Particle motion in gases — AQA GCSE Combined Science
Test yourself on Particle motion in gases with AQA GCSE practice questions.
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Particle motion in gases explained
Gas molecules move rapidly in straight lines until they collide with each other or with the container walls.
Read the full explanation
These collisions are frequent, random and elastic, and they exert a pressure on the walls. Temperature is a measure of the average kinetic energy of the molecules, not of the total kinetic energy or the speed of any single molecule. Raising the temperature increases the average kinetic energy, so molecules move faster on average and collide with the walls more often and more forcefully, increasing the gas pressure if the volume is fixed. For example, heating a sealed rigid container of gas raises its pressure because the average molecular kinetic energy and collision rate both increase.
Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.
Gas pressure arises from particles colliding with the container walls. Each collision exerts a tiny force, and the total force per unit area is the pressure. If the volume is fixed, heating the gas transfers energy to the particles, so their mean kinetic energy and average speed increase. Faster particles hit the walls more often and each impact is harder, so the pressure rises. Cooling has the opposite effect: particles move more slowly, collide less often and less forcefully, so pressure falls. For a fixed mass of gas at constant volume, pressure is directly proportional to absolute temperature in kelvin, so doubling the kelvin temperature doubles the pressure. A sealed rigid container, such as an aerosol can left in a hot car, illustrates the effect and explains why such containers carry warnings.
explain how the motion of the molecules in a gas is related to both its temperature and its pressure
Gas molecules move randomly at high speed, colliding with each other and with the container walls. Temperature measures the average kinetic energy of these molecules: heating a gas increases that average kinetic energy, so the molecules move faster. Pressure arises from the total force of molecular collisions on each unit area of the container wall. Faster molecules hit the walls more often and harder, so raising the temperature at fixed volume raises the pressure. For example, heating a sealed rigid can of gas makes its molecules faster, increasing the collision rate and force on the walls, so the pressure rises. Cooling reverses this: molecules slow, collisions become less frequent and less forceful, and pressure falls.
explain qualitatively the relation between the temperature of a gas and its pressure at constant volume.
At constant volume, the pressure of a fixed mass of gas increases as its temperature increases, and decreases as it cools. Heating raises the average kinetic energy of the molecules, so they move faster. In a fixed container the molecules therefore strike the walls more often and each collision involves a greater change of momentum, so the total force on each unit area of wall rises: pressure increases. Cooling has the opposite effect. For example, a sealed rigid aerosol can thrown into a fire heats up; its molecules speed up, wall collisions become more frequent and forceful, and the pressure rises enough to burst the can. The relationship is qualitative: pressure rises with temperature, but no calculation is required.
Your focus
- Describe the random motion and elastic collisions of gas molecules.
- Explain how temperature is related to the average kinetic energy of gas molecules.
- Use kinetic theory to explain how heating a fixed volume of gas increases its pressure.
Show all 12 objectives
- Describe how heating or cooling a gas at constant volume changes the pressure it exerts.
- Explain the pressure change in terms of particle speed, collision frequency and collision force.
- Apply the direct proportionality between pressure and absolute temperature to a fixed mass of gas at constant volume.
- Describe how gas molecules move and how their motion produces pressure on the container walls.
- Explain that temperature measures the average kinetic energy of gas molecules and that heating increases their average speed.
- Link increased molecular speed at constant volume to more frequent, more forceful wall collisions and therefore higher pressure.
- State qualitatively how the pressure of a fixed mass of gas at constant volume changes when its temperature changes.
- Explain the change in pressure in terms of molecular speed, collision frequency and force on the container walls.
- Apply the qualitative relationship to everyday contexts such as a sealed container being heated or cooled.
Particle motion in gases exam tips
Marking Points
- Describe gas molecules as moving continuously, randomly and in straight lines between collisions.
- State that collisions between gas molecules and with the container walls are elastic and produce pressure on the walls.
- Relate the temperature of a gas to the average kinetic energy of its molecules, not to the kinetic energy of an individual molecule.
- Explain that increasing temperature increases average molecular kinetic energy, leading to more frequent and more forceful collisions with the walls and therefore higher pressure at constant volume.
- Distinguish between average kinetic energy and total kinetic energy when describing a fixed mass of gas.
- Pressure is the force exerted by gas particles per unit area on the container walls.
