Answers · The particle model

Solids, liquids and gases — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. PQ1 · Practice

Model answer. The water takes a new shape but still takes up 300 cm³, and the air fills the rest. A liquid has a fixed volume but no fixed shape: as the bottle is laid down, the water flows into the lowest part and takes the shape of that part of the bottle, spread out and shallower, but there is still 300 cm³ of it. A gas has no fixed shape or volume of its own: the air spreads out to fill whatever space is left, still the other 700 cm³. Stood up again, the bottle would show the water at the same level as before.

A full answer includes

I chose: the water takes a new shape but still takes up 300 cm³; air fills the rest.
I wrote that a liquid has a fixed volume, so the water is still 300 cm³.
I wrote that a liquid has no fixed shape: the water flows into the lowest part of the bottle.
I wrote that the air, a gas, spreads out to fill the other 700 cm³.
I did not write that the water’s volume changes when its shape changes.

Also accept

c: the water flows to a new shape but stays 300 cm³ (a fixed volume); the air, a gas, fills the other 700 cm³.

Watch for

“The water spreads along the bottle, so it now takes up more than 300 cm³.” — Spreading out changes the water’s shape, not its amount. A liquid has a fixed volume: it is still 300 cm³ however the bottle lies.
“The water keeps its upright shape, so it still takes up exactly 300 cm³.” — The volume is right, but a liquid has no fixed shape. As the bottle tips, the water flows into the lowest part and takes the shape of that part of the bottle.
“The air is squashed into one bubble, so the water now takes up 700 cm³.” — Nothing squashes the air, and the water cannot grow. The water keeps its 300 cm³, and the air, a gas, still spreads out to fill the other 700 cm³.

2. PQ2 · Practice

Options as numbered on the pupil sheet: 1 — solid only; 2 — liquid only; 3 — gas only; 4 — liquid and gas; 5 — solid and liquid; 6 — solid, liquid and gas.

Model answer. A solid has a fixed shape and a fixed volume, and it cannot be compressed. A liquid keeps its volume, but it flows and takes the shape of its container; it can hardly be compressed at all. A gas has no fixed shape and no fixed volume: it flows, spreads out to fill its whole container, and is easily compressed. So a fixed shape belongs to a solid only; a fixed volume to a solid and a liquid; flowing to a liquid and a gas; spreading out to fill the container, and being easily compressed, to a gas only.

A full answer includes

I matched “Has a fixed shape” to solid only.
I matched “Takes the shape of its container but keeps its volume” to liquid only.
I matched “Spreads out to fill its whole container” to gas only.
I matched “Is easily compressed” to gas only.
I matched “Flows” to liquid and gas.
I matched “Has a fixed volume” to solid and liquid.
I did not match “Flows” to liquid only.

Also accept

Fixed shape – solid; takes the shape of its container, keeps its volume – liquid; fills its container – gas; easily compressed – gas; flows – liquid and gas; fixed volume – solid and liquid.

Watch for

“A gas has a fixed volume, like a liquid, so “Has a fixed volume” goes with all three states.” — A gas spreads out to fill whatever container it is in, so its volume is the volume of the container. Let some air out of a bottle, and the rest still fills the bottle.
“Only a liquid flows.” — A gas flows too: air flows through a straw, and gas flows along the pipes to a cooker. Liquids and gases both flow; solids do not.
“A liquid is easily compressed, like a gas.” — Squeeze a sealed bottle filled right to the top with water, and it hardly gives at all. Only a gas is easily compressed.

3. PQ3 · Practice

Options as numbered on the pupil sheet: 1 — melting; 2 — freezing (solidification); 3 — boiling; 4 — evaporation; 5 — condensation; 6 — not a change of state.

Model answer. Butter in a warm pan goes from solid to liquid: melting. Molten candle wax goes from liquid to solid as it cools: freezing, also called solidification, even though wax sets at about 60 °C, far warmer than water freezes. Dew forms because water vapour in the air touches the cold grass and turns into liquid water: condensation. The fish tank loses water at room temperature, far below 100 °C, without bubbling, because water leaves its surface as a gas: evaporation. Water bubbling hard all through the pan at 100 °C is boiling. The coffee granules do not melt: they dissolve, their particles spreading out among the water particles. The coffee has not turned into a liquid of its own, so that is not a change of state.

