AQA GCSE Physics

Revision Notes

3.3.1 Kinetic Theory

Temperature & Kinetic Energy

  • Molecules in a gas are in constant random motion at high speeds
  • The temperature of the gas is related to the average kinetic energy of the molecules
    • The hotter the gas, the higher the average kinetic energy
    • The cooler the gas, the lower the average kinetic energy
  • Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
    • With the walls of its container
    • With other molecules
  • The random motion of tiny particles in a fluid is known as Brownian motion

Random motion of gas particles, IGCSE & GCSE Chemistry revision notes

Random motion of gas molecules in a container

Temperature & Pressure

  • The motion of molecules in a gas changes according to the temperature and pressure of the gas


  • As the temperature of a gas increases, the average speed of the molecules also increases
  • Since the average kinetic energy depends on their speed, the kinetic energy of the molecules also increases if its volume remains constant


  • Molecules of gas in a container will collide with the container walls
  • Pressure is defined as the force exerted per unit area of the container

Pressure equation, downloadable AS & A Level Physics revision notes

  • Therefore a gas at high pressure has more frequent collisions on the container walls and therefore overall more force per unit area
  • If the gas is heated up, the molecules will travel at a higher speed
    • This means they will collide with the walls more often
    • This creates an increase in pressure
  • Therefore, at a constant volume, an increase in temperature increases the pressure of a gas and vice versa
  • Diagram A shows molecules in the same volume collide with the walls of the container more with an increase in temperature
  • Diagram B shows that since the temperature is proportional to the pressure, the graph against each is a straight line

States of Matter Temperature and Pressure, downloadable IB Chemistry revision notes

At constant volume, an increase in the temperature of the gas increases the pressure due to more collisions on the container walls

Exam Tip

You are required to be able to describe the links between pressure & volume and pressure & temperature qualitatively. This means that the correct use of terms such as ‘collision’, ‘kinetic energy’ and ‘frequency’, will be really important.

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