
It earns 2; the long paragraph earns 1 and the other two earn 0.

A one-point part: Explain why sweat cools the skin. ‘Award 1 point for: the fastest molecules leave as vapor carrying energy away, so the water left behind is cooler.’ A student’s answer earns the point in one sentence, then adds a second sentence saying the same thing in other words.

How many points does the answer earn?

1. A. 0 points
   A second correct sentence saying the same thing costs nothing.
2. B. ✓ 1 point
3. C. 2 points
   One idea said twice earns its point once.
4. D. 3 points
   The part is worth one point, for one idea.

Why: The part awards its one point once.
The first sentence earned the point; the second sentence, the same idea in other words, adds nothing, so the answer earns 1 point.

Part: Explain why an insect called a water strider can stand on the surface of a pond. Guide: ‘Award 1 point for: a molecule at the surface has no neighbors above, so its hydrogen bonds pull it sideways and inward, and the surface layer holds together like a taut skin that supports the strider. Do not award the point for: hydrogen bonds described as bonds inside a water molecule.’ A four-sentence answer scored 0. (1) Water striders live on ponds and slow streams. (2) A surface molecule has neighbors beside and beneath it but none above, so its hydrogen bonds pull it sideways and inward, and the surface holds together like a taut skin under the strider’s feet. (3) These hydrogen bonds are the covalent bonds that join the oxygen to its two hydrogens. (4) The strider’s legs are long and spread its weight.

Which sentence lost the point?

1. A. Sentence 1
   Sentence 1, where striders live, is neither the earning idea nor a contradiction of it: it earns nothing and costs nothing.
2. B. Sentence 2
   Sentence 2 carries the whole Award idea: it is the sentence that earned the point.
3. C. ✓ Sentence 3
4. D. Sentence 4
   Sentence 4, long legs spreading the weight, is true and not about the question: it earns nothing and costs nothing.

Why: Sentence 2 earned the point.
Sentence 3 calls the hydrogen bonds covalent bonds inside a molecule, which contradicts it and matches the Do not award line, so the point is lost.
Sentences 1 and 4 are not about the question and cost nothing.

Part: Explain why a drop of water clings to a glass mirror. Guide: ‘Award 1 point for: water molecules hydrogen-bond to the partial charges on the glass, so the drop is held to it. Do not award the point for: water sharing electrons with the glass.’ A student writes four sentences. (1) Drops often cling to a mirror after a shower. (2) The glass carries partial charges, so the water molecules hydrogen-bond to it and the drop is held to the glass. (3) This is adhesion, water attracted to something other than water. (4) Cohesion, water attracted to water, is a different property.

How many points does the answer earn?

1. A. 0 points
   None of sentences 1, 3 and 4 contradicts the hydrogen bonding or matches the Do not award line, so none costs anything.
2. B. ✓ 1 point
3. C. 2 points
   Naming adhesion in sentence 3 is correct but earns nothing more: the part awards one point for one idea.
4. D. 3 points
   The part is worth one point, not one per correct sentence.

Why: Sentence 2 earned the point.
Sentences 1, 3 and 4 are correct or not about the question, and none of them contradicts the hydrogen bonding or matches the Do not award line, so the answer keeps its 1 point.

Two identical dark cans, one of water and one of dry sand, stood under one lamp for 30 minutes; the sand warmed by 25 °C and the water by 8 °C. Four students each wrote one sentence for part (b), Explain why the two temperatures changed by different amounts, and each student thinks the sentence earns the point.

Which sentence earns the point for part (b)?

1. A. Water has a high specific heat capacity, so it warms slowly under the lamp; that is why the water in the can warmed less than the sand did.
   Naming the property never says where the added energy goes.
2. B. ✓ Much of the lamp’s energy went into pulling apart hydrogen bonds between water molecules, not into speeding them up, so it warmed less.
3. C. The water reflects some of the lamp’s light back upward, so it takes in less energy than the dark sand does and warms less.
   Both cans are dark and take in the lamp’s light; the water reflects no more of it than the sand does.
4. D. The lamp’s energy breaks the O–H bonds inside each molecule, so less of the energy is left over to warm the water in the can.
   The O–H bonds inside each molecule stay whole; the energy goes into pulling apart the hydrogen bonds between molecules.

Why: The sentence says where the added energy goes: into pulling apart the hydrogen bonds between water molecules.
It follows that to the result: the water warmed less.
That is the Award idea in the student’s own words.

Part (a) of the question about the two cans, Describe how the water’s temperature change compared with the sand’s: the sand went from 22 °C to 47 °C, the water from 22 °C to 30 °C. Each of four students finished the sentence ‘After 30 minutes under the same lamp, the sand had warmed by 25 °C, while the water …’ in a different way.

Which ending earns the point for part (a)?

1. A. … was held back from warming by the hydrogen bonds between its molecules.
   A reason is part (b)’s point; part (a) asks for the comparison.
2. B. … had not warmed at all by the end of the 30 minutes.
   The water did warm, from 22 °C to 30 °C, so ‘not at all’ is false.
3. C. ✓ … had warmed by only 8 °C, far less than the sand.