A full answer includes

I matched the butter going runny to melting.
I matched the candle wax setting hard to freezing (solidification).
I matched the dew on the grass to condensation.
I matched the falling fish-tank level to evaporation.
I matched the pasta water bubbling hard at 100 °C to boiling.
I matched the coffee granules disappearing to “not a change of state”.
I did not choose melting for the coffee granules.

Also accept

Butter – melting; wax – freezing (solidification); dew – condensation; fish tank – evaporation; pasta water – boiling; coffee – not a change of state (it dissolves).

Watch for

“The coffee granules melt in the hot water.” — Melting is a solid turning into a liquid of its own, like butter in a pan. The coffee dissolves: its particles spread out among the water particles, so it is still coffee, mixed with water.
“The dew evaporated onto the grass.” — Evaporation is liquid to gas. Dew forms the other way round: water vapour in the air touches the cold grass and turns into liquid. That is condensation.
“The water in the fish tank boils away.” — Boiling happens at the boiling point, 100 °C for water, all through the liquid, with bubbles. The tank is at room temperature: water leaves only from its surface, which is evaporation.
“The pasta water is evaporating, because it turns into a gas.” — It does turn into a gas, but all through the liquid, with bubbles, at its boiling point: that is boiling. Evaporation happens only from the surface, below the boiling point.
“The wax cannot be freezing, because it is still warm.” — Freezing, or solidification, is any liquid turning into a solid, at whatever temperature that happens. Candle wax sets at about 60 °C.

4. PQ4 · Practice

Options as numbered on the pupil sheet: 1 — separate single atoms; 2 — molecules of joined atoms; 3 — positive ions and free electrons; 4 — positive and negative ions.

Model answer. Helium is a noble gas: its atoms do not join up, so helium gas is separate single atoms, far apart, with negligible forces between them. Water is made of molecules: each one is two hydrogen atoms joined to one oxygen atom. Copper is a metal: positive ions held in fixed positions by strong forces, with electrons moving freely between them, which is why a copper wire conducts. Salt, sodium chloride, is a compound of a metal and a non-metal: positive sodium ions and negative chloride ions, held close together in a regular pattern. Whatever the particles are, the forces and distances between them, and how they move, decide whether the substance is a solid, a liquid or a gas.

A full answer includes

I matched the helium to separate single atoms.
I matched the water to molecules of joined atoms.
I matched the copper wire to positive ions and free electrons.
I matched the salt to positive and negative ions.
I did not give the copper negative ions.

Also accept

Helium – single atoms; water – molecules; copper – positive ions with free electrons; salt – positive and negative ions.

Watch for

“Helium is made of molecules.” — Helium atoms do not join up, so helium gas is single atoms.
“Water is made of single atoms.” — Each water particle is a molecule: two hydrogen atoms joined to one oxygen atom.
“Copper is made of positive and negative ions.” — A metal has no negative ions. Its free electrons move between fixed positive ions, which is why it conducts.
“Salt is made of positive ions and free electrons.” — Salt has no free electrons. Its positive and negative ions are held in fixed positions, so solid salt does not conduct.
“Salt is made of molecules.” — Salt is a compound of a metal and a non-metal, so it is made of ions, not molecules: positive sodium ions and negative chloride ions.

5. PQ5 · Practice

Model answer. Box C. In a liquid the particles are close together, mostly touching, but in an irregular arrangement, because they move about, sliding past each other. The liquid settles in the bottom of its container with an uneven top. A is a solid: a regular pattern, touching. B is a gas: a few particles, far apart, scattered at random. D is not a correct diagram of any state: its particles are in neat rows like a solid’s, but with gaps between them. Particles in a regular pattern are touching, as in a solid; particles far apart are in no pattern, as in a gas.

