Temperature (Edexcel IGCSE Science)

Temperature & speed

Spec point — 6.4 · understand why an increase in temperature results in an increase in the average speed of gas molecules

• Imagine molecules of gas that are free to move around in a box

• The molecules in the gas move around randomly at high speeds, colliding with surfaces and exerting pressure upon them

• The temperature of a gas is a measure of the average speed of the molecules:

  • The higher the temperature of the gas, the faster the molecules move
  • This is because they have a greater average speed

Gas molecules move about randomly at high speeds

Temperature & kinetic energy

Spec point — 6.5 · know that the Kelvin temperature of a gas is proportional to the average kinetic energy of its molecules

• The temperature of a substance is related to the average kinetic energy of its molecules

• The higher the temperature, the higher the average kinetic energy of the molecules, and vice versa

  • This means they move faster at higher temperatures
  • This applies to all states of matter, although the motion of particles in a solid is different from that of particles in a gas

• For a gas, the internal energy can be taken as the sum of the kinetic energies of all the molecules

  • In a gas, the intermolecular forces are negligible, so the contribution from potential energy can be taken as zero
  • This is different from a solid or liquid, where the internal energy includes contributions from both kinetic and potential energy

As the container is heated up, the gas molecules move faster with higher kinetic energy. The energy stored within the system - the internal energy - therefore increases

• If the temperature of a gas is increased, the particles move faster and gain kinetic energy

  • They collide more often with each other and the container walls, leading to an increase in pressure

• The temperature (in Kelvin) is proportional to the average kinetic energy of the molecules:

$T \propto K E$

Worked Example When a liquid evaporates, molecules escape from the surface of the liquid.

What happens to the temperature of the liquid and the average kinetic energy of the molecules within it?

	Temperature / K	Average kinetic energy of the molecules
A	Increases	Increases
B	Decreases	Decreases
C	Stays the same	Stays the same
D	Decreases	Increases

Answer: B

• When evaporation takes place, the more energetic molecules leave the surface of the liquid

• Since the more energetic molecules have left, the average kinetic energy per molecule must decrease

  • Therefore, A& D are not correct

• Temperature is proportional to the average kinetic energy per molecule, therefore, the temperature also decreases