Why: Part (a) awards its point for the comparison: the water warmed by 8 °C, from 22 °C to 30 °C, against the sand’s 25 °C, far less.
The ending states it, with the numbers, so it earns the point.

Hemoglobin is a protein in blood that binds oxygen.

What lets hemoglobin do that job?

1. A. ✓ Its shape
2. B. Its color
   Color is what hemoglobin looks like; color does not decide what the molecule can bind.
3. C. How many copies the blood holds
   More copies bind more oxygen in total.
   Each copy binds because of its shape.

Why: A protein’s shape lets it do its job: hemoglobin’s shape lets it bind oxygen.

The task: ‘Make a claim about how coating the inside of a glass tube with wax affects how high water climbs in it.’ A student writes: ‘Wax changes how high the water climbs.’

Does the sentence earn the claim point?

1. A. Yes
   The sentence gives no direction.
   Higher or lower is the assertion the task wants.
2. B. ✓ No

Why: A claim asserts one specific thing.
‘Changes’ commits to neither higher nor lower, so the sentence asserts nothing the task did not already say.

The task: ‘Make a claim about how coating the inside of a glass tube with wax affects how high water climbs in it.’ A student writes: ‘Water climbs less high in the waxed tube than in the clean tube.’

Does the sentence earn the claim point?

1. A. ✓ Yes
2. B. No
   The sentence asserts one specific thing with a direction: less high in the waxed tube.

Why: The task asks how wax affects the height.
‘Less high in the waxed tube’ is one specific assertion about that, so it earns the point.

The task: ‘Make a claim about how coating the inside of a glass tube with wax affects how high water climbs in it.’ A student writes: ‘Water climbs the clean tube because glass carries partial charges.’

Does the sentence earn the claim point?

1. A. Yes
   The sentence is about the clean tube.
   It asserts nothing about the wax.
2. B. ✓ No

Why: The task names the wax.
A sentence about the clean glass makes no claim about the effect of wax, so it earns nothing.

The task: ‘Make a claim about how coating the inside of a glass tube with wax affects how high water climbs in it.’ A student writes: ‘In the waxed tube the water stands no higher inside than outside.’

Does the sentence earn the claim point?

1. A. ✓ Yes
2. B. No
   The sentence commits to one outcome for the waxed tube: no climb at all.

Why: ‘Stands no higher inside than outside’ commits to one outcome for the waxed tube.
That is a claim about the effect of the wax, so it earns the point.

The task: ‘Make a claim about how coating the inside of a glass tube with wax affects how high water climbs in it.’ Three students each write one sentence.

Which sentence makes a claim about the effect of the wax?

1. A. Glass and wax are both solids, and both can be shaped into a tube.
   Both being solids does not tell you how high the water climbs.
2. B. The water may behave differently in the waxed tube than in the clean one.
   ‘May behave differently’ commits to nothing.
3. C. ✓ Water climbs higher in the clean tube than in the waxed tube.

Why: A claim asserts one specific thing about the thing named.
Only ‘higher in the clean tube than in the waxed tube’ does that.

Water in a pan is warmed on a stove.

What happens to the water molecules?

1. A. ✓ They move faster
2. B. They move more slowly
   Added energy makes molecules move faster, not more slowly.
3. C. They stop moving
   Molecules in liquid water never stop moving.
   Warming speeds them up.

Why: Energy added to water makes its molecules move faster.

The task: ‘A student claims that a drop of water on a waxed leaf stays as a rounded bead. Support the student’s claim.’ Three sentences are offered.

Which sentence is the fact the support starts from?

1. A. The drop stays as a rounded bead.
   That is the claim itself, not a fact that supports it.
2. B. ✓ Wax carries no partial charges.
3. C. Water is a liquid.
   Water is a liquid on wax and on glass alike; the sentence separates nothing.

Why: The support starts from a fact about the wax.
Wax carries no partial charges, so nothing on the wax attracts water.
The bead rests on that fact.

The task: ‘A student claims that a drop of water on a waxed leaf stays as a rounded bead. Support the student’s claim.’ A student has written the fact: ‘Nothing on the surface of wax carries a partial charge.’

Which sentence links that fact to the bead?

1. A. So the wax is nonpolar, which is another way of saying that the surface of the wax carries no partial charges at all.
   ‘Nonpolar’ restates the fact in another word.
   The sentence reaches no bead.
2. B. So the drop is round, and a round drop sitting on a leaf is exactly what a bead of water on a leaf looks like.
   ‘So the drop is round’ jumps to the claim with no link from the fact.
3. C. ✓ So nothing on the wax attracts the water; the water molecules attract one another instead and pull the drop into a bead.

Why: The link runs from the fact to the claim.
Nothing on the wax attracts the water.
So the water molecules attract one another instead, and that pull makes the bead.

The task: ‘A student claims that sweat cools the skin more slowly in humid air than in dry air. Support the student’s claim.’

Which answer earns the point?