A full answer includes

I chose box C.
I wrote that in a liquid the particles are close together, mostly touching.
I wrote that the arrangement is irregular, because the particles slide past each other.
I wrote that a liquid settles in the lower part of its container.
I did not choose box D, with neat rows and gaps.

Also accept

C: close together, mostly touching, but no regular pattern.

Watch for

“Box D shows a liquid: its particles are a little further apart than in a solid.” — In a liquid the particles are still close together, mostly touching, and they are not in rows, because they keep sliding past each other. Neat rows belong to a solid, and wide gaps belong to a gas.
“Box B shows a liquid, because the particles of a liquid move about.” — The particles of a liquid move about, but they stay close together, in the bottom of the container. A few particles far apart, scattered through the whole box, are a gas.
“Box A shows a liquid, because its particles touch.” — The particles of a liquid do touch, but box A also has them in neat rows, a regular pattern, filling the box. That is a solid.

6. PQ6 · Practice

Options as numbered on the pupil sheet: 1 — The water freezes: the particles settle into fixed positions; 2 — Steam: the particles are far apart, moving fast in random directions; 3 — Ice: the particles vibrate about fixed positions, in a regular pattern; 4 — The steam condenses: the particles come close together; 5 — Water: the particles are close together, sliding past each other.

Model answer. First the steam: its particles are far apart, moving fast in random directions. As it cools, the steam condenses: the particles come close together, and the steam becomes liquid water. In the water the particles are close together, mostly touching, but they still move about, sliding past each other. Cooled further, the water freezes, also called solidification: the particles settle into fixed positions. In the ice they vibrate about those fixed positions, in a regular pattern. The water molecules are the same the whole time; only their arrangement, separation and motion change.

A full answer includes

Step 1: I put steam, with the particles far apart and moving fast.
Step 2: I put “the steam condenses”.
Step 3: I put water, with the particles close together, sliding past each other.
Step 4: I put “the water freezes”.
Step 5: I put ice, with the particles vibrating about fixed positions.
I did not put the particles in fixed positions straight after the steam condenses.

Also accept

steam (far apart, fast); condenses; water (close together, sliding); freezes; ice (vibrate about fixed positions).

Watch for

“When steam condenses, its particles go straight into fixed positions.” — Condensing makes a liquid: the particles come close together but keep sliding past each other. They settle into fixed positions only when the water freezes.
“In the ice the particles stop moving.” — They vibrate about their fixed positions all the time. Freezing stops them sliding past each other, not vibrating.

7. PQ7 · Practice

Model answer. Correct: in a solid, strong forces hold the particles in fixed positions; in a gas, the forces between the particles are negligible, so the particles spread out to fill the container; a gas can be compressed because its particles are far apart, with empty space between them. A liquid does have forces between its particles: they hold the particles close, which is why a liquid keeps its volume, but they are not strong enough to hold them in fixed positions, so the liquid flows. When a gas is compressed, its particles stay the same size; the empty space between them shrinks. A liquid cannot be compressed noticeably: its particles move about, but they are already close together, touching.

A full answer includes

I chose: in a solid, strong forces hold each particle in a fixed position.
I chose: in a gas, the forces are negligible, so the particles spread out.
I chose: a gas can be compressed because its particles are far apart.
I did not choose “no forces” for a liquid: its forces hold the particles close, but not in fixed positions.
I did not choose that gas particles get smaller: only the space between them shrinks.
I did not choose that a liquid is easily compressed: its particles are already close together.
My explanation links one property to the forces or the distances between the particles.

Also accept

a, b and c: strong forces fix the particles of a solid; negligible forces let a gas spread out; far-apart particles let a gas be compressed.

Watch for

“A liquid flows because there are no forces between its particles.” — The forces in a liquid hold the particles close together, which is why it keeps its volume. They are too weak to hold the particles in fixed positions, so they slide past each other.
“A gas can be compressed because its particles get smaller.” — The particles do not change size. What shrinks is the empty space between them.
“A liquid is easily compressed, as its particles are not fixed.” — Moving about does not leave room to squash. The particles of a liquid are already close together, touching, so a liquid cannot be compressed noticeably.

