Answers · Gases under pressure
Gases under pressure — return questions key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. SR1 · Return
Model answer. The volume decreases. The same particles are pushed closer together, into less space. They hit the walls of the container more often, so the gas pushes harder. The volume stops decreasing when the push of the gas balances the push from outside.
A full answer includes
- The volume decreases as the pressure increases.
- The same particles are pushed closer together, into less space.
- They hit the walls more often, so the gas pushes harder, until its push balances the push from outside.
Also accept
- Volume decreases; the particles are closer together, with more frequent collisions with the walls, so a bigger push, until the pushes balance.
Watch for
- “The volume decreases because the particles get smaller.” — Squeezing changes the spaces between the particles, not the particles.
2. SR2 · Return
Model answer. When the gas is cooled at constant pressure, its volume decreases: it contracts. Its particles move more slowly, so they hit the walls less hard and less often. The push from outside is now bigger, so the gas is pushed into less space until the pushes balance again. If instead the pressure on it is increased at constant temperature, its volume also decreases.
A full answer includes
- Cooled at constant pressure: the volume decreases (the gas contracts).
- The particles move more slowly, so they hit the walls less hard and less often; the push from outside wins, and the volume decreases until the pushes balance.
- More pressure at constant temperature: the volume decreases.
Also accept
- Cooling: volume decreases, because slower particles hit the walls less hard and less often until the pushes balance. More pressure: volume decreases.
Watch for
- “Cooling stops the gas particles moving.” — Cooling slows the particles down; it doesn't stop them. A cold gas is still a gas, with moving particles.