Practice questions · Topic 1.5

Unit 1 · Practice for the Topic 1.5 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 duck spreads oil from a gland near its tail over its feathers. Rain beads up on the oiled feathers and rolls off. In a separate test, a pinch of table salt dropped into a glass of water disappears into the water within a minute.

How should the duck’s oil and the salt be classified?

A. The oil is hydrophobic and a lipid; the salt is hydrophilic B. The oil is hydrophilic and a lipid; the salt is hydrophobic C. The oil is hydrophobic and the salt is hydrophobic; only the salt is a lipid D. Both are hydrophilic; the oil beads up because it is light

Question 2

A student says: “Water dissolves a substance only when it can hydrogen-bond to it.” Yet table salt, which is made of ions and has no hydrogen to offer, dissolves in water.

Which statement explains how the salt dissolves?

A. The ions form hydrogen bonds to the water, so the student is right B. Water’s partial charges pull on the ions’ full charges, so no hydrogen bond is needed C. The ions have no charge, and uncharged particles slip between the water molecules D. Water breaks the ions into smaller pieces that carry partial charges

Question 3

Candle wax is a hydrocarbon: long chains of carbon and hydrogen atoms and nothing else. Drops of melted wax that fall into a bowl of water set as solid beads and stay separate from the water.

Why does the water exclude the wax?

A. The wax carries a full charge along its chains, and water’s partial charges push that charge away B. The C–H bonds of the wax are polar, so the wax molecules are attracted to one another far more strongly than they are to water C. The wax is too heavy for the water molecules to lift into the liquid, so it stays as beads on the surface D. The wax has no charge or partial charge for water to pull on, so the water molecules stay attracted to one another and exclude it

Question 4

Two fatty acids are compared. Each has a carboxyl group, –COOH, at one end. One has a tail of 4 carbons; the other has a tail of 18 carbons.

Which fatty acid is more strongly hydrophobic, and why?

A. The 4-carbon one, because a shorter chain is excluded from water more easily than a long one B. Neither; both carry a polar carboxyl group, so both mix into water completely and stay mixed C. The 18-carbon one, because a larger share of the molecule is nonpolar tail with nothing for water to pull on D. Both equally, because each has exactly one carboxyl group and that group decides the behavior

Question 5

Models of three fatty acids, each with a 14-carbon tail. Each corner of a zigzag is one carbon atom; a pair of parallel strokes marks a double bond.

The drawing shows models of three fatty acids, each with a 14-carbon tail. Each corner of a zigzag is a carbon atom, and a pair of parallel strokes marks a double bond.

Which statement about the three tails is correct?

A. Tail 1 is unsaturated, because a zigzag is a chain of kinks B. Tail 2 has one double bond and is unsaturated; tail 3 has three C. Tail 2 has one double bond and is saturated, because one is too few to matter D. Tail 3 is saturated, because its double bonds fill every carbon with hydrogen

Question 6

Two hydrocarbon waxes are compared at 20 °C. Wax P is made of straight chains about 30 carbons long and is a hard solid. Wax Q is made of straight chains about 10 carbons long and is a runny liquid. Both have single bonds only.

Why is the long-chain wax solid while the short-chain wax is liquid?

A. The long chains carry partial charges along their whole length and hydrogen-bond to one another B. The long chains lie against neighbors along more of their length, so the weak attractions between them add up C. The short chains carry full charges that push the molecules apart from one another D. The long chains contain double bonds, and double bonds lock neighboring molecules together

Question 7

Sunflower oil has about two double bonds in each fatty-acid tail. Palm oil has about one double bond in every two tails. Both are compared at 20 °C.

Which is more liquid at 20 °C, and why?

A. Palm oil, because fewer double bonds let its tails move more freely B. Both the same, because a double bond changes the mass of a tail by very little C. Sunflower oil, because each double bond puts a kink in a tail and a more unsaturated lipid is more liquid D. Sunflower oil, because double bonds make its tails polar and water keeps them apart

Question 8

A jar of coconut oil is a white solid in a pantry at 20 °C. On a hot afternoon at 30 °C the same jar is a clear liquid. Nothing about its molecules has changed.

Why did the oil become liquid?

A. Warming made the tails move more, so they pulled apart and slid past one another B. Warming added double bonds to the tails, so the tails kinked C. Warming broke the fatty acids off the glycerol, so the fat could flow D. Warming made the tails polar, so water in the air pulled them apart

Question 9

A camel’s hump is a store of fat, not water. A student writes: “The camel gets its energy from the hump when the bonds in the fat break, releasing the energy stored in them.”

Which statement corrects the student?

