Students should be able to: explain how the motion of the molecules in a…

The idea

Pressure comes from collisions: molecules strike the walls of the container and rebound, and the total force from all those impacts, divided by the wall area, is the pressure. Two factors set it, how hard each impact is and how many impacts happen each second. Temperature controls both, because temperature is a measure of the mean kinetic energy of the molecules. Hold the volume constant and raise the temperature: the molecules move faster, so they hit harder and hit more often, and the pressure rises.

What earns the marks

• Stating that pressure is caused by molecules colliding with the walls of the container.
• Linking a rise in temperature to a rise in the mean kinetic energy of the molecules.
• Stating both that each collision exerts a greater force and that collisions become more frequent.
• Concluding that the pressure increases when the volume is held constant.

Common mistakes and how to put them right

• Saying the molecules gain more energy without ever mentioning collisions with the walls.

• Claiming the number of molecules changes inside a sealed container.

• Giving the explanation for a volume change when the question asked about a temperature change.

• Calling pressure a force rather than a force acting over an area.

Exam tips

• Start every gas explanation with the sentence that pressure is caused by collisions with the container walls; everything else hangs off it.
• Check which quantity the question holds constant and say so explicitly in your answer.
• Qualitative means no calculation is needed, but the chain of reasoning still has to be complete to score.