Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Definition
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration as a result of random motion. Particles move both ways, but more leave the high-concentration region than return.
Gases
Gas particles move rapidly through large spaces, so gases mix readily. A coloured gas or odour spreads even without stirring. Diffusion is faster at higher temperature because average speed is greater.
Liquids
Diffusion also occurs in liquids, but it is slower because particles are close together and interact more. A coloured crystal spreading through still water provides visible evidence.
Equilibrium
Eventually concentration becomes uniform. Random motion continues, but there is no net movement because equal numbers cross in opposite directions.
Valid evidence
Temperature gradients may create convection, which is bulk fluid movement rather than diffusion. A good experiment avoids disturbance and uses equal conditions when comparing rates.
Practical or data skill
Observe a coloured solute spreading through still water at two temperatures. Discuss how convection could affect the warmer result.
Examination tip
A full definition includes net movement, higher to lower concentration and random motion.
Review questions and suggested answers
Question 1
Does movement stop at equilibrium?
Suggested answer
No. Random movement continues with no net movement.
Question 2
Why is gas diffusion faster than liquid diffusion?
Suggested answer
Gas particles move rapidly and are widely separated.
Question 3
How does temperature affect diffusion?
Suggested answer
Higher temperature increases particle speed and diffusion rate.