A. The energy is released when the fat reacts with oxygen and new, stronger bonds form, so the products hold less energy B. The hump holds no usable energy; fat only insulates, and the camel’s energy comes from the sugar in its food C. The energy is released when hydrolysis splits the fat into glycerol and fatty acids D. The student is right: breaking any bond releases the energy stored in it, and fat has many bonds

Question 10

Estrogen is a molecule of four carbon rings joined edge to edge with no long tail. It is made in the ovaries, travels in the blood and changes what cells in the uterus do. Cholesterol has the same four-ring frame and sits among the tails of animal cell membranes.

Which statement about the two molecules is correct?

A. Both are steroid hormones, since every steroid carries a signal B. Both are steroids; estrogen is a hormone and cholesterol is not C. Only estrogen is a steroid; cholesterol is a fat D. Only cholesterol is a steroid; estrogen is a hormone, so it is a different kind of molecule

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

A reindeer stands in snow for months, and the fat in its lower legs, near the hooves, is kept at about 0 °C while the fat in its body core stays near 38 °C. The fatty-acid tails in the leg fat contain many more double bonds than the tails in the core fat. The leg fat stays soft in the cold.

(a) Identify what a double bond does to the shape of a fatty-acid tail. (1 point)

Hint: Picture a saturated tail beside an unsaturated one.

A full-credit answer: At a double bond the tail kinks and runs on at an angle; a tail with single bonds only runs straight.

Check the box for each point your answer earns

a double bond puts a kink (bend) in the tail, and a tail with only single bonds runs straight.

Accept: ‘bend’ for kink.

Do not award: a double bond described only as ‘fewer hydrogens’, or as making the tail shorter.

Common slip: Describing the double bond only by the hydrogens it removes. What matters for the fat is the shape: the double bond bends the tail.

(b) Describe how straight tails pack together compared with kinked tails. (1 point)

Hint: Picture many tails side by side and ask how close each can get to its neighbors.

A full-credit answer: Straight tails lie close against one another along their whole length, whereas kinked tails cannot lie close against their neighbors, so they are held further apart.

Check the box for each point your answer earns

straight tails pack close along their length; kinked tails cannot pack closely.

Accept: ‘the kinks hold the tails apart’.

Do not award: kinked tails packing more tightly, or no difference in packing.

Common slip: Having kinks lock the tails together like hooks. A kink stops a tail lying flat against its neighbor, so kinked tails pack worse, not better.

(c) Explain why a fat whose tails have more double bonds stays liquid at a lower temperature. (1 point)

Hint: What does the strength of the attraction between two tails depend on?

A full-credit answer: Any two tails attract weakly when they lie close, and more strongly the more of their length is in contact. Because kinked tails cannot lie close, the weak attractions between them are weaker, so the molecules slide past one another instead of being held in place, and the fat stays liquid down to a lower temperature.

Check the box for each point your answer earns

kinked tails cannot lie close, so the attractions between tails are weaker and the molecules slide past one another (the fat stays liquid).

Accept: the packing step and the attraction step in either order, provided both appear.

Do not award: ‘double bonds are weaker bonds’, or ‘double bonds hold more energy’.

Common slip: Saying the double bonds themselves are weak or break easily. Nothing breaks; the kinks keep whole tails apart, so the attractions between tails are weaker.

(d) Predict what would happen to the fat near the hooves at 0 °C if it were built like the core fat, with few double bonds. (1 point)

Hint: Look back at (b) and (c): what do straight tails do that kinked tails cannot?

A full-credit answer: At 0 °C that fat would set hard instead of staying soft, so the reindeer’s lower legs would stiffen.

Check the box for each point your answer earns

the fat would set hard (become solid, far less liquid) at 0 °C.

Accept: ‘it would be solid’ or ‘it would go hard like the core fat’.

Do not award: it would stay soft, or it would melt.

Common slip: Predicting no change because the temperature is the same for both fats. The temperature is the same; the tails are different, and straight tails set hard in the cold.

(e) A third sample of fat, from the reindeer’s neck, sits at about 20 °C in the animal and is soft at that temperature, but it sets hard when cooled to 0 °C. Predict whether its tails have more or fewer double bonds than the leg fat’s tails, and justify your prediction. (1 point)

Hint: What does a fat that sets hard at 0 °C tell you about its tails?

A full-credit answer: The neck fat’s tails have fewer double bonds than the leg fat’s, because a fat that sets hard at 0 °C must have tails that pack closely and attract one another strongly once cooling slows them, and that needs straighter tails with fewer kinks. The leg fat stays soft at 0 °C because its many kinks keep its tails apart even when cold.

Check the box for each point your answer earns

fewer double bonds than the leg fat, because setting hard at 0 °C means the tails pack closely and attract strongly, which needs straighter tails with fewer kinks.