1. A. Humid air already holds a lot of water vapor, and the sweat on the skin is made of the same kind of molecule as that vapor.
   The fact stops one step short.
   Nothing reaches the cooling.
2. B. ✓ Humid air already holds a lot of water vapor, so fewer sweat molecules leave as vapor, so less energy leaves the skin.
3. C. The student is right: humid days feel sticky, and sticky skin shows that the sweat is not doing its job well.
   ‘Feels sticky’ repeats the claim in other words and gives no fact.

Why: The fact: humid air already holds much water vapor.
The link: fewer sweat molecules leave as vapor, so less energy leaves the skin.
The sentence reaches the claim: slower cooling.

The task: ‘A student claims that ice floats on liquid water. Support the student’s claim.’

Which answer earns the point?

1. A. Ice is cold, and cold things float, so the coldest water in a pond rises to the top and freezes into a layer there.
   Cold on its own floats nothing; a cold stone sinks.
2. B. Ice is a solid, and a solid holds its shape, so the ice holds itself up on top of the pond instead of mixing into it.
   A solid usually sinks in its own liquid.
   Holding a shape does not reach floating.
3. C. ✓ In ice, hydrogen bonds hold the molecules farther apart than in liquid water, so ice is less dense, so it floats.

Why: The fact: hydrogen bonds hold the molecules in ice farther apart.
So ice is less dense than liquid water.
So ice floats: the fact reaches the claim.

A student claims that a wet arm dries faster in a breeze than in still air. In still air the arm took 6 minutes to dry; in a breeze, 2 minutes.

Is the claim supported by the two times?

1. A. ✓ Yes
2. B. No
   2 minutes in the breeze against 6 minutes in still air is faster drying in the breeze.

Why: The arm dried in 2 minutes in the breeze and 6 minutes in still air.
Faster in the breeze is what the claim says, so the two times support it.

A student claims that a wet arm dries faster in a breeze than in still air. In still air the arm took 6 minutes to dry; in a breeze, 2 minutes. The claim is judged supported.

Which sentence is the ground for that judgement?

1. A. Skin has a large surface, and a large surface gives the water many places to sit.
   The skin was the same in both trials; its size separates nothing.
2. B. ✓ The arm dried in 2 minutes in the breeze and 6 minutes in still air.
3. C. Moving air feels cool on wet skin, and a cool feeling is what the student noticed.
   How the air feels is not a measurement from the two trials.

Why: The ground is the datum that decides the judgement.
Only the two drying times compare the breeze with still air.

Two identical dark cans, one of water and one of dry sand, stand under one lamp for 30 minutes. The sand warms by 23 °C and the water by 8 °C. A student claims that the two warm by the same amount.

Which answer earns the Evaluate point?

1. A. The sand warmed by 23 °C and the water by 8 °C, so the sand ended up much warmer than the water did.
   The two temperature changes are the ground, but no judgement of the claim is made.
2. B. The claim is not supported, because the student did not measure the two cans carefully enough to be sure.
   The judgement stands on a guess about the measuring, not on the data.
3. C. ✓ The claim is not supported: the sand warmed by 23 °C and the water by only 8 °C under the same lamp.

Why: Evaluate needs the judgement and its ground.
‘Not supported’ is the judgement.
23 °C against 8 °C is the ground.

A student claims that a drop of water spreads wider on glass than on wax paper. On glass a drop spread to 14 mm across; on wax paper, to 6 mm.

Which answer earns the Evaluate point?

1. A. ✓ Supported: the drop spread to 14 mm on glass and only 6 mm on wax paper.
2. B. Supported: glass is smooth, and a smooth surface lets the drop slide outward.
   ‘Glass is smooth’ is not a datum from the two measurements.
   It compares nothing.
3. C. The drop spread to 14 mm on glass and 6 mm on wax paper, a difference of 8 mm.
   The two widths are the ground, but the claim is never judged.

Why: The judgement: supported.
The ground: 14 mm on glass against 6 mm on wax paper.

Students timed how long a drop of water took to dry at three temperatures (table). At 12 °C the range is 38 to 46 minutes; at 22 °C it is 34 to 44 minutes.

Do the ranges at 12 °C and 22 °C overlap?

1. A. ✓ Yes
2. B. No
   38 to 46 and 34 to 44 share every value from 38 to 44.

Why: Two ranges overlap when they share values.
38 to 46 and 34 to 44 both contain 38 to 44, so they overlap.

Students timed how long a drop of water took to dry. At 22 °C the range is 34 to 44 minutes; at 32 °C it is 18 to 24 minutes.

Do the ranges at 22 °C and 32 °C overlap?

1. A. Yes
   18 to 24 ends before 34 to 44 begins.
   The two ranges share no value.
2. B. ✓ No

Why: 18 to 24 and 34 to 44 share no value, so they do not overlap.

Drying times for a drop of water: 12 °C, 38 to 46 minutes; 22 °C, 34 to 44 minutes; 32 °C, 18 to 24 minutes. Two times are different when their ranges do not overlap.

