Warm it, cool it — answer key

1. SQ6 · Practice

Model answer. When the air was warmed to 80 °C, its volume increased from 50 to 60 cm³. When it was cooled to 2 °C, its volume decreased to 47 cm³. The pressure was kept the same: there were no weights on the plunger, so only the atmosphere pushed on it each time.

A full answer includes

• Warmed: the volume increased (from 50 to 60 cm³).
• Cooled: the volume decreased (from 50 to 47 cm³).
• Kept the same: the pressure, because there were no weights on the plunger in all three readings.

Also accept

• increased; decreased; the pressure (no weights on the plunger)

Watch for

• “The temperature was kept the same.” — The temperature is what was changed on purpose. The thing kept the same was the pushing on the plunger: no weights each time.

2. SQ7 · Practice

Options as numbered on the pupil sheet: 1 — Warmed air; 2 — Cooled air; 3 — Both.

Model answer. 1 warmed, 2 cooled, 3 warmed, 4 cooled, 5 both. Warming makes the particles move faster, so they hit harder and push the plunger out until the pushes balance. Cooling slows them down, so they hit less hard and the plunger moves in. In both cases the sealed syringe holds the same particles, and they keep their size.

A full answer includes

• Faster particles, and a plunger that moves out: the warmed air (1 and 3).
• Hitting less hard, and a plunger that moves in: the cooled air (2 and 4).
• The same particles, the same size: both (5).

Also accept

• 1 warmed, 2 cooled, 3 warmed, 4 cooled, 5 both.

Watch for

• “Warmed particles are bigger.” — Warmed particles move faster, not bigger. The spaces between them grow as the plunger moves out.

3. SQ8 · Practice

Model answer. 2, 4 and 5. Warming makes the particles move faster: they have more kinetic energy. At first they hit the plunger harder and more often, so the air pushes up harder than the atmosphere pushes down, and the plunger moves out. As it does, the particles have more space, so each bit of the wall is hit less often and the push of the air falls back. The plunger stops higher, where the pushes balance again, with bigger spaces between the particles. Statement 1 is wrong: the particles keep their size. Statement 3 is wrong: the syringe is sealed, so no air is added. Statement 6 is wrong: only the atmosphere pushes on the plunger, before and after.

A full answer includes

• I chose 2, 4 and 5: faster particles, harder and more frequent hits at first, bigger spaces between the particles.
• I explained that faster particles (more kinetic energy) hit the walls harder and more often, so the air pushes harder than the outside and the plunger moves out.
• I explained that with more space, each bit of the wall is hit less often, so the push of the air falls back until the pushes balance again, with the plunger higher.

Also accept

• 2, 4, 5. Faster particles hit harder and more often, so the plunger moves out; with more space the hits on each bit of wall become less frequent until the pushes balance, higher up.

Watch for

• “The gas expands because each particle expands.” — A gas expands because its particles move faster and push the plunger out. Each particle stays as it was.
• “Heating makes more gas.” — No air can get into the sealed syringe. The same particles take up more space.

4. SQ9 · Practice

Model answer. (a) Two things changed between her readings: the temperature went up, and the weight increased the pressure. The weight squashed the air more than the warming expanded it, so the volume went down. She can't tell what warming does on its own. (b) She should change only one thing: keep the same weights on the plunger (for example, none) for both readings, and change only the temperature. Then any change in volume is caused by the temperature.

A full answer includes

• I said two things changed at once: the temperature and the pressure (the weight).
• I said the weight made the volume smaller, so her reading can't show the effect of warming alone.
• I suggested keeping the pressure the same, with no weights (or the same weights) on the plunger for both readings, and changing only the temperature.

Also accept

• Pressure and temperature both changed; warm the air with the same weights (none) on the plunger and change only the temperature.

Watch for

• “Her reading went down, so warming must make gases shrink.” — With no weight added, warming to 80 °C gives 60 cm³: a larger volume. Her weight hid the effect of warming by squashing the air at the same time.

5. SQ10 · Practice

Options as numbered on the pupil sheet: 1 — Graph A; 2 — Graph B; 3 — Graph C; 4 — Graph D.

Model answer. Pressure: graph B. More pressure means a smaller volume, and each extra weight squashed the air by less than the one before, so the curve gets less steep. Temperature: graph A. A higher temperature means a larger volume; in the warm and cold results the volume rose steadily with the temperature (47, 50, then 60 cm³). C would mean the volume doesn't change at all, and D would mean equal drops in volume for equal increases in pressure.

A full answer includes

• Pressure: B, the curve that falls steeply and then less and less steeply (in the squeeze results the volume fell by 17, then 8, then 5 cm³).
• Temperature: A, the straight line that rises (the volume rose with the temperature: 47, 50, then 60 cm³).

Also accept

• Pressure B; temperature A.

Watch for

• “Pressure: D, a straight falling line.” — The volumes fell by 17, then 8, then 5 cm³ for equal extra weights. Steps that get smaller and smaller give a curve, not a straight line.
• “Temperature: C, because the amount of gas stays the same.” — The amount stays the same, but the space it takes up changes: warmer air expands.

6. ST2 · Apply

Model answer. (a) The hot water warms the air inside the ball. The same amount of air expands: its volume increases, and it pushes the dent out. (b) Warmer particles move faster, so they hit the inside of the shell harder and more often. The air inside now pushes outwards harder than the push from outside (the air and the water), so the dented part is pushed out, like the plunger in the warm bath, until the ball is round again.

A full answer includes

• I said the air inside is warmed, so it expands: its volume increases, while the amount of air stays the same.
• I said the particles move faster, so they hit the inside of the ball harder and more often.
• I said the air inside then pushes outwards harder than the push from outside (the air and the water), so the dent is pushed out until the ball is round again. (Naming just the water, the air or the atmosphere is fine.)

Also accept

• (a) It warms and expands. (b) Faster particles hit the shell harder and more often, so the air inside pushes out harder than the air and water outside push in, until the ball is round again.

Watch for

• “Hot water gets into the ball and pushes the dent out.” — The ball isn't cracked, so nothing gets in. The air already inside expands, because its particles move faster.
• “The air particles get bigger in the heat.” — Heat speeds the particles up; it doesn't make them grow. Faster particles hit the shell harder and more often.