LearnovaAll Physics notes
AQA PhysicsParticle model of matter

Gas pressure and temperature

Explain gas pressure using particle motion.

Start here

The key idea

Gas pressure is caused by particles colliding with the walls of their container.

Gas Pressure And Temperature
hotter particles collide harder and more often

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

Gas pressure

Gas particles move quickly in random directions.They collide with the walls of their container, and each collision exerts a tiny force.Gas pressure is the total force these collisions exert per unit area of the container walls.The more frequent and forceful the collisions, the higher the pressure.

2

Temperature and particle energy

The temperature of a gas is related to the average kinetic energy of its particles.Heating a gas gives the particles more kinetic energy, so they move faster.The kelvin temperature is directly proportional to the average kinetic energy of the particles, so doubling the kelvin temperature doubles their average kinetic energy.Absolute zero, 0 kelvin (about -273 degrees C), is where particles have minimum energy.

3

Pressure and temperature at constant volume

If a fixed volume of gas is heated, the particles move faster and hit the walls more often and harder, so the pressure increases.If the gas is cooled, the particles slow down, collide less often and with less force, so the pressure falls.This is why a sealed container can burst if heated too strongly.

4

Pressure and volume at constant temperature

At constant temperature, pressure x volume is constant for a fixed mass of gas: P x V = constant.If the volume is reduced, the same number of particles hit a smaller area more often, so the pressure rises.Doing work on a gas by compressing it quickly can also raise its temperature, as in a bicycle pump warming up.

Key terms

Definitions to learn

Gas pressure

The force per unit area from particle collisions with the walls.

Absolute zero

0 kelvin, about -273 degrees C, the lowest possible temperature.

Kelvin scale

A temperature scale starting at absolute zero; add 273 to degrees C.

Compression

Reducing the volume of a gas, which raises its pressure.

Average kinetic energy

The mean energy of motion of the gas particles.

Worked example

Explain why the pressure of a sealed gas increases when its temperature increases.

1

Describe the change in particle motion.

2

Link this to collisions with the walls.

Final answer

Particles move faster, collide more often and with greater force, so the pressure increases.

Exam habit

Explain pressure in terms of particle collisions: frequency, force, and area.For Boyle's law calculations, state p1V1 = p2V2 explicitly before substituting. Always state what is kept constant.

Watch out

Do not say particles expand; the spacing may change, but the particles themselves do not get bigger.