At which temperature is the drying time different from the time at 22 °C?

1. A. 12 °C
   38 to 46 overlaps 34 to 44, so the 12 °C time is not different.
2. B. Both 12 °C and 32 °C
   Only the 32 °C range, 18 to 24, does not overlap 34 to 44.
3. C. ✓ 32 °C

Why: The 12 °C range overlaps the 22 °C range, so those two times are not different.
The 32 °C range does not overlap, so that time is different.

Water climbed three brands of paper towel in 60 seconds: brand A 40 ± 4 mm (36 to 44), brand B 43 ± 3 mm (40 to 46), brand C 30 ± 2 mm (28 to 32). Two heights are different when their ranges do not overlap.

Which brand gives a height different from brand A’s?

1. A. Brand B
   Brand B’s range, 40 to 46 mm, overlaps brand A’s 36 to 44 mm.
2. B. ✓ Brand C
3. C. Brands B and C
   Brand B’s range overlaps brand A’s.
   Only brand C’s range does not overlap it.

Why: Brand B’s range overlaps brand A’s, so those two heights are not different.
Brand C’s range does not overlap brand A’s, so that height is different.

Water climbed three brands of paper towel in 60 seconds: brand A 40 ± 4 mm (36 to 44), brand B 43 ± 3 mm (40 to 46), brand C 30 ± 2 mm (28 to 32). A student decides that only brand C gives a height different from brand A’s.

Which line does that decision rest on?

1. A. Brand C’s average, 30 mm, is the furthest from brand A’s average of 40 mm.
   Averages alone decide nothing.
   Brand B’s average also differs from brand A’s, yet the two ranges overlap.
2. B. Brand C is the cheapest of the three brands of paper towel.
   Price is not in the data and decides nothing about height.
3. C. ✓ Brand C’s range, 28 to 32 mm, does not overlap brand A’s range of 36 to 44 mm.

Why: A Determine decision rests on the ranges.
Brand C’s range shares no value with brand A’s, so only brand C’s height is different.

Students stand strips of paper towel in water at 10 °C, 20 °C and 30 °C and measure how high the water climbs in 60 seconds. A student writes: ‘Water temperature has no effect on how high the water climbs in 60 seconds.’

Is the sentence the null hypothesis for the experiment?

1. A. ✓ Yes
2. B. No
   The sentence names the thing changed, the thing measured, and no effect.

Why: The null hypothesis says the thing changed (water temperature) has no effect on the thing measured (the height climbed).
This sentence says exactly that.

Students stand strips of paper towel in water at 10 °C, 20 °C and 30 °C and measure how high the water climbs in 60 seconds. A student writes: ‘Warmer water climbs higher.’

Is the sentence the null hypothesis for the experiment?

1. A. Yes
   The sentence claims an effect with a direction.
   That is an alternative hypothesis.
2. B. ✓ No

Why: The null hypothesis claims no effect.
‘Warmer water climbs higher’ claims an effect, so it is an alternative hypothesis.

Students stand strips of paper towel in water at 10 °C, 20 °C and 30 °C and measure how high the water climbs in 60 seconds. A student writes: ‘The temperature of the water has no effect on the height the water reaches in 60 seconds.’

Is the sentence the null hypothesis for the experiment?

1. A. ✓ Yes
2. B. No
   Different words, same claim: the thing changed has no effect on the thing measured.

Why: The sentence names the water temperature, the height reached, and no effect, so it is the null hypothesis in other words.

Students stand strips of paper towel in water at 10 °C, 20 °C and 30 °C and measure how high the water climbs in 60 seconds. A student writes: ‘Water temperature has an effect on how high the water climbs.’

Which hypothesis is the sentence?

1. A. ✓ The alternative hypothesis
2. B. The null hypothesis
   The null hypothesis says no effect.
   This sentence says there is an effect.

Why: A claim that the thing changed does have an effect on the thing measured is the alternative hypothesis.

Students time how long sweat takes to dry on skin in still air and in moving air from a fan.

Which sentence states the null hypothesis?

1. A. Moving air makes sweat dry faster than still air does.
   The sentence claims an effect with a direction: an alternative hypothesis.
2. B. ✓ Moving air has no effect on how long sweat takes to dry.
3. C. Sweat dries on skin whether the air is moving or still.
   The sentence names the thing changed but claims nothing about the drying time.

Why: The null hypothesis: the thing changed (moving or still air) has no effect on the thing measured (the drying time).

A molecule from a plant is a single chain of about 1,200 identical units of glucose, a sugar, joined one after another.

Which description fits?

1. A. ✓ One glucose unit is a monomer; the whole chain is a polymer.
2. B. One glucose unit is a polymer; the whole chain is a monomer.
   The single unit is the monomer, and the chain of many units is the polymer.
3. C. Both one glucose unit and the whole chain are monomers.
   A chain of many joined units is not a single unit.
4. D. Neither is a polymer; a polymer needs more than one kind of unit.
   A polymer does not need different kinds of unit; many copies of one unit make a polymer.