8. PQ8 · Practice

Model answer. (a) In a solid the particles are in a regular pattern, close together and touching. Each vibrates about a fixed position. (b) In a liquid the particles are still close together, mostly touching, but in an irregular arrangement. They move about at random, sliding past each other, so the arrangement keeps changing. (c) In a gas the particles are far apart, in a random arrangement, and move fast in random directions, in straight lines until they hit another particle or a wall. (d) In a real gas the distance from one particle to the next is about ten times the size of a particle. Drawn to scale, a box would hold only one or two circles, so diagrams draw the gas particles closer together than they really are. Each circle is one particle, so a diagram shows only a few of them: a real sample has billions upon billions.

A full answer includes

(a) I wrote that in a solid the particles are in a regular pattern.
(a) I wrote that they are close together, touching.
(a) I wrote that they vibrate about fixed positions.
(b) I wrote that in a liquid the arrangement is irregular.
(b) I wrote that the particles are close together, mostly touching.
(b) I wrote that they move about, sliding past each other.
(c) I wrote that in a gas the arrangement is random.
(c) I wrote that the particles are far apart.
(c) I wrote that they move fast, in random directions.
(d) I wrote that in a real gas the particles are much further apart than a diagram can show.
(d) I wrote that a diagram shows only a few particles, while a real sample has a vast number.
I did not describe the particles of a liquid as being in rows with gaps.
I did not write that the particles of a gas are bigger than those of a liquid.

Also accept

(a) regular, touching, vibrate; (b) irregular, mostly touching, slide past each other; (c) random, far apart, fast in random directions; (d) real gas particles are much further apart, and far more numerous, than drawn.

Watch for

“(b) In a liquid the particles are in rows, a little further apart than in a solid.” — The particles of a liquid are still close together, mostly touching, and in no pattern, because they keep sliding past each other.
“(a) The particles of a solid do not move.” — They vibrate about their fixed positions all the time. They cannot move from place to place, which is why a solid keeps its shape.
“(c) The particles of a gas are bigger than those of a liquid.” — The particles are the same in every state, the same size. In a gas they are further apart, not bigger.

9. PQ9 · Practice

Model answer. Mo is incorrect. Each grain of granulated sugar is a tiny crystal, a solid: it keeps its own fixed shape and fixed volume, and it cannot be compressed. The stream flows only because the separate crystals slide and roll over one another. In the bowl the crystals pile up into a heap with sloping sides, instead of settling with a level surface, as a liquid would. Inside each crystal, the particles are close together in fixed positions, in a regular pattern, where they vibrate, so the crystal keeps its shape. In a liquid, it is the particles themselves that slide past each other.

A full answer includes

I wrote that each sugar crystal is a solid.
I wrote that each crystal keeps a fixed shape and a fixed volume.
I wrote that the stream flows only because the separate crystals slide and roll over one another.
I wrote that the heap does not settle with a level surface, as a liquid would.
I wrote that inside each crystal the particles are in fixed positions, in a regular pattern, where they vibrate.
I did not write that the sugar melts, or becomes a liquid, when it is poured.

Also accept

Each crystal is a solid with a fixed shape and volume; the crystals slide over one another; the heap is not level; inside a crystal the particles are in fixed positions.

Watch for

“Anything that pours out of a container is a liquid.” — Pouring is not enough. Look at one crystal: it keeps its own shape and volume, which a liquid cannot. A heap of sugar also keeps its sloping sides instead of settling level.
“Sugar is part solid and part liquid.” — Dry sugar has no liquid in it at all. It is a huge number of tiny solid crystals, each with its particles in fixed positions.

10. PQ10 · Practice

Model answer. (a) The lava is freezing, also called solidification: it goes from liquid to solid. Freezing means a liquid turning into a solid, at whatever temperature that happens. Water happens to freeze at 0 °C, but lava sets at around 1000 °C. (b) The cloud is condensation. Sea water boils where it touches the lava, but the water vapour it makes is invisible. As the vapour rises into cooler air, it turns into tiny drops of liquid water, and those drops are the white cloud. (c) The pools dried by evaporation. Boiling happens at the boiling point, all through the liquid, with bubbles; the pools never bubbled, and the water left only from their surface, far below its boiling point.

