Practice questions · Topic 1.1

Unit 1 · Practice for the Topic 1.1 end-of-topic test

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

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 free-response questions, write your answer in full sentences. Use a hint if you are stuck, then open the full-credit answer and score your own work against it.

Question 1

A potassium atom hands one electron completely to an iodine atom. The electron is now held by the iodine alone.

What are the two atoms now, and why do they stay close together?

A. Two ions, K⁺ and I⁻, each carrying a full charge; the opposite charges attract B. Two atoms joined by a covalent bond; the moved electron is the shared pair C. Two atoms carrying partial charges, δ+ on potassium and δ− on iodine D. Two atoms held by a hydrogen bond between the potassium and the iodine

Question 2

Chlorine pulls shared electrons harder than hydrogen does. Chlorine gas, Cl₂, is two chlorine atoms sharing one pair of electrons. Hydrogen chloride, HCl, is a hydrogen atom and a chlorine atom sharing one pair.

Which of these bonds is polar?

A. The Cl–Cl bond only, because chlorine pulls shared electrons hard B. The H–Cl bond only, because chlorine pulls the pair harder than hydrogen C. Both bonds, because every bond that contains chlorine is polar D. Neither bond, because a shared pair always sits in the middle

Question 3

Nitrogen pulls shared electrons harder than carbon does. A molecule contains a C–N covalent bond.

Which labels belong on the two atoms of this bond?

A. C δ− and N δ+ B. C a full + and N a full − C. C δ+ and N δ− D. No label on either; the electrons are shared

Question 4

A molecule of hydrogen peroxide, H₂O₂. Its atoms are numbered 1 to 4; the solid lines are covalent bonds.

The drawing shows a molecule of hydrogen peroxide, H₂O₂, with its atoms numbered 1 to 4. Oxygen pulls shared electrons harder than hydrogen does, and the two oxygen atoms pull equally on the pair they share.

Which atoms carry the label δ−?

A. Atoms 1 and 4 B. Atoms 2 and 3 C. Atom 2 only D. All four atoms

Question 5

A methanol molecule (left) beside a water molecule (right). Solid lines are covalent bonds. A student has drawn two dashed lines, 1 and 2, as possible attractions between the molecules.

The drawing shows a methanol molecule, CH₃OH, beside a water molecule. Solid lines are covalent bonds. Dashed line 1 runs from the hydrogen of methanol’s O–H group to the water’s oxygen. Dashed line 2 runs from one of the hydrogens bonded to methanol’s carbon to the water’s oxygen.

Which dashed line shows a hydrogen bond?

A. Line 1 only B. Line 2 only C. Both line 1 and line 2 D. Neither line

Question 6

A hospital sterilizer heats water until it turns to steam. A student writes: “The heat splits the water into hydrogen gas and oxygen gas, and that mixture is the steam.”

Which statement corrects the student?

A. The steam is whole water molecules; the heat broke the hydrogen bonds between molecules, and the covalent bonds inside each molecule held B. The steam is hydrogen atoms and oxygen atoms; the heat broke the covalent O–H bonds, which are the weakest bonds in water C. The steam is whole molecules of water; the heat broke the covalent bonds that had joined each molecule to its neighbors D. The steam is single oxygen atoms; the hydrogen atoms stayed behind in the liquid, so the liquid became rich in hydrogen

Question 7

Three observations. I: after rain, blades of grass, which are a polar surface, stay coated in a clinging film of water. II: water poured slowly from a jug falls as one unbroken stream. III: a drop of water placed on a clean glass plate spreads out into a thin film.

Which of these observations depends on cohesion?

A. I only B. II only C. III only D. I and III

Question 8

A sugar cube is stood in a shallow dish of colored water. Sugar molecules carry many –OH groups, which are polar. Within a minute the color has climbed to the top of the cube, well above the water in the dish.

Which attraction pulls the water onto the surfaces of the sugar?

A. Cohesion: the water molecules hydrogen-bond to one another and climb the cube as a group B. Surface tension: the taut surface layer of the water in the dish pushes water up the cube C. Adhesion: the water molecules hydrogen-bond to the partial charges on the sugar’s –OH groups D. Adhesion: the sugar’s –OH groups pull water because they are nonpolar

Question 9

Three liquids, 100 g of each, sit in identical sealed flasks on identical hot plates that deliver the same energy every minute. After 3 min the water has warmed from 20 °C to 26 °C, liquid X from 20 °C to 35 °C, and liquid Y from 20 °C to 44 °C.

Which liquid has the highest specific heat capacity, and why did its temperature rise so little?