Why: The small repeating unit, one glucose, is the monomer, and the chain of 1,200 of them is the polymer.
A polymer can be built from identical units.

Which of these is a macromolecule but not a polymer?

1. A. a chain of 500 sugar units
   A chain of 500 joined sugar units is a polymer as well as a macromolecule.
2. B. a single glucose unit
   A single glucose unit is small: it is a monomer, not a macromolecule.
3. C. ✓ a fat molecule
4. D. a water molecule
   A water molecule has only three atoms and is far too small to be a macromolecule.

Why: A fat molecule is large enough to count as a macromolecule, but it is not a chain of repeating units, so it is not a polymer.

Two sugar units are joined by dehydration synthesis. The pieces removed from them join as one water molecule.

Which pieces are removed?

1. A. An –OH from each of the two units
   Two –OH groups hold two oxygens; a water molecule has only one.
2. B. An H from each of the two units
   Two hydrogens contain no oxygen and cannot make a water molecule.
3. C. ✓ An –OH from one unit and an H from the other
4. D. An –OH and an H, both from the same unit
   One piece comes off each unit, and the new bond forms between the two places the pieces came off.

Why: One unit loses its whole –OH and the other loses the H of its –OH.
Together those two pieces leave as exactly one water molecule, and the units bond where the pieces came off.

Monomers P and Q are joined by dehydration synthesis. P loses only the H of its –OH.

What happened to Q?

1. A. ✓ Q lost its whole hydroxyl group, –OH
2. B. Q also lost only an H
   Two hydrogens cannot make a water molecule.
3. C. Q lost nothing; P supplied the whole water molecule
   One monomer cannot supply both pieces of the water; one piece comes off each.
4. D. Q gained a hydroxyl group
   Gaining a group happens in the breaking reaction, not in the joining one.

Why: The released water is one H from one monomer joined to one –OH from the other.
P gave the H, so Q gave the –OH.

A water molecule joins the bond between monomers M and N, and the bond breaks. Afterwards, N carries an extra hydroxyl group.

What does M carry?

1. A. A whole extra water molecule
   The water molecule splits, so M gets only one of its two pieces.
2. B. ✓ The water’s H, joined to the oxygen M kept, so M now ends in –OH too
3. C. The water’s whole –OH as well, so M gains the same piece as N
   One water molecule splits into only one –OH and one H; N took the –OH, so M can only get the H.
4. D. Nothing extra; N took the whole water molecule
   The water splits between the two monomers rather than going whole to one of them.

Why: In hydrolysis the water molecule splits into an H and an –OH, and each piece goes to a different monomer.
N took the –OH, so M took the H, which joined the oxygen M kept: both monomers end in –OH, but M got only the H from the water.

Which statement describes hydrolysis?

1. A. Water is released and a covalent bond forms.
   Water released and a bond forming describes dehydration synthesis, the joining reaction.
2. B. Water is added and a covalent bond forms.
   Adding water breaks a bond; it does not form one.
3. C. Water is released and a covalent bond breaks.
   Breaking a bond takes a water molecule in; it does not release one.
4. D. ✓ Water is added and a covalent bond breaks.

Why: Hydrolysis adds a water molecule to the bond between two monomers, and that bond breaks.

You eat a slice of bread. Hours later, free glucose from it is in your blood.

What did hydrolysis in your gut do?

1. A. Released the energy your cells use by breaking the bonds
   Hydrolysis frees the units but does not, on its own, give your cells usable energy; a cell gets that by breaking glucose down further in later reactions.
2. B. Joined the sugar units into chains your cells can use
   Joining units into chains is dehydration synthesis, not hydrolysis.
3. C. Removed water from the bread so it could be absorbed
   Hydrolysis adds water at every bond it breaks rather than removing it.
4. D. ✓ Freed single glucose units small enough for your blood to carry

Why: Hydrolysis adds water to the bonds between the sugar units and breaks them, freeing single glucose units that your blood can carry to your cells.

Quick recall: two units join into one molecule by dehydration synthesis.

How many water molecules are released?

1. A. Two
   One –OH leaves one unit and one –H leaves the other; together they make just one water molecule.
2. B. ✓ One
3. C. None
   Joining does release water: an –OH from one unit and an –H from the other leave as one water molecule.

Why: An –OH leaves one unit and an –H leaves the other, and they join as one water molecule; breaking the bond again takes one water molecule back in.

A polymer is fully broken apart into its monomers.

Compared with the polymer, the pile of monomers weighs…

1. A. less, because bonds were broken
   Nothing leaves when a bond is broken by hydrolysis; a water molecule is added.
2. B. the same, because atoms are never created or destroyed
   Although atoms are never created or destroyed, water molecules have joined the pile, so the monomers weigh more than the polymer alone.
3. C. ✓ more, by the water added at every broken bond
4. D. more, by one water molecule in total
   One water molecule is added at every bond that breaks, not one in total.

Why: Each broken bond takes in one water molecule, and that water’s atoms end up in the monomers, so the pile weighs more than the polymer by all the water added.

