Structure of Water and Hydrogen Bonding

Unit 1 · Topic 1.1 end-of-topic test

Answer every question. For each multiple-choice question choose one answer and press Check; the feedback tells you what a wrong choice assumed. For the two free-response questions, write your answer in full sentences, then open the scoring guide and mark your own work against it.

Question 1

A drawing of a molecule shows a pair of electrons sitting between a carbon atom and a hydrogen atom, with both atoms holding on to that pair.

What does the drawing show?

A. A covalent bond B. A hydrogen bond C. An attraction between two ions D. A partial charge

Question 2

A lithium atom hands one electron completely to a fluorine atom.

What charge does each atom carry afterward?

A. Lithium slightly positive (δ+) and fluorine slightly negative (δ−) B. Lithium a full positive charge and fluorine a full negative charge C. Lithium a full negative charge and fluorine a full positive charge D. Neither carries a charge, because the pair of atoms stays neutral overall

Question 3

Oxygen pulls shared electrons harder than nitrogen does; nitrogen pulls harder than carbon; carbon and hydrogen pull about equally. A molecule of ammonia has three N–H bonds. A molecule of methane has four C–H bonds.

Which of these bonds are polar?

A. The C–H bonds only, because carbon pulls the shared electrons much harder than hydrogen B. Both kinds, because any covalent bond that contains a hydrogen atom is polar C. The N–H bonds only, because nitrogen pulls the shared electrons harder than hydrogen D. Neither kind, because both are covalent bonds and covalent bonds share evenly

Question 4

In a molecule of hydrogen fluoride, H–F, the fluorine atom pulls the shared pair of electrons much harder than the hydrogen atom does.

Which describes the charges on the two atoms?

A. Fluorine slightly negative (δ−), hydrogen slightly positive (δ+) B. Fluorine a full negative charge and hydrogen a full positive charge, as ions C. Fluorine slightly positive (δ+), hydrogen slightly negative (δ−) D. No charge on either atom, because the electrons are shared

Question 5

A water molecule, H₂O, is one oxygen atom covalently bonded to two hydrogen atoms.

Why is the oxygen end of the molecule slightly negative?

A. Oxygen has taken one electron completely from each of the two hydrogens B. Oxygen carries more protons than each hydrogen does, and protons are negative C. Water is an ion, so the whole molecule carries a negative charge D. Oxygen pulls the shared electrons of each O–H bond harder than hydrogen does

Question 6

Part of a larger molecule. Positions 1, 2 and 3 mark a carbon atom, an oxygen atom and a hydrogen atom; the solid lines are covalent bonds.

The drawing shows part of a larger molecule: a carbon atom bonded to an oxygen atom, which is bonded to a hydrogen atom. Oxygen pulls shared electrons harder than either carbon or hydrogen does. Position 2 is the oxygen and position 3 is the hydrogen.

Which partial-charge labels belong at positions 2 and 3?

A. Position 2 δ+, position 3 δ− B. Position 2 δ−, position 3 δ+ C. Position 2 a full −, position 3 a full + D. Position 2 δ−, position 3 δ−

Question 7

Three water molecules. Solid lines are covalent bonds; the dashed lines 1, 2 and 3 are attractions a student has drawn.

The drawing shows three water molecules. The solid lines are covalent O–H bonds. A student has drawn three dashed lines, numbered 1, 2 and 3, to show attractions between the molecules.

Which dashed line shows a hydrogen bond?

A. Line 2 only B. Line 3 only C. Line 1 only D. Lines 1, 2 and 3

Question 8

Water boils at 100 °C. Hydrogen sulfide, H₂S, is a molecule of about the same size and shape, but its molecules do not form hydrogen bonds with one another. It boils at −60 °C.

Why is water's boiling point so much higher?

A. Boiling water has to break the O–H covalent bonds inside each molecule B. Water molecules are lighter, so the attractions between them are stronger C. Electrons move between water molecules, making ions that attract one another D. Many hydrogen bonds together hold water's molecules to one another

Question 9

A drop of water placed on a waxed car hood, a nonpolar surface, pulls itself into a rounded bead instead of spreading out.

What holds the water molecules of the bead together?

A. Hydrogen bonds between the water molecules, which is cohesion B. Hydrogen bonds between the water and the wax, which is adhesion C. Covalent bonds that form between neighboring water molecules D. The high specific heat capacity of water, which keeps the drop cool

Question 10

The same drop of water placed on a clean glass plate spreads out into a thin film instead of pulling into a bead.

