4PH1

Ideal Gases

Solids, Liquids & Gases · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable
Low
Stable7%

Gas law calculations (Boyle's Law, pressure-temperature) and kinetic theory explanations appear regularly.

Pressure and volume at constant temperature

  • Squeezing a fixed amount of gas into a smaller box (at the same temperature) makes the pressure rise:
    • the molecules still move at the same average speed
    • but they have less distance to travel between collisions with the wall
    • so they hit the walls more often per second, and more frequent collisions mean higher pressure
  • Letting the gas expand makes the pressure fall by the reverse logic

Pressure and temperature at constant volume

  • Heating a sealed container of gas (volume fixed) makes the pressure rise:
    • the molecules gain kinetic energy and move faster
    • they hit the walls more often and each impact carries more force
    • both effects raise the pressure
  • Cooling the gas reverses the effect, giving slower molecules, fewer and softer collisions, lower pressure
Exam tip

Explaining why pressure rises when temperature rises (constant volume)

What comes up: A question (usually 3 marks) asking you to explain, in terms of particles, why the pressure of a gas in a sealed container increases when the temperature increases.

Write (three marks): (1) Increasing temperature raises the speed (and kinetic energy) of the molecules. (2) The molecules collide with the walls more frequently (more collisions per second). (3) Each collision is harder, so the force on the walls increases, giving a higher pressure.

Watch out: You must link the answer to the walls. Simply writing "particles gain more energy" or "particles move faster" without mentioning wall collisions scores only the first mark and drops the next two.

Volume and temperature at constant pressure

  • Heating a gas in a container that can change shape (a balloon, a piston that can slide) makes the volume rise:
    • the molecules push harder on every wall
    • if a wall is free to move, it moves outward until the pressure rebalances against whatever is pushing back from outside
  • The volume of a gas is proportional to its Kelvin temperature at fixed pressure