A cell joins 100 monomers into one chain.

Which statement about mass is correct?

1. A. The chain weighs the same as the 100 monomers did.
   Every join released one water molecule, and that mass left the chain.
2. B. ✓ The chain weighs less than the 100 monomers did, by 99 water molecules.
3. C. The chain weighs more than the 100 monomers did, because bonds add mass.
   A bond is a shared pair of electrons and adds no mass; forming it releases water, so the chain gets lighter.
4. D. The chain weighs less than the 100 monomers did, by 100 water molecules.
   100 monomers in a row make 99 joins, not 100.

Why: 100 monomers in a row are held by 99 joins, and each join released one water molecule, so the chain weighs less than the monomers by 99 water molecules.

Inside one cell, one chain is being extended while an identical chain nearby is being taken apart.

Which way does water move in each?

1. A. ✓ Water is released where the chain is being extended and taken in where it is being taken apart.
2. B. Water is taken in where the chain is being extended and released where it is being taken apart.
   Joining a unit releases water, and breaking a chain takes water in: the water goes the opposite way in each reaction.
3. C. Water is released by both, because water always leaves when bonds change.
   Only the joining reaction releases water; the breaking reaction takes it in.
4. D. Water is taken in by both, because both reactions need water to work.
   Only the breaking reaction takes water in; the joining reaction releases it.

Why: Extending the chain is dehydration synthesis, which releases one water molecule at every new bond.
Taking the chain apart is hydrolysis, which takes one water molecule in at every bond it breaks.

A cell builds a long chain from units that are not sugars. As the cell adds each unit, one water molecule leaves.

What does this show about the reaction building the chain?

1. A. It is hydrolysis, because water takes part in the reaction.
   Hydrolysis takes water in and shortens a chain; this chain is growing and giving water off.
2. B. It is a different reaction, because the unit is not a sugar.
   The same reaction joins every kind of unit into a chain; only the unit changes.
3. C. The units are joined by hydrogen bonds, not covalent bonds.
   Dehydration synthesis joins units with covalent bonds, whatever the unit.
4. D. ✓ It is dehydration synthesis, the same reaction that joins sugar units.

Why: Every chain a cell builds is built by dehydration synthesis: one water molecule leaves at each new covalent bond, whatever the unit.
Hydrolysis would take water in and shorten the chain.

Look at the model.

Which reaction does it show, and what are the products?

1. A. ✓ Hydrolysis; a single unit and a two-unit piece
2. B. Hydrolysis; three single units
   Only one water molecule is drawn, beside one bond, so only that bond breaks.
3. C. Dehydration synthesis; a four-unit chain
   The water is on the starting side, being added, not on the product side, being released.
4. D. Dehydration synthesis; a single unit and a two-unit piece
   Water beside a bond on the starting side means hydrolysis, not dehydration synthesis.

Why: The water is on the starting side, approaching the bond between units 1 and 2, so this is hydrolysis.
Breaking that one bond leaves unit 1 on its own and units 2 and 3 still joined.

The model shows two units joined through an oxygen. The break is marked between the oxygen and unit 2.

Which piece gains the –OH from the water?

1. A. unit 1, which keeps the oxygen
   The side that keeps the oxygen already has an oxygen there; it gains only the water’s H, which completes an –OH.
2. B. ✓ the two-unit piece, units 2 and 3
3. C. both pieces gain an –OH from the water
   One water molecule splits into only one –OH; its other piece is an H.
4. D. neither; the water stays whole
   In hydrolysis the water splits, and its two pieces go to different sides of the broken bond.

Why: The break is between the oxygen and unit 2, so unit 1 keeps the oxygen and gains the water’s H.
The other piece, units 2 and 3, gains the water’s –OH.

A cell is adding units, one at a time, to the end of a growing chain.

Which way does water move as each new unit joins?

1. A. Two water molecules leave at each join, one for the H and one for the –OH
   The H and the –OH that leave make one water molecule between them, not two.
2. B. One water molecule is taken in at each join
   Joining units releases water; water is taken in only when a chain is broken apart.
3. C. ✓ One water molecule leaves at each join
4. D. One water molecule leaves for each unit, including the first
   The first unit placed makes no join, so no water leaves until the second unit joins it.

Why: Each new unit joins the chain by dehydration synthesis, and the pieces of one water molecule leave at each join.

Two monomers join into one molecule, and the product weighs less than the two monomers did together.

Where is the missing mass?

1. A. In the new covalent bond between the two monomers
   A bond is a shared pair of electrons and has no mass of its own.
2. B. Nowhere; it was destroyed when the bond formed
   Atoms are rearranged in a reaction, never destroyed.
3. C. In the –H and –OH groups still on the product
   The –H and –OH did not stay on the product; they left together as one water molecule.
4. D. ✓ In the water molecule that left as the monomers joined

Why: A whole –OH from one monomer and an H from the other left as the monomers joined, and made one water molecule.
The missing mass is the mass of that water molecule.

Look at the model.

Which reaction does it show?