What does this show about the glass, and why does the water spread?

A. The glass is warm, so the molecules move faster and slide apart B. The glass is nonpolar, so the water is pushed flat against it C. The glass carries partial charges, and the water hydrogen-bonds to them D. The glass carries partial charges, which break the hydrogen bonds in the drop

Question 11

A water strider stands on a pond without breaking through the surface. Someone adds a drop of soap to the water near it, and the strider sinks.

Which property of the water did the soap reduce?

A. Adhesion B. Surface tension C. Specific heat capacity D. Heat of vaporization

Question 12

One end of a dry cotton string hangs into a cup of water. Cotton is a polar material. An hour later, water has traveled up the string well above the level of the water in the cup.

Which properties are at work, and what does each do?

A. Adhesion alone: every water molecule sticks to the cotton and climbs by itself B. Surface tension pulls the water up; adhesion holds it at the top C. Cohesion pulls water to the cotton; adhesion drags the column behind D. Adhesion pulls water to the cotton; cohesion drags the column behind

Question 13

Two identical hot plates each deliver the same energy every minute. One heats 100 g of water; the other heats 100 g of cooking oil. Readings: 0 min, water 20 °C, oil 20 °C. 2 min, water 24 °C, oil 28 °C. 4 min, water 28 °C, oil 36 °C.

What do the readings show about the two liquids?

A. Water has the higher specific heat capacity: it needs more energy to warm by one degree B. Oil has the higher specific heat capacity: it warmed more in the same time C. The oil took in more energy from its hot plate than the water did from its plate D. The two liquids have the same specific heat capacity, because both received the same energy

Question 14

100 g of water and 100 g of cooking oil sit on identical hot plates that deliver the same energy every minute. The oil's molecules do not form hydrogen bonds with one another. After four minutes the oil has warmed twice as many degrees as the water.

Why did the water's temperature rise more slowly than the oil's?

A. Water reflects most of the heat coming from the hot plate, so much less energy gets into the water than into the oil B. Energy added to water is partly spent pulling hydrogen bonds apart rather than only making the molecules move faster C. Energy added to water is spent breaking the O–H covalent bonds inside each of its molecules instead of making them move D. Water molecules are heavier than oil molecules, so the same energy makes them move more slowly and warm less

Question 15

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 by morning?

A. Both greenhouses cool by the same amount, because the barrels are sealed B. The greenhouse with barrels cools less, because water blocks heat from leaving through the glass C. The greenhouse with barrels cools more, because water pulls heat out of the air D. The greenhouse with barrels cools less, because the water gives up its heat slowly

Question 16

A nurse wipes one patient's arm with one gram of water and another patient's arm with one gram of rubbing alcohol. The alcohol has evaporated completely within a minute; the water takes several minutes.

Which arm lost more energy to the evaporating liquid, and which quantity tells you so?

A. The alcohol arm; the alcohol evaporated faster, so it took more with it B. Both the same; the same mass evaporated from each arm C. The water arm; water has the higher heat of vaporization D. The water arm; water has the higher specific heat capacity

Question 17

On a warm, breezy day a hiker wraps one water bottle in a wet cloth and an identical bottle in a dry cloth. An hour later, the water in the wet-wrapped bottle is cooler than the water in the other bottle.

Why is the wet-wrapped bottle cooler?

A. The fastest water molecules leave the wet cloth as vapor and carry their energy away B. The wet cloth blocks the warm air from reaching the bottle, so no energy can get in C. The water in the cloth has a high specific heat capacity, so it soaks up the bottle's heat D. Evaporating water breaks the O–H covalent bonds, and breaking them takes in energy

Question 18

One large molecule drawn as a long chain folded back on itself. Solid lines are covalent bonds; the dashed line is the attraction in question.

The drawing shows one large molecule: a long chain folded back on itself. On the upper part of the chain an oxygen atom carries a hydrogen; on the lower part of the same chain there is another oxygen atom. A dashed line runs from that hydrogen to the second oxygen.

What does the dashed line represent?