A. Liquid Y; it warmed the most, so it took in the most energy from its hot plate B. Water; part of the energy added to it is spent pulling hydrogen bonds apart, so less goes into faster motion C. Liquid X; it warmed by a middling amount, so its value lies highest of the three D. Water; its molecules are the heaviest of the three, so the same energy speeds them up the least

Question 10

A student lays a dry steel needle flat on the still surface of a bowl of water. The needle rests on the surface instead of sinking, even though steel is far denser than water.

Why does the surface of the water hold the needle up?

A. The water molecules at the surface hydrogen-bond to the steel needle, and this adhesion between water and steel holds the needle up B. The water molecules at the surface form covalent bonds with one another, making a solid skin that the needle rests on C. The water molecules at the surface have neighbors beside and below but none above, so hydrogen bonds pull them sideways and inward into a taut layer D. The water molecules at the surface move faster than the ones below them, and their constant motion pushes the needle upward

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 · Conceptual Analysis · 5 points

On hot days, worker honeybees carry water into the hive, spread it in thin films over the wax comb, and fan the films with their wings. The inside of the hive stays near 35 °C even when the air outside reaches 45 °C. Some hot days are also humid, with far more water vapor in the air, and the bees fan the same water films on those days too.

(a) Identify the property of water that the bees are using to cool the hive. (1 point)

Hint: The films are thin, spread wide and fanned: what is that arrangement for?

A full-credit answer: The bees are using evaporative cooling, which cools the comb as the water in the films evaporates into the air.

Check the box for each point your answer earns

evaporative cooling, named and tied to the water films evaporating.

Accept: ‘the water evaporates and cools the comb’ with the name.

Do not award: specific heat capacity or surface tension; the comb is cooled by water leaving as vapor.

Common slip: Naming specific heat capacity because water is being used to keep a temperature steady. The films are thin and fanned so that they evaporate; the cooling comes from the water that leaves.

(b) Describe which water molecules leave a film as vapor, and what they take with them. (1 point)

Hint: The molecules in liquid water move at many different speeds.

A full-credit answer: Only the fastest molecules leave the film, because only they have enough energy to break their hydrogen bonds to their neighbors. They carry their energy away with them, so the water left behind is, on average, slower and therefore cooler.

Check the box for each point your answer earns

only the fastest molecules leave, and they carry their energy away, leaving the remaining water cooler.

Accept: ‘the molecules with the most energy escape, so the average energy of the rest falls’.

Do not award: ‘the water absorbs heat and evaporates’ with no mention of which molecules leave or what they carry.

Common slip: Writing that evaporation ‘uses up heat’ with no molecules in the sentence. The point is that the fastest molecules are the ones that escape, and their energy goes with them.

(c) Explain why a water molecule needs so much energy to leave the liquid. Start from the O–H bonds inside the molecules. (1 point)

Hint: What must a molecule at the surface break free from before it can leave the liquid?

A full-credit answer: Oxygen pulls the shared electrons of each O–H bond harder than hydrogen does, so the oxygen is δ− and each hydrogen is δ+. The δ+ hydrogen of one molecule is attracted to the δ− oxygen of a neighbor, which is a hydrogen bond. A molecule can leave as vapor only by breaking every hydrogen bond to its neighbors, and that takes a great deal of energy, which is why water’s heat of vaporization is high.

Check the box for each point your answer earns

the polar O–H bond gives partial charges, the partial charges make hydrogen bonds between molecules, and a molecule must break those hydrogen bonds to leave.

Accept: the same chain in plain words, provided the hydrogen bonds are between molecules and are what the leaving molecule breaks.

Do not award: energy spent breaking the covalent O–H bonds, or ‘hydrogen bonds’ with no source in the polar bond.

Common slip: Starting at ‘hydrogen bonds hold the molecules together’. The point needs where the hydrogen bonds come from: oxygen pulling the shared electrons harder, and the partial charges that creates.

(d) Predict what happens to the temperature inside the hive on a hot day that is also humid, compared with a hot dry day. (1 point)

Hint: How does wet laundry dry on a humid day compared with a dry day?

A full-credit answer: On a humid day the hive temperature rises higher than on a dry day, even though the bees spread and fan the same amount of water.

Check the box for each point your answer earns

the hive warms more (rises higher) on the humid day.

Accept: ‘the cooling works less well, so the hive is hotter’.

Do not award: the hive stays at 35 °C, or the hive is cooler on the humid day.

Common slip: Predicting that the hive stays at 35 °C because the bees are doing the same work. The bees spread the same water, but less of it leaves as vapor.