1. A. Hydrolysis, because the water is on the product side
   Water on the product side means the water was made as the monomers joined, which is dehydration synthesis, not hydrolysis.
2. B. ✓ Dehydration synthesis, because the water is on the product side
3. C. Hydrolysis, because a water molecule is drawn in the model
   Both reactions involve water, so a water molecule being drawn does not by itself say which reaction it is; the side it is on does.
4. D. Dehydration synthesis, because a water molecule is drawn in the model
   The reaction is right but the reason is not: a water molecule appears in models of both reactions, and only its side tells them apart.

Why: The water is drawn on the right, the product side, so it left as the two monomers joined: dehydration synthesis.

In the model, the break is marked between the oxygen and unit 3.

Which piece gains the water’s H?

1. A. ✓ The piece with units 1 and 2, which keeps the oxygen
2. B. The piece with units 3 and 4, which did not keep the oxygen
   The side that lost the oxygen gains the water’s –OH, not just an H.
3. C. Both pieces gain an H from the water
   The water splits into one H and one –OH, and they go to different sides.
4. D. Neither piece; the water stays whole
   In hydrolysis the water splits, and its two pieces go to the two sides of the broken bond.

Why: The break is between the oxygen and unit 3, so units 1 and 2 keep the oxygen.
That side gains the water’s H, finishing an –OH; units 3 and 4 gain the water’s –OH.

Every bond in a carbohydrate is broken. What is left is many small, identical molecules, each a single sugar unit.

What are those small molecules?

1. A. Polymers, because each came out of one large molecule
   A polymer is a chain of joined units, and these are single units.
2. B. Water molecules, released as the bonds broke
   Breaking bonds takes water in rather than releasing it, and the molecules left over are sugar units, not water.
3. C. ✓ Monosaccharides, the monomers the carbohydrate was built from
4. D. Glucose, because every carbohydrate is built from glucose
   Glucose is only one of the monosaccharides, and nothing here says which sugar this carbohydrate was built from.

Why: Break a carbohydrate all the way down and you are left with its monomers, and the monomers of carbohydrates are single sugar units: monosaccharides.

Which of these is a monosaccharide?

1. A. half of a glucose unit
   Half of a glucose unit is not a sugar at all.
2. B. ✓ one glucose unit
3. C. two glucose units joined together
   Two joined sugar units are no longer a single sugar unit.
4. D. a chain of many glucose units
   A chain of many units is a polysaccharide, not a single unit.

Why: A monosaccharide is a single sugar unit that cannot be split into a smaller sugar, and one glucose unit is exactly that.

Quick recall: two sugar units join into one molecule by dehydration synthesis.

What leaves the two units as they join?

1. A. ✓ One water molecule: a whole –OH from one unit and an H from the other
2. B. One oxygen atom: the oxygen between the two units is lost as the bond forms
   The oxygen that stays is not lost: it becomes the bridge between the two units.
   What leaves is one water molecule.
3. C. Nothing: the two units bond to each other, and every atom of both stays in the product
   Not every atom stays: an –OH and an H leave, together as one water molecule, and only then does the bond form.

Why: A whole –OH leaves one unit and an H leaves the other, the two leave together as one water molecule, and a covalent bond forms through the oxygen that stayed.

A plant cell joins 500 glucose units end to end into one molecule.

What does the cell make, and how much water comes off?

1. A. One polysaccharide, and no water
   Every join is a dehydration synthesis, and each one releases a water molecule.
2. B. 500 separate monosaccharides, and 499 water molecules
   The units do not stay separate; the covalent bonds between them make one molecule.
3. C. One monosaccharide, and 500 water molecules
   Many joined units make a polysaccharide, not a monosaccharide, and 500 units have 499 joins, not 500.
4. D. ✓ One polysaccharide, and 499 water molecules

Why: Every join is a dehydration synthesis that releases one water molecule, and 500 units in a row have 499 joins.
The 499 covalent bonds make the units one large molecule: a polysaccharide.

Which of these is a polysaccharide?

1. A. one glucose unit
   One unit is a monosaccharide.
2. B. two glucose units joined together
   Two joined units are not many.
3. C. 300 glucose units dissolved in water, not joined
   Units dissolved separately in water are not joined into one molecule.
4. D. ✓ a chain of 300 glucose units joined together

Why: A polysaccharide is many monosaccharides joined to one another by covalent bonds into one molecule, and a chain of 300 joined glucose units is exactly that.

Look at the two polysaccharides, X and Y.

Which statement is correct?

1. A. ✓ X is linear and Y is branched.
2. B. X is branched and Y is linear.
   X bends but nothing comes off its sides, while Y has two side chains.
3. C. Both X and Y are linear.
   Two short chains come off Y along its length, so Y is branched.
4. D. Both X and Y are branched.
   X’s bends are not branches; nothing comes off X’s sides.

Why: X bends but has nothing coming off its sides, so it is linear.
Y has two side chains coming off along its length, so it is branched.

Two glucose stores hold the same number of glucose units. Store A is one linear chain. Store B is heavily branched.

