Squeeze a gas — answer key

1. SQ1 · Practice

Model answer. b. The volume fell from 50 to 33, then 25, then 20 cm³: smaller with every weight, but each extra weight squashed the air by less than the one before (by 17, then 8, then 5 cm³). The temperature stayed at 20 °C and the sealed syringe kept the same amount of air, so only the pressure changed.

A full answer includes

• I chose b, “The volume decreased with each weight, but by less each time”: it fell by 17, then 8, then 5 cm³.
• I stated what was kept the same: the temperature (20 °C) and the amount of air (the same particles).

Also accept

• b; the temperature and the amount of air (the number of particles) were kept the same.

Watch for

• “Each weight squashes the air by the same amount.” — Work out the differences: 17, then 8, then 5 cm³. Each weight adds the same extra push, but the push of the air grows faster and faster as its space shrinks (half the space, twice the hits), so a smaller squeeze balances each extra weight.
• “The volume can't change, because no air can get out.” — The amount of air stays the same, but the space it takes up can change. The same particles can be pushed closer together.

2. SQ2 · Practice

Options as numbered on the pupil sheet: 1 — The plunger moves in, so the same particles have less space.; 2 — The air pushes up harder on the plunger.; 3 — The plunger stops when the push of the air balances the push from outside.; 4 — The push from outside becomes bigger than the push of the air.; 5 — The particles hit the walls and the plunger more often..

Model answer. d, a, e, b, c. The weight makes the push from outside bigger than the push of the air, so the plunger moves in. The same particles now have less space, so they hit the walls and the plunger more often, and the air pushes back harder. Once its push balances the push from outside, the plunger stops: it doesn't go all the way down.

A full answer includes

• Order: the push from outside becomes bigger; the plunger moves in, so the particles have less space; they hit the walls and the plunger more often; the air pushes up harder; the plunger stops when the pushes balance.
• I explained that the plunger stops because the push of the squeezed air grows until it balances the push from outside.

Also accept

• Outside push bigger, plunger moves in, more hits, air pushes harder, pushes balance (d a e b c).

Watch for

• “The plunger stops because the air has run out of space.” — There is still lots of empty space between the particles. The plunger stops because the air now pushes back as hard as the weights and the atmosphere push down.

3. SQ3 · Practice

Model answer. 1, 2 and 4. The sealed syringe still holds the same particles, now closer together, so they hit the plunger more often. Statement 3 is wrong: particles don't shrink; the spaces between them get smaller. Statement 5 is wrong: the cap lets nothing out, which is why the plunger returns to 50 cm³ when the weights come off. Statement 6 is wrong: the temperature didn't change, so the particles move just as fast on average.

A full answer includes

• I chose 1, 2 and 4: the same number of particles, closer together, hitting the plunger more often.
• I corrected one rejected statement: the particles keep their size (3); the sealed syringe lets no particles out (5); or, at the same temperature, the particles move just as fast (6).

Also accept

• 1, 2, 4. The particles don't shrink, none escape, and at the same temperature they move just as fast.

Watch for

• “Squeezed particles get smaller.” — A gas can be squeezed because there is empty space between its particles. The space shrinks, not the particles.
• “Crowded particles slow down.” — Their average speed depends on the temperature, which stayed at 20 °C. They hit the plunger more often because they have less space to cross between hits.

4. SQ4 · Practice

Model answer. (a) Her test isn't fair, because two things change at once: the weights increase the pressure while the warming bath raises the temperature. A change of temperature on its own changes the volume of a gas, so she can't tell how much of each change in volume the weights caused. She must keep the bath at 20 °C. (b) Some air leaked out through the split cap while it was squeezed. The same amount of air, at the same temperature and pressure, would have come back to 50 cm³. With fewer particles left, the air takes up less space, so the plunger comes back lower.

A full answer includes

• I said two things changed at once: the pressure (the weights) and the temperature (the warming bath).
• I said that a change of temperature on its own changes the volume, so she can't tell how much of the change the weights caused.
• I said the 44 cm³ reading shows that some air leaked out: with fewer particles, at the same temperature and pressure, the air takes up less space than at the start.

Also accept

• (a) Pressure and temperature both changed, so the effect of the weights can't be separated; keep the bath at 20 °C. (b) Air escaped: fewer particles take up less space at the same temperature and pressure.

Watch for

• “It doesn't matter if the bath warms, because the weights still squash the air.” — A change of temperature also changes the volume, so each reading would mix two effects.
• “The air just stays squashed after the weights come off.” — With the weights off, the same air pushes the plunger back out until the pushes balance, at its starting volume. A lower reading means some air got out.

5. SQ5 · Practice

Model answer. The pupil is right that the air takes up less space and pushes back harder, but wrong about why. Particles don't shrink when a gas is squeezed: they stay the same size, and only the empty spaces between them get smaller. With the same particles in less space, they hit the walls and the plunger more often. There are more collisions every second, so the air pushes harder on the plunger.

A full answer includes

• I said the particles do not shrink: they stay the same size.
• I said the spaces between the particles get smaller, so the same particles have less space.
• I said the particles hit the walls and the plunger more often, so the air pushes harder.

Also accept

• Particles keep their size; with less space they collide with the plunger more often, so the air pushes harder.

Watch for

• “The particles push back harder because they are squashed like springs.” — Gas particles aren't springs. The air pushes harder because the same particles hit the plunger more often when they have less space.

6. ST1 · Apply

Model answer. (a) As the bubble rises, there is less water above it, so the pressure on it decreases. The amount of air and the temperature stay the same, so the volume of the bubble increases. It's like taking weights off the plunger. (b) For a moment, the air particles inside push outwards harder than the water pushes in, so the bubble grows. As it grows, the same particles have more space and hit the inside of the bubble less often, so their push gets smaller. At each depth the bubble grows until the pushes balance; as it rises further, the push from outside keeps falling, so the bubble keeps growing.

A full answer includes

• I said the pressure on the bubble decreases as it rises, so its volume increases; the temperature and the amount of air stay the same.
• I said the air inside then pushes outwards harder than the water pushes in, so the bubble grows.
• I said that as it grows, the same particles have more space and hit the inside of the bubble less often, until their push balances the smaller push from outside.

Also accept

• Less pressure from the water near the surface, so the volume increases; the same particles spread into more space and hit less often until their push matches the smaller push from outside.

Watch for

• “The bubble grows because the particles get bigger as they rise.” — Same particles, more space: the water pushes less near the surface, so the particles spread out.
• “The bubble grows because more air gets into it.” — No air enters or leaves the bubble. The same amount of air takes up more space because the pressure on it is lower.