Answers · Gases under pressure
Gases under pressure — final check key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. SK1 · Final check
Model answer. (a) The volume decreases: the harder she pushes, the less space the air takes up. The amount of air stays the same, because her finger stops any getting out, and so does the temperature, because she pushes slowly. (b) The same particles are pushed closer together, so they hit the walls and the plunger more often. The air pushes back harder, so she has to push harder to squeeze it further. When she lets go, the air pushes on the plunger harder than the atmosphere pushes from outside, so the handle moves out. The particles spread out and hit less often until the pushes balance. That happens at the starting volume, because the amount of air and its temperature are the same as at the start.
A full answer includes
- I said the volume of the trapped air decreases as the pressure on it increases.
- I stated the two conditions kept the same: the amount of air (the number or mass of air particles) and the temperature. (The particles also keep their size, but that is not a condition you keep the same.)
- I said the same particles are pushed closer together and hit the walls and the plunger more often, so the air pushes back harder and the handle gets harder to push.
- I said that when she lets go, the air pushes harder than the atmosphere, so the handle moves out until the pushes balance; it ends where it started because the amount of air and its temperature are the same as at the start.
Also accept
- (a) Volume decreases; kept the same: the amount of air (number or mass of particles) and the temperature. (b) Closer particles collide with the plunger more often, so a bigger push; released, the air pushes the handle out until its push balances the atmosphere again, at the starting volume.
Watch for
- “The air pushes back because it gets hot.” — She pushes slowly, so the air stays at room temperature. The extra push comes from the same particles hitting the plunger more often.
- “The handle comes back because the air particles are springy.” — The particles aren't springs. The squeezed air pushes harder than the atmosphere until it has spread back out to the volume where the pushes balance.
2. SK2 · Final check
Model answer. (a) The air in the bottle was cooled, so its volume decreased. Two things stayed the same: the amount of air, because the cap was on, and the pressure of the air outside the bottle (atmospheric pressure): the air in the freezer pushed on the bottle just as hard as the air in the room. So the change was caused by the cold. (b) In the cold, the particles inside moved more slowly, so they hit the inside of the bottle less hard and less often. The air inside now pushed less hard than the air outside, so the air outside pushed the sides in. (The stiff plastic pushes back, so the sides cave in only a little.) (c) The statement is wrong on both points. Cooling slows the particles down, but they keep moving: the air is still a gas. The particles don't shrink; the spaces between them get smaller.
A full answer includes
- I said the air was cooled, so its volume decreased.
- I stated two things that stayed the same: the amount of air (the cap was on) and the pressure of the air outside the bottle (atmospheric pressure). “The pressure” alone is not enough: the push of the air inside got smaller.
- I said the particles inside moved more slowly, so they hit the inside of the bottle less hard and less often; the air inside then pushed less hard than the air outside, so the air outside pushed the sides in.
- I said the statement is wrong: the particles slow down but keep moving, and they keep their size; only the spaces between them get smaller.
Also accept
- (a) Volume decreased; the amount of air and the pressure of the air outside (atmospheric pressure) stayed the same. (b) Slower particles hit the walls less hard and less often, so the air inside pushed less hard than the air outside, which pushed the sides in. (c) Wrong: particles slow down but keep moving, and keep their size.
Watch for
- “The cold sucked air out of the bottle.” — The cap was on, so the same air is inside. It takes up less space because its particles move more slowly and push less.
- “The pressure of the air inside the bottle stayed the same.” — The push from inside got smaller: slower particles hit the walls less hard and less often. That's why the air outside could push the sides in. What stayed the same was the pressure of the air outside.
- “Cold particles stop moving and shrink.” — Cold particles move more slowly but keep moving, and they keep their size.