A full answer includes

(a) I identified the change as freezing (solidification).
(a) I wrote that freezing means a liquid turning into a solid, at whatever temperature that happens.
(a) I did not write that freezing only happens at 0 °C.
(b) I identified the change as condensation.
(b) I wrote that steam is invisible, and the white cloud is tiny drops of liquid water.
(c) I identified the change as evaporation.
(c) I wrote that no bubbles formed, so the water did not boil.
(c) I wrote that the water left only from the surface of the pools, below its boiling point.

Also accept

(a) freezing (solidification): any liquid to solid, at any temperature; (b) condensation: the cloud is liquid drops, steam is invisible; (c) evaporation: no bubbles, only from the surface.

Watch for

“(a) Freezing only happens at 0 °C.” — 0 °C is the freezing point of water only. Each substance freezes at its own temperature: iron sets at about 1500 °C, lava at about 1000 °C.
“(b) You can see steam as a white cloud.” — Water vapour is invisible. The white cloud is made of tiny drops of liquid water, formed when the vapour condensed in the cooler air.
“(c) Any liquid turning into a gas is boiling.” — Boiling happens at the boiling point, all through the liquid, with bubbles. Below its boiling point a liquid turns into a gas only at its surface: that is evaporation.

11. PQ11 · Practice

Model answer. In the iron, the particles are close together and the forces between them are strong. The forces hold each particle in a fixed position, where it can only vibrate, so the block keeps its shape. (In a metal the particles are ions, with electrons moving freely between them.) In the water, the particles are also close together, and the forces between them hold them close, so the water keeps its volume. But the forces are weaker, not strong enough to hold the particles in fixed positions. The particles slide past each other, so the water flows and spreads out over the table top.

A full answer includes

I wrote that in the iron, strong forces hold each particle in a fixed position.
I wrote that the iron’s particles can only vibrate, so the block keeps its shape.
I wrote that in the water, the forces still hold the particles close together.
I wrote that this is why the water’s volume stays the same.
I wrote that the forces in the water are too weak to hold the particles in fixed positions.
I wrote that the water’s particles slide past each other, so the water flows.
I did not write that there are no forces between the particles of water.

Also accept

Iron: strong forces, fixed positions, only vibrate, so a fixed shape; water: forces hold the particles close (fixed volume) but not in fixed positions, so they slide past each other and it flows.

Watch for

“The iron particles do not move at all.” — They vibrate about their fixed positions all the time. They just cannot move from place to place.
“There are no forces between the particles of water.” — There are: they hold the particles close together, which is why the spill keeps its volume. They are only too weak to hold the particles in fixed positions.

12. PQ12 · Practice

Model answer. One molecule is drawn 4 mm across, and real neighbours are about ten molecule-sizes apart, so the centres should be about 10 × 4 mm = 40 mm apart, 4 cm. A box 10 cm wide would then hold only two or three circles across. That is why a particle diagram of a gas is not to scale: the particles are drawn much closer together than they really are, so that the diagram can show enough of them. A real gas is mostly empty space.

Checked automatically

Distance from centre to centre, drawn to scale: 40 mm (within ±4 mm)
- Common wrong answer 4 : That would make the circles touch, as in a solid or a liquid. In a gas, neighbours are about ten times their own size apart: 10 × 4 mm.
- Common wrong answer 0.4 : You divided by ten. The gaps in a gas are bigger than the particles, not smaller: multiply, 10 × 4 mm.
- Common wrong answer 400 : You multiplied by 100. The distance is about ten times the size: 10 × 4 mm.
- Common wrong answer 14 : You added 10 to 4. The distance is about ten times the size of a molecule: multiply, 10 × 4 mm.

A full answer includes

I multiplied: 10 × 4 mm.
I wrote about 40 mm.
I wrote that a page-sized box would then hold only a few circles, so gas diagrams draw the particles much closer together than they really are.
I did not divide by 10.

Also accept

About 40 mm (4 cm).