- At constant volume, raising the temperature increases the average kinetic energy and average speed of the particles.
- Faster particles collide with the walls more frequently and each collision exerts a larger force, so pressure increases.
- Cooling a fixed volume of gas reduces particle speed, collision frequency and impact force, so pressure decreases.
- For a fixed mass of gas at constant volume, pressure is directly proportional to absolute temperature measured in kelvin.
- Kelvin temperature is calculated by adding 273 to the Celsius temperature, so 27 °C is about 300 K.
- Gas molecules move randomly and rapidly, colliding with each other and with the walls of the container.
- Temperature is a measure of the average kinetic energy of the gas molecules, so a higher temperature means a higher average speed.
- Pressure is caused by the force of molecular collisions acting on the container walls per unit area.
- At constant volume, increasing temperature increases the average kinetic energy and speed of the molecules.
- Faster molecules collide with the walls more frequently and with greater change of momentum, so the force on the walls and the pressure increase.
- A complete explanation links temperature to average kinetic energy, then to collision rate and force, then to pressure.
- At constant volume, the number of molecules and the volume of the container are fixed.
- Increasing temperature increases the average kinetic energy and average speed of the molecules.
- Faster molecules collide with the container walls more frequently.
- Each collision with a faster molecule produces a greater change of momentum, so the average force on the walls is larger.
- Greater collision frequency and greater force per collision both increase the pressure, so pressure rises with temperature.
- The relationship is direct and qualitative: as temperature increases, pressure increases; as temperature decreases, pressure decreases.
Examiner Tips
- 💡Use the phrase 'average kinetic energy' rather than 'kinetic energy' when linking molecular motion to temperature.
- 💡When explaining pressure changes, refer to both the frequency and the force of collisions with the container walls.
- 💡Sketch or describe molecular motion using straight-line paths between collisions to show random motion clearly.
- 💡Link each change in pressure to both collision frequency and the force of each collision, since either alone gives an incomplete explanation.
- 💡Convert temperatures to kelvin before comparing or calculating pressures, and show the conversion clearly.
- 💡Use the particle model consistently: refer to particles, collisions, kinetic energy and wall force rather than saying the gas 'wants more space'.
- 💡Use the chain temperature → average kinetic energy → average speed → collision rate and force on walls → pressure.
- 💡Refer explicitly to collisions with the walls per unit area, because that is what pressure measures.
- 💡Use comparative language such as faster, more frequent and greater force rather than simply saying the molecules move more.
- 💡State clearly that volume and number of molecules stay constant before explaining the change.
- 💡Use the sequence temperature → average kinetic energy → speed → collision frequency and force → pressure.
- 💡Include both more frequent collisions and greater force per collision to secure the explanation.
Common Mistakes
- Saying that all molecules in a gas have the same kinetic energy: correct this by stating that molecules have a range of speeds and kinetic energies, and temperature relates to their average.
- Describing gas pressure as molecules pushing each other: correct this by explaining that pressure arises from collisions of molecules with the container walls.
- Claiming that temperature measures the total kinetic energy of the gas: correct this by stating that temperature measures average kinetic energy per molecule.
- Saying that particles expand or get bigger when heated: correct this by stating that the particles themselves do not change size; their average speed and kinetic energy increase.
- Using degrees Celsius in a pressure–temperature calculation: correct this by converting to kelvin first, because the proportionality only holds for absolute temperature.
- Claiming that pressure increases because there are more particles: correct this by explaining that the number of particles is unchanged at constant volume; the increase is due to more frequent and more forceful collisions.
- Saying that molecules expand or get bigger when heated: correct this by stating that the molecules themselves do not change size; their average speed and kinetic energy increase.
- Claiming that pressure is caused by molecules pushing each other rather than hitting the walls: correct this by explaining that pressure comes from collisions with the container walls per unit area.
- Confusing temperature with total kinetic energy: correct this by stating that temperature relates to the average kinetic energy of the molecules, not the total.
- Saying pressure increases because molecules expand: correct this by stating that molecules do not expand; their average speed and kinetic energy increase.
- Saying pressure increases because there are more molecules: correct this by stating that at constant volume the number of molecules is fixed; only their motion changes.
- Treating the relationship as proportional and calculating values: correct this by giving a qualitative explanation, since the specification asks only for a qualitative relation.