Examiner tips

How to score full marks

  • 1Always convert temperature to kelvin (add 273) when using the relationship with particle energy.
  • 2Explain pressure changes in terms of collision frequency and force, not just particle speed.
  • 3For constant temperature use P x V = constant, so P1 x V1 = P2 x V2.
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1A gas is compressed at constant temperature. Explain why its pressure increases.
Mark scheme
  1. 1.Consider collision frequency in a smaller volume.
Particles travel a shorter distance between wall collisions, so collisions occur more frequently.
2State one assumption needed when comparing pressure and volume using Boyle's law.
Mark scheme
  1. 1.Identify the controlled variable.
The temperature must remain constant.
3A gas has volume 60 cm3 at 100 kPa. Find its volume at 150 kPa if temperature is constant.
Mark scheme
  1. 1.Use p1V1 = p2V2.
  2. 2.Rearrange for V2.
40 cm3
4Explain in terms of particles what causes the pressure of a gas.[2 marks]
Mark scheme
  1. 1.Recall how particles interact with the walls.
Gas particles collide with the container walls (1); the force of these collisions per unit area creates the pressure (1)
5Convert 27 degrees C to kelvin.[1 mark]
Mark scheme
  1. 1.Add 273 to the Celsius value.
27 + 273 = 300 K (1)
6Explain why the pressure of a fixed volume of gas increases when it is heated.[3 marks]
Mark scheme
  1. 1.Link heating to particle speed.
  2. 2.Link faster particles to collisions.
  3. 3.Link collisions to pressure.
Heating gives the particles more kinetic energy so they move faster (1); they hit the walls more often and with greater force (1); so the force per unit area, and therefore the pressure, increases (1)
7A gas has a volume of 0.020 m3 at a pressure of 100000 Pa. It is compressed at constant temperature to 0.010 m3. Calculate the new pressure.[3 marks]
Mark scheme
  1. 1.Use P1 x V1 = P2 x V2.
  2. 2.Rearrange to P2 = (P1 x V1) / V2.
  3. 3.Substitute the values.
P1 x V1 = P2 x V2 (1); P2 = (100000 × 0.020) / 0.010 (1) = 200000 Pa (1)
8A sealed rigid can of gas is left in direct sunlight on a hot day and eventually bursts. Explain why, in terms of particles and pressure, and state why the volume staying fixed is important.[4 marks]
Mark scheme
  1. 1.Link heating to particle kinetic energy.
  2. 2.Link to collision rate and force.
  3. 3.Link to rising pressure in fixed volume.
  4. 4.Connect to bursting.
The sun heats the gas, increasing the average kinetic energy of the particles so they move faster (1); they collide with the walls more frequently and with greater force (1); because the volume is fixed the pressure rises (P proportional to T at constant volume) (1); eventually the pressure exceeds what the can can withstand and it bursts (1)
9Convert -73 degrees C to kelvin and state what this temperature represents in terms of particle motion.[3 marks]
Mark scheme
  1. 1.T(K) = T(degrees C) + 273.
  2. 2.Interpret the kelvin value in relation to absolute zero.
  3. 3.Describe particle motion.
T = -73 + 273 = 200 K (1); at this temperature the particles still have kinetic energy and are still moving (1); only at absolute zero (0 K) would particle motion cease (though in practice quantum effects prevent this) (1)
10A fixed mass of gas has a volume of 0.600 m3 at 300 K and 120 kPa. The gas is heated to 450 K at constant pressure. Calculate the new volume.[3 marks]
Mark scheme
  1. 1.At constant pressure V/T = constant, so V1/T1 = V2/T2.
  2. 2.Rearrange to V2 = V1 x T2 / T1.
  3. 3.Substitute V1 = 0.600, T1 = 300, T2 = 450.
V1 / T1 = V2 / T2 (1); V2 = V1 x T2 / T1 = 0.600 × 450 / 300 (1) = 0.900 m3 (1)
11Explain why a gas in a bicycle pump becomes warm when the pispiston is pushed in quickly. In your answer, refer to work done and internal energy.[4 marks]
Mark scheme
  1. 1.Pushing the pispiston does work on the gas.
  2. 2.Work done increases the internal (kinetic) energy of the gas particles.
  3. 3.Increased kinetic energy means higher temperature.
  4. 4.The effect is noticeable because compression is rapid so energy cannot escape.
When the pispiston is pushed in, a force is applied through a distance, doing work on the gas (1); this work transfers energy into the kinetic store of the gas particles, increasing the internal energy of the gas (1); higher internal energy means the particles move faster on average, which corresponds to a rise in temperature (1); the pump warms noticeably because the compression is fast so little energy is lost to the surroundings before the temperature rises (1)
12A car tyre contains 0.030 m3 of air at 250 kPa and 290 K. After a long motorway journey the temperature rises to 320 K. Assuming the volume of the tyre stays the same, calculate the new pressure and explain whether this is a safety concern.[4 marks]
Mark scheme
  1. 1.At constant volume P/T = constant, so P1/T1 = P2/T2.
  2. 2.P2 = P1 x T2 / T1.
  3. 3.Substitute values.
  4. 4.Compare new pressure to original and comment on safety.
P1/T1 = P2/T2 (1); P2 = 250000 × 320 / 290 (1) = 275862 Pa ≈ 276 kPa (1); the pressure has risen by about 26 kPa (about 10%), which could cause the tyre to burst if the walls are already under high stress; this is why tyre pressure should be checked when tyres are cold (1)
13Describe and explain, using a full particle model argument, what happens to the pressure, volume and particle motion of a fixed mass of gas when it is: (a) heated at constant volume, and (b) compressed at constant temperature. In each case, explain the changes at a particle level and link them to the macroscopic observable change.[4 marks]
Mark scheme
  1. 1.(a) Constant volume, heating: particles gain KE, collide harder/more often, pressure rises.
  2. 2.(b) Constant T, compressed: volume smaller, same speed particles but shorter path between walls, collision rate rises, pressure rises.
  3. 3.Use P = F/A concept for both.
  4. 4.No change in number of particles in either case.
(a) When the gas is heated at constant volume, the particles gain kinetic energy and move faster; they strike the walls more frequently and each collision exerts a greater force; since the area of the walls does not change, the force per unit area — the pressure — increases; T increases, P increases, V stays the same (1 for particle argument, 1 for macroscopic outcome); (b) when the gas is compressed at constant temperature the particles' speeds remain unchanged (temperature constant = same average KE), but the container is smaller so the particles travel shorter distances between successive wall collisions, increasing the collision frequency per unit area; this raises the pressure; V decreases, P increases, particle speed unchanged (1 for particle argument, 1 for macroscopic outcome) — award 4 marks total
Official exam-board sources
Specification Past papers
Browse all Physics topics