AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The molecules of a gas are in constant random motion. The temperature of…
Full specification wording
Particle motion in gases: The molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules.
Gas molecules move rapidly in straight lines until they collide with each other or with the container walls.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gas molecules move rapidly in straight lines until they collide with each other or with the container walls. 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.
What earns the marks
- 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.
Common mistakes and how to put them right
Saying that all molecules in a gas have the same kinetic energy.
Put it right 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.
Put it right by explaining that pressure arises from collisions of molecules with the container walls.
Claiming that temperature measures the total kinetic energy of the gas.
Put it right by stating that temperature measures average kinetic energy per molecule.
Exam 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.
Try the lesson
The start of the Particle motion in gases: The molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes how the molecules of a gas behave?
Listen
Gas molecules move rapidly in straight lines until they collide with each other or with the container walls. 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. A strong answer earns marks for these 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. A common slip: Saying that all molecules in a gas have the same kinetic energy. Instead, stating that molecules have a range of speeds and kinetic energies, and temperature relates to their average. Exam tip: Use the phrase 'average kinetic energy' rather than 'kinetic energy' when linking molecular motion to temperature.
Key words
Say what each word means before you turn the card.
Random molecular motion
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Temperature and energy
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Kinetic theory and pressure
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See every definition
- Random molecular motion
- Gas molecules move continuously, randomly and in straight lines between collisions with each other or with the container walls.
- Temperature and energy
- 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.
- Kinetic theory and pressure
- Collisions between gas molecules and with the container walls are elastic and exert a pressure on the walls.
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