AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Explain qualitatively the relation between the temperature of a gas and…
Full specification wording
Particle motion in gases: 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.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 4 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Saying pressure increases because molecules expand.
Put it right by stating that molecules do not expand; their average speed and kinetic energy increase.
Saying pressure increases because there are more molecules.
Put it right by stating that at constant volume the number of molecules is fixed; only their motion changes.
Exam tips
- 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.
Try the lesson
The start of the Particle motion in gases: explain qualitatively the relation between the temperature of a gas and its pressure at constant volume. lesson, made from checked teaching notes for this specification point.
Have a go first
A sealed container holds a fixed volume of gas. What happens to the gas pressure if the temperature of the gas is increased?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Saying pressure increases because molecules expand. Instead, stating that molecules do not expand; their average speed and kinetic energy increase. Exam tip: State clearly that volume and number of molecules stay constant before explaining the change.
Key words
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Pressure–temperature relationship at constant volume
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Average kinetic energy and molecular speed
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Collision frequency and force on container walls
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See every definition
- Pressure–temperature relationship at constant volume
- At constant volume, the pressure of a fixed mass of gas increases as its temperature increases, and decreases as it cools.
- Average kinetic energy and molecular speed
- Increasing temperature increases the average kinetic energy and average speed of the molecules.
- Collision frequency and force on container walls
- Faster molecules collide with the container walls more frequently, and each collision produces a greater change of momentum, so the average force on the walls is larger.
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