From which store can the cell take more glucose units at the same time?

1. A. Store A
   Units come off the working ends of chains only, and a linear chain has just one.
2. B. ✓ Store B
3. C. Both the same
   The total held sets how much glucose there is, not how many working ends it can come off at once.
4. D. Neither; every store gives up one unit at a time
   Units can come off every working end of a store at the same time, not one unit at a time for the whole store.

Why: Units come off the working ends of chains.
Store B’s branches give it many working ends, so many units can be taken off at once; the linear store A has only one.

Two glucose stores hold the same number of glucose units. When a cell starts taking units off them, forty come off store P at once but only one comes off store Q.

Which store is more likely branched, and why?

1. A. ✓ P, because it has many working ends for units to come off at once
2. B. Q, because branches slow the release of units
   Every branch adds a working end where a unit can be taken off, so branches speed release rather than slowing it.
3. C. P, because a branched store holds more glucose
   Both stores hold the same number of units; branching changes how many places a unit can come off at once, not how much is held.
4. D. Neither can be told; both hold the same number of units
   The number of units coming off at once does tell the stores apart, even though they hold the same total.

Why: Units come off the working ends of chains.
Forty units at once means forty working ends, and only a branched store has that many; one at a time is a linear chain with its single working end.

A hummingbird has to power flight within a second of waking, using glucose from a store in its liver and muscles.

Which polysaccharide is that store?

1. A. starch
   Starch is the glucose store a plant makes, in a potato for example, and a hummingbird is an animal.
2. B. ✓ glycogen
3. C. cellulose
   Cellulose is the fiber of plant cell walls, not a store, and animals do not make it.
4. D. glucose
   Glucose is the single sugar the store is built from and releases; the store itself is a polysaccharide, many glucose units joined.

Why: Animals store glucose in liver and muscle as glycogen, a heavily branched polysaccharide of glucose.
Starch is the store a plant makes, and cellulose is a plant’s fiber, not a store.

A potato, a slice of liver and a cotton thread.

Which list names the main polysaccharide in each, in that order?

1. A. ✓ starch, glycogen, cellulose
2. B. glycogen, starch, cellulose
   Plants store glucose as starch, and animals store it as glycogen.
3. C. cellulose, glycogen, starch
   A potato holds starch, not cellulose, and cotton is cellulose, not starch.
4. D. starch, cellulose, glycogen
   Liver holds glycogen, not cellulose, and cotton is cellulose, not glycogen.

Why: A potato holds a plant’s glucose store, starch; liver holds an animal’s glucose store, glycogen; a cotton thread is cellulose fiber.

Which of the three polysaccharides has no branched chains at all?

1. A. starch
   Most of the chains in starch are branched, with only some unbranched ones mixed in.
2. B. glycogen
   Glycogen is the most heavily branched of the three.
3. C. ✓ cellulose
4. D. both starch and cellulose
   Starch does have branched chains; only cellulose has none.

Why: Cellulose is unbranched chains of glucose, lying straight and packed side by side.
Starch is mostly branched, and glycogen is branched even more heavily.

Quick recall: a branched chain of glucose units, like the chains in starch.

Why does a branched store come apart quickly?

1. A. Its bonds are weaker than the bonds in an unbranched chain
   The covalent bonds are the same in both chains; the difference is how many ends there are to work at.
2. B. ✓ It has many ends, and units come off only at the ends

Why: Units come off only at the ends of a chain, and a branched chain has many ends, so many units can come off at once.

Two large molecules from different organisms are broken all the way down. Both molecules give nothing but glucose, and the same amount of it. One molecule was linear; the other was branched.

What can you fairly conclude?

1. A. The two molecules are one and the same, since they broke down into the same units.
   Linear and branched are different arrangements, so these are different molecules even though both are built from glucose.
2. B. The two molecules probably do the same job, because they are made of the same monomer.
   Sharing a monomer does not make two polymers behave alike; the arrangement of the units sets the job.
3. C. ✓ The two molecules share a monomer but differ in arrangement, so they may do different jobs.
4. D. The branched one must have been built from a different sugar.
   Both gave nothing but glucose, so the monomer is the same in both.

Why: Identical breakdown products mean the monomer is the same.
Linear and branched are different arrangements of that monomer, and arrangement is what sets the job.

A student writes: “Cellulose is strong and starch is weak, so cellulose must be made from a different, stronger sugar.”

What is wrong with this?

1. A. Nothing; cellulose is built from a different sugar
   Cellulose and starch are both polymers of glucose; there is no second sugar.
2. B. ✓ Both polymers are glucose; only the arrangement of the units differs
3. C. Cellulose is stronger because it is more branched than starch
   Cellulose is the unbranched one; its strength comes from straight chains packing side by side, not from branching.
4. D. Starch is not a polymer, so it cannot be strong
   Starch is a polymer of glucose; it is built as a store rather than a fiber.

Why: Cellulose and starch are both polymers of glucose.
Cellulose is strong because its unbranched chains pack side by side into fibers; starch is branched and built to come apart.