Watch for

“The circles should just touch, 4 mm apart.” — Touching particles belong to a solid or a liquid. In a gas, neighbours are about ten times their own size apart.
“The circles should be 0.4 mm apart.” — You divided by ten. The particles of a gas are further apart than their size, not closer: multiply.
“The circles should be 14 mm apart.” — You added 10 to 4. The distance is about ten times the size of a molecule: multiply.

13. PQ31 · Practice

Model answer. Diagram B. A change of state does not change the particles, so the circles for steam must be the same size as the circles for the water. In a gas the particles are far apart, in a random arrangement, because they move fast in random directions, and they spread out to fill the whole container. Diagram A is incorrect because it draws the particles bigger: steam takes up much more room than the water it came from because its particles are far apart, not because each particle grows. Diagram C puts the particles in neat rows, a regular pattern, which belongs to a solid. Diagram D packs them into the lower part of the box, as a liquid would settle.

A full answer includes

I chose diagram B.
I wrote that the particles are the same in every state, so the circles stay the same size.
I wrote that steam takes up more room because its particles are far apart, not because they are bigger.
I did not choose diagram D: a gas spreads out to fill its whole container.

Also accept

B; A is incorrect because the particles are the same size in every state: a gas takes up more room because its particles are far apart.

Watch for

“Diagram A: the circles of steam should be bigger, because steam takes up more room.” — The particles are the same in every state, so the circles stay the same size. Steam takes up more room because its particles are far apart.
“Diagram C: the circles of a gas can be in neat rows, as long as they are far apart.” — A regular pattern belongs to a solid. The particles of a gas move fast in random directions, so their arrangement is random.
“Diagram D: the steam collects in the lower part of the box, like the water did.” — A liquid settles in the lower part of its container, but a gas spreads out to fill the whole container.

14. PT1 · Apply

Model answer. (a) The particles of air are far apart, with empty space between them, and the forces between them are negligible. Squeezing air into the tank pushes the particles much closer together, into less space, so the air is compressed: 2400 litres fit into 12. The particles themselves do not shrink. (b) In water the particles are already close together, touching, held close by the forces between them. There is no empty space to squeeze out, so water cannot be compressed noticeably: the tank would hold only 12 litres of it. (c) The air left in the tank is still a gas, so it still spreads out to fill the whole 12 litres. With half as many particles in the same space, they are further apart, on average, than before. Their arrangement is still random, and they still move fast in random directions, colliding with each other and with the walls. (d) Condensation: her breath contains water vapour, a gas, which touches the cool glass of the mask and turns into liquid water.

A full answer includes

(a) I wrote that in air the particles are far apart, with empty space between them.
(a) I wrote that the forces between them are negligible.
(a) I wrote that squeezing pushes the particles closer together, so the air is easily compressed.
(a) I did not write that the particles get smaller.
(b) I wrote that in water the particles are already close together, touching.
(b) I wrote that there is no empty space to squeeze out, so water cannot be compressed noticeably.
(c) I wrote that the air left still fills the whole tank.
(c) I wrote that its particles are now further apart, on average.
(c) I wrote that their arrangement is still random.
(c) I wrote that they still move fast, in random directions.
(d) I identified the change as condensation.
(d) I wrote that the water came from water vapour in her breath, which touched the cool glass.

Also accept

(a) air particles far apart, negligible forces, pushed closer; (b) water particles already touching, no space; (c) still fills the tank, further apart, still random and fast; (d) condensation of water vapour from her breath.

Watch for

“(a) The air particles are squashed smaller in the tank.” — The particles stay the same size. They are pushed closer together: it is the empty space between them that shrinks.
“(c) The air left in the tank collects in one part of it.” — A gas always spreads out to fill its whole container. The particles left are spread through the whole tank, further apart than before.
“(c) With fewer particles in the tank, each one moves more slowly.” — How fast the particles move does not depend on how many there are. The particles left still move fast, in random directions.
“(d) The mist is seawater that has leaked through the glass.” — Glass does not let water through, and the mask mists even when its seal is perfect. The water comes from her breath: water vapour condensing on the cool glass.