A. A covalent bond, because both atoms belong to the same molecule B. A hydrogen bond, which can form within one large molecule C. A repulsion, because the hydrogen and the oxygen both carry partial charges D. An attraction between two ions, one on each part of the chain

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Analyze Model or Visual Representation · 4 points

The model shows three water molecules near the surface of a pond. Solid lines are covalent bonds. Dashed lines are attractions between molecules. Molecule 1 has its partial charges marked; molecules 2 and 3 do not. One dashed line is labeled X and one solid line is labeled Y.

Three water molecules. Molecule 1 carries partial-charge labels; X marks one dashed line and Y marks one solid line.

(a) Describe what X and Y each represent, and compare their strengths. (1 point)

A full-credit answer: X is a hydrogen bond: the attraction between a δ+ hydrogen of one water molecule and the δ− oxygen of a neighboring molecule. Y is a covalent O–H bond inside one molecule, a shared pair of electrons. A single hydrogen bond is far weaker than a covalent bond.

Check the box for each point your answer earns

X is a hydrogen bond, the attraction between a hydrogen of one water molecule and the oxygen of a neighboring molecule; Y is a covalent O–H bond inside one molecule; a single hydrogen bond (X) is far weaker than a covalent bond (Y).

Accept: X described as an attraction between a slightly positive hydrogen and a slightly negative oxygen on the next molecule; Y described as a shared pair of electrons; either 'X is much weaker than Y' or 'Y is much stronger than X'.

Do not award the point if X is called a covalent bond, or if X is described as stronger than Y.

Common slip: Calling X a covalent bond because it is drawn between atoms. A dashed line between molecules is an attraction, not a shared pair of electrons, and it is the weaker of the two.

(b) Explain why attraction X exists. Start from the O–H bonds inside the molecules. (1 point)

A full-credit answer: Oxygen pulls the shared electrons of each O–H bond harder than hydrogen does, so the oxygen is slightly negative (δ−) and each hydrogen slightly positive (δ+). The δ+ hydrogen of one molecule and the δ− oxygen of the next carry opposite partial charges, and opposite charges attract: that attraction is X.

Check the box for each point your answer earns

oxygen pulls the shared electrons of each O–H bond harder than hydrogen does, so the oxygen is slightly negative (δ−) and each hydrogen slightly positive (δ+); the δ+ hydrogen of one molecule and the δ− oxygen of the other carry opposite partial charges, which attract.

Accept: 'the O–H bonds are polar, so opposite partial charges on neighboring molecules attract', provided both the uneven pull (or polarity) and the opposite partial charges are named.

Do not award the point for 'opposite charges attract' alone, without saying where the partial charges come from.

Common slip: Writing ‘opposite charges attract’ and stopping. The point needs where the partial charges come from: oxygen pulling the shared electrons harder than hydrogen.

(c) Molecule 2 has no partial charges marked. Represent them by stating which label, δ+ or δ−, belongs on each of its three atoms. (1 point)

A full-credit answer: The oxygen of molecule 2 carries δ−, and each of its two hydrogens carries δ+.

Check the box for each point your answer earns

δ− on the oxygen of molecule 2 and δ+ on each of its two hydrogens (all three atoms labeled correctly).

Do not award the point for full charges (+ and −), for δ+ on the oxygen, or for a label on only one of the two hydrogens.

Common slip: Writing full + and − signs, or labeling only one hydrogen. The charges are partial, and both hydrogens carry δ+.

(d) A water strider stands on the surface of the pond without breaking through. Explain how the attractions shown in the model produce the property of the surface that holds the strider up. (1 point)

A full-credit answer: A water molecule at the surface has neighbors beside and below it but none above, so its hydrogen bonds pull it sideways and inward. The surface layer holds together like a taut skin, which is surface tension, and the strider’s weight is not enough to break through it.

Check the box for each point your answer earns

a water molecule at the surface has neighbors beside and below it but none above, so its hydrogen bonds pull it sideways and inward; the surface layer holds together like a taut skin, which is surface tension, and the strider's weight is not enough to break through it.

Accept: 'many hydrogen bonds together hold the surface molecules to one another strongly, so the surface resists being broken (surface tension)', provided hydrogen bonds between water molecules are named as the cause.

Do not award the point for naming surface tension without linking it to hydrogen bonds between water molecules, or for saying the strider floats because it is less dense than water.