Accept: ‘fewer kinks, so the tails can lock together at 0 °C, which the leg fat’s tails cannot’.

Do not award: more double bonds than the leg fat, or fewer with no reason from packing or attraction.

Common slip: Reasoning from where the fat sits in the body instead of from what it does at 0 °C. Setting hard when cold means the tails can pack and hold one another, and that takes straight tails.

Free-response score: 0 of 5

Free response 2 · Analyze Model or Visual Representation · 4 points

The model shows two lipid molecules. Molecule 1 has three fatty-acid tails joined to a small three-carbon molecule. Molecule 2 has two fatty-acid tails and a head, containing a phosphate group marked P, joined to the same kind of three-carbon molecule. One of molecule 2’s tails bends where a double stroke is drawn.

Two lipid molecules. Molecule 1 has three tails on a small three-carbon molecule. Molecule 2 has two tails and a head, marked P for its phosphate group, on the same kind of three-carbon molecule; one of its tails bends where marked by a double stroke.

(a) Identify molecule 1 and molecule 2, and name the small three-carbon molecule they share. (1 point)

A full-credit answer: Molecule 1 is a fat, molecule 2 is a phospholipid, and both are built on glycerol.

Check the box for each point your answer earns

fat, phospholipid, glycerol.

Do not award: the two molecules swapped, or a steroid named for either.

Common slip: Calling molecule 2 a fat with a missing tail. The head with its phosphate group is what makes it a phospholipid.

(b) A probe measures how strongly water is attracted to each part of the two molecules. Explain why the probe finds attraction at the head of molecule 2 and no attraction along the tails of either molecule. (1 point)

A full-credit answer: The phosphate group in the head carries a full negative charge, and the head often carries other charged or polar groups too, so water’s partial charges pull on it: the head is hydrophilic. The tails are hydrocarbon, with nonpolar C–H bonds and no charge or partial charge, so water has nothing to pull on and is not attracted to them: the tails are hydrophobic.

Check the box for each point your answer earns

the head’s phosphate group carries a full charge (and polar groups), which water’s partial charges pull on; the hydrocarbon tails carry no charge or partial charge, so water is not attracted.

Accept: ‘the head is charged and polar, the tails are nonpolar’ with water’s partial charges as the reason.

Do not award: the head described as polar with partial charges only, or the tails described as repelled by a charge.

Common slip: Describing the phosphate group as carrying partial charges. It carries a full negative charge, as an ion does; a polar group carries partial charges only.

(c) Many copies of each molecule are stirred into separate beakers of water. Predict how each kind of molecule arranges itself, and justify each prediction. (1 point)

A full-credit answer: Molecules of the fat gather into oily droplets or a layer, because no part of a fat has a charge or partial charge for water to pull on, so water excludes the whole molecule. The phospholipids line up as a lipid bilayer, two sheets laid tail to tail with heads facing the water on both sides, because water pulls on the heads and not the tails, and a double sheet keeps every tail away from the water.

Check the box for each point your answer earns

fats gather into droplets or a layer because water excludes the whole molecule; phospholipids form a bilayer (two sheets tail to tail, heads out) because water pulls on the heads but not the tails.

Accept: ‘a single sheet would leave tails touching water, so two sheets form’ as the bilayer reason.

Accept: a single-layered ball with heads out and tails packed inside, provided the reason is that every tail is kept from the water.

Do not award: a bilayer of fat molecules, or phospholipids spread evenly through the water.

Common slip: Drawing the phospholipids as a single sheet. A single sheet leaves the tails on its underside touching water; two sheets laid tail to tail keep every tail dry.

(d) An animal stores molecule 1 in large amounts and later breaks it down for fuel. Explain where the usable energy comes from when the animal’s cells break the fat down. (1 point)

A full-credit answer: The energy comes out when the fat reacts with oxygen: the new bonds formed in carbon dioxide and water are stronger than the bonds broken, so the products hold less energy than the fat and oxygen did, and that difference is released for the cell to use. Breaking bonds on its own takes energy in; hydrolysis of the fat into glycerol and fatty acids frees them but releases no usable energy.

Check the box for each point your answer earns

energy is released in the reaction with oxygen because the new bonds formed are stronger than the bonds broken, leaving the products lower in energy.

Accept: ‘the products hold less energy than the starting molecules; the difference is released’.

Do not award: ‘energy stored in the bonds is released when they break’, or hydrolysis named as the energy-releasing step.

Common slip: Placing the energy in the fat’s bonds and releasing it by breaking them. Breaking a bond takes energy in; the release comes from the new, stronger bonds that form.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

Take the end-of-topic test