Particle Motion in Gases
Understand Particle Motion in Gases for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Gas particles move randomly, and their average kinetic energy — and so the gas temperature — increases when the gas is heated.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The molecules of a gas are in constant random motion. Their temperature is related to the average kinetic energy of the molecules.
Raising the temperature increases the average kinetic energy, so the molecules move faster. This is a direct relationship: temperature is a measure of the average kinetic energy of the particles.
Moving molecules collide with the walls of their container and with each other. Each collision exerts a small force on the wall, and the total effect of all these collisions produces the pressure of the gas. Pressure therefore depends on how often and how hard the molecules strike the walls.
Revision notes
Random motion
The molecules of a gas are in constant random motion, moving in all directions at a range of speeds.
They collide with each other and with the walls of the container. Between collisions they travel in straight lines.
Temperature and kinetic energy
The temperature of a gas is related to the average kinetic energy of its molecules.
Raising the temperature increases the average kinetic energy, so the molecules move faster on average. Cooling decreases it.
How pressure arises
Each collision between a molecule and the container wall exerts a small force on it.
The total effect of the very large number of such collisions produces the pressure of the gas. More frequent or more forceful collisions mean a higher pressure.
Key points
- Gas molecules are in constant random motion.
- Temperature relates to average kinetic energy.
- Higher temperature means faster molecules.
- Molecules collide with the container walls.
- Each collision exerts a force on the wall.
- The total effect of collisions produces pressure.
Worked examples
Example 1
Explain how the molecules of a gas produce pressure on the walls of a container. [3 marks]
Model answer
The molecules are in constant random motion and collide with the walls (1 mark). Each collision exerts a small force on the wall (1 mark). The total effect of the very large number of collisions produces the pressure of the gas (1 mark).
Example 2
Explain what happens to the molecules of a gas when its temperature is increased. [2 marks]
Model answer
The average kinetic energy of the molecules increases (1 mark), so they move faster on average (1 mark).
Example 3
State what the temperature of a gas is a measure of. [1 mark]
Model answer
The average kinetic energy of the molecules of the gas (1 mark).
Common mistakes
Saying molecules move in one direction.
The motion is random, in all directions and at a range of speeds.
Saying all molecules have the same speed.
There is a range of speeds; temperature relates to the AVERAGE kinetic energy.
Saying pressure comes from molecules pushing each other.
Pressure on the container arises from collisions with the walls.
Forgetting to say average.
Temperature relates to the average kinetic energy, not to every molecule equally.
Exam tips
- Always use the word average when linking temperature to kinetic energy.
- Describe pressure through collisions with the walls.
- Say constant random motion when describing gas molecules.
- Build the pressure explanation in three steps: motion, collisions, total force.
Key terms
- Random motion
- Movement in all directions with no preferred path.
- Average kinetic energy
- The mean energy of motion of the molecules.
- Pressure
- Force per unit area, produced by molecular collisions.
- Collision
- A molecule striking a wall or another molecule.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.