Common slip: Naming surface tension without saying what makes it. The point is earned by linking the taut surface to hydrogen bonds pulling the surface molecules sideways and inward.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

On a hot, dry afternoon a runner's skin is covered in sweat, which is mostly water. As the sweat evaporates, the runner's skin temperature stays close to normal even though the working body is producing a great deal of heat. Later the runner enters a valley where the air is already full of water vapor. Sweat now forms drops that run off the skin instead of evaporating.

(a) Describe how evaporating sweat cools the runner's skin. (1 point)

A full-credit answer: Evaporating sweat cools the skin because only the fastest water molecules break free of their hydrogen bonds and leave as vapor, and each one carries its energy away with it. The liquid left behind is slower on average, so it and the skin it touches are cooler: evaporative cooling.

Check the box for each point your answer earns

water molecules leaving the skin as vapor carry energy away with them, so the liquid left behind, and the skin it touches, are cooler (evaporative cooling).

Accept: 'only the fastest molecules escape, and they take their energy with them, leaving the rest cooler'. The reason so much energy goes with each gram (the hydrogen bonds) belongs to part (b) and is not needed here.

Do not award the point for 'sweat is cold' or 'liquid sweat absorbs heat' with no molecule leaving as vapor.

Common slip: Saying sweat cools because it is wet or cold. Liquid sweat sitting on the skin does nothing; the cooling happens only when molecules leave as vapor.

(b) Explain why so much energy leaves the skin with each gram of sweat that evaporates. Start from the attractions between water molecules. (1 point)

A full-credit answer: Water molecules are held to their neighbors by hydrogen bonds, the attractions between the δ+ hydrogen of one molecule and the δ− oxygen of another that come from the polar O–H bonds. A molecule can leave as vapor only by breaking those hydrogen bonds, so each gram of water that evaporates takes a large amount of energy with it: water’s high heat of vaporization.

Check the box for each point your answer earns

water molecules are held to their neighbors by hydrogen bonds (attractions between the δ+ hydrogen of one molecule and the δ− oxygen of another, which come from the polar O–H bonds); a molecule can leave as vapor only by breaking its hydrogen bonds, so a large amount of energy is needed for each gram of water turned to gas (water's high heat of vaporization).

Accept: 'each escaping molecule must break its hydrogen bonds to its neighbors, which takes a lot of energy, so each gram that evaporates carries a lot of energy away', with or without the name heat of vaporization.

Do not award the point for breaking the O–H covalent bonds inside water molecules, for naming heat of vaporization without saying that hydrogen bonds must be broken, or for repeating part (a) (molecules leave and carry energy away) with no hydrogen bonds.

Common slip: Having evaporation break the covalent O–H bonds. Molecules leave whole; what each one breaks is its hydrogen bonds to its neighbors.

(c) Predict what happens to the runner's body temperature in the valley, where sweat runs off instead of evaporating. (1 point)

A full-credit answer: The runner’s body temperature rises; the body can no longer hold its temperature down, and the runner overheats.

Check the box for each point your answer earns

the runner's body temperature rises (the body can no longer hold its temperature down; the runner overheats).

Accept: 'the skin gets hotter' or 'the runner cannot cool down'.

Do not award the point for 'it stays the same because the runner still sweats' or for 'it falls because the skin is wet'.

Common slip: Predicting no change because the runner is still sweating. Sweat that runs off as liquid has done no cooling.

(d) Justify your prediction, using the attractions between water molecules and what happens to the sweat in the valley. (1 point)

A full-credit answer: Sweat that runs off as liquid has not broken its hydrogen bonds to become vapor, so it carries almost no energy away. With little evaporation, little energy leaves the skin while the body keeps producing heat, so body temperature climbs and the steady internal condition, homeostasis, is harder to hold.

Check the box for each point your answer earns

sweat that runs off as liquid has not broken its hydrogen bonds to become vapor, so it carries almost no energy away; with little evaporation, little energy leaves the skin while the body keeps producing heat, so body temperature climbs and the body's steady internal condition (homeostasis) is harder to hold.

Accept: 'air already full of water vapor slows evaporation; less water turns to vapor, so less energy is removed and the body's heat builds up', provided the link between evaporation and energy removed is stated.

Do not award the point for 'humid air is hotter' or 'the sweat holds heat against the skin' as the reason.

Common slip: Blaming the humid air for being hotter, or the wet sweat for holding heat in. The reason is what the sweat no longer does: leave as vapor and carry energy away.

Free-response score: 0 of 4

Multiple choice checked: 0 of 18 correct.

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