Lesson overview
Gases under pressure: What happens to a gas when you squeeze or heat it?
Trap one sample of air in a sealed syringe. Load the plunger with weights, then warm and cool it in a water bath, and watch the same particles fill more or less space.
Suggested sequence
- Lesson 1 · Squeeze a gas. Pressure and volume at the same temperature, explained with particles. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Warm it, cool it. Temperature and volume at the same pressure, explained with particles. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Squeeze a gas
- Describe the effect of pressure on the volume of a gas at constant temperature
- Explain, using kinetic particle theory, the effect of pressure on the volume of a gas
Part 2: Warm it, cool it
- Describe the effect of pressure on the volume of a gas at constant temperature
- Describe the effect of temperature on the volume of a gas at constant pressure
- Explain, using kinetic particle theory, the effect of temperature on the volume of a gas
Misconceptions to listen 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.
- “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.
- “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.
- “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.
- “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.
- “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.
Scope
The effects of pressure and temperature on the volume of a gas (1.1.4) and their explanation with kinetic particle theory (Supplement 1.1.6), with a recap of gas particles from 1.1.2. The experiment uses one ideal sealed sample of air; gas-law equations and numerical gas calculations are not part of this module.