(e) Justify your prediction in (d) using what happens to the water films in humid air. (1 point)

Hint: What is the air above the films already full of on a humid day?

A full-credit answer: In humid air, the air already holds a great deal of water vapor, so far less of the water in the films evaporates. Fewer fast molecules leave and carry their energy away, so less heat leaves the comb and the hive warms.

Check the box for each point your answer earns

humid air already holds much water vapor, so less water evaporates from the films and less energy is carried away from the comb.

Accept: ‘the water stays liquid on the comb instead of leaving as vapor, so it takes almost no heat away’.

Do not award: ‘humid air is hotter’, or ‘the water films trap heat’.

Common slip: Confusing humid with hot. Humid air carries more water vapor, so the films cannot lose their fastest molecules into it; the water stays and the heat stays.

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

A student leaves 500 g of dry sand and 500 g of water in identical open trays in the sun for 30 minutes, then moves both trays into the shade for 30 minutes, reading the temperature of each every half hour. The readings: at 0 min, sand 20 °C and water 20 °C; at 30 min (end of sun), sand 41 °C and water 26 °C; at 60 min (end of shade), sand 24 °C and water 25 °C. Most of the mass of a person is water.

(a) Describe how the temperature of each sample changed over the hour. (1 point)

A full-credit answer: In the sun the sand warmed by 21 °C and the water by only 6 °C; in the shade the sand cooled by 17 °C while the water cooled by only 1 °C. The sand’s temperature swung far more than the water’s in both directions.

Check the box for each point your answer earns

the sand warmed far more in the sun (21 °C against 6 °C) and cooled far more in the shade (17 °C against 1 °C), with values from the table.

Accept: ‘the water’s temperature changed much less than the sand’s, both warming and cooling’ with at least one pair of values.

Do not award: a description of only the warming half, or values with no units.

Common slip: Describing only the warming and stopping. The cooling readings are the second half of the pattern: the water gave up its heat slowly as well as taking it in slowly.

(b) Explain why the water’s temperature changed so much less than the sand’s, even though both trays took in the same sunlight. Start from the attractions between water molecules. (1 point)

A full-credit answer: Water molecules are held to one another by hydrogen bonds. Energy added to water is partly spent pulling hydrogen bonds apart rather than only making the molecules move faster, so each degree of warming costs water far more energy than it costs sand: water has a high specific heat capacity. When the water cools, the same large amount of energy has to leave for each degree it drops, so it cools slowly too.

Check the box for each point your answer earns

energy added to water is partly spent pulling hydrogen bonds apart rather than only speeding the molecules up, so water needs more energy per degree (a high specific heat capacity) and its temperature changes slowly.

Accept: ‘water has a high specific heat capacity’ provided the hydrogen bonds are given as the reason.

Do not award: ‘water holds heat’ or ‘water reflects sunlight’ with no hydrogen bonds.

Common slip: Naming specific heat capacity and stopping. The point needs the mechanism: part of the energy goes into pulling hydrogen bonds apart instead of into faster motion.

(c) A person rests in the same sun for 30 minutes. Predict how the change in their body temperature compares with the change in the sand’s temperature. (1 point)

A full-credit answer: The person’s body temperature rises far less than the sand’s did, by a fraction of a degree rather than by 21 °C.

Check the box for each point your answer earns

the person’s body temperature changes much less than the sand’s (a small rise, well under the sand’s 21 °C).

Accept: ‘it barely changes’ or ‘it rises only slightly’.

Do not award: a rise as large as the sand’s, or ‘it stays exactly the same’.

Common slip: Predicting that body temperature does not change at all. Water slows the change; it does not stop it.

(d) Justify your prediction in (c), and name the process that keeping internal conditions steady is part of. (1 point)

A full-credit answer: Most of a person’s mass is water, so warming the body by one degree takes about as much energy as warming the same mass of water, far more than the same mass of sand needs. The heat taken in over 30 minutes therefore changes the body’s temperature only slowly. Keeping internal conditions steady like this is homeostasis, and water’s high specific heat capacity helps with it.

Check the box for each point your answer earns

most of the body is water, which needs a large amount of energy per degree, so its temperature changes slowly; this helps keep internal conditions steady, which is homeostasis.

Accept: ‘the body behaves like the tray of water, not the tray of sand’ together with the name homeostasis.

Do not award: homeostasis named with no link to water, or ‘the body sweats’ as the only reason.

Common slip: Answering with sweating alone. Sweating is a different property at work; the question is about how slowly the body warms because it is mostly water.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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