Diffusion: the temperature

The idea

Temperature is one of the factors that affects the rate of diffusion. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of particles. Raising the temperature increases the average kinetic energy of the particles, so they move faster and collide more frequently. This means the particles spread out and reach a uniform concentration more quickly, so the rate of diffusion increases.

For example: A drop of food colouring spreads through hot water faster than through cold water because the dye particles and water molecules have more energy.

What earns the marks

• States that increasing temperature increases the rate of diffusion.
• Explains that higher temperature gives particles more kinetic energy so they move faster.
• Links faster particle movement to more frequent collisions and quicker net movement from higher to lower concentration.
• Uses a suitable example, such as food colouring spreading faster in hot water than in cold water.
• Describes the direction of diffusion as net movement from higher to lower concentration.
• Recognises that diffusion is driven by the random motion of particles.

Common mistakes and how to put them right

• Thinking that temperature changes the direction of diffusion.

  Put it right: Temperature changes the rate, while net movement remains from higher to lower concentration.

• Saying that particles expand or get bigger when heated.

  Put it right: The particles themselves do not expand, they gain kinetic energy and move faster.

• Confusing diffusion with active transport.

  Put it right: Diffusion is passive and does not require energy from the cell, whereas active transport uses energy.

• Believing that only the diffusing particles speed up.

  Put it right: All particles in the system gain kinetic energy, which increases the frequency of collisions.

Exam tips

• Always link temperature to kinetic energy and then to the speed of particle movement.
• Use the phrase net movement from higher to lower concentration to show understanding of diffusion.
• When interpreting a graph, describe the trend and then explain it using particle motion rather than just repeating the data.
• Give a named example, such as food colouring in water, to support your explanation.