Diffusion – IGCSE Chemistry Definition

Diffusion is one of the clearest demonstrations that particles are in constant random motion. The 0620 syllabus tests it directly in multiple-choice and structured questions, often linked to the kinetic particle theory. You must be able to define diffusion precisely, explain why rate changes with temperature or molecular mass, and apply the concept to real examples.

The 0620 definition

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient. It happens because of the random motion of particles.

Key point: diffusion occurs in gases and liquids, but not in solids. Solid particles vibrate in fixed positions and cannot move past each other.

Factors affecting the rate of diffusion

Factor	Effect on rate	Explanation
Temperature	Increases rate	Particles have more kinetic energy, move faster
Molecular mass	Lower mass = faster	Lighter particles travel faster at the same temperature
State of matter	Gas > liquid	Gas particles are further apart with fewer collisions
Concentration gradient	Steeper = faster	Greater difference drives faster net movement

The ammonia–HCl experiment

A classic 0620 practical question: cotton wool soaked in ammonia (NH₃, Mr = 17) is placed at one end of a glass tube and cotton wool soaked in hydrochloric acid (HCl, Mr = 36.5) at the other. A white ring of ammonium chloride forms closer to the HCl end.

Why? NH₃ has a lower relative molecular mass, so its molecules diffuse faster and travel further along the tube before meeting HCl molecules.

NH₃(g) + HCl(g) → NH₄Cl(s)

Worked exam question

Bromine vapour is released at the bottom of a gas jar of air. After several minutes the brown colour is seen throughout the jar. (a) Name this process. (1) (b) Explain, in terms of particles, why the brown colour eventually fills the jar. (2) (c) State and explain how heating the gas jar would affect the result. (2)

Mark scheme

(a) Diffusion [1]

(b) Bromine particles move randomly / have kinetic energy [1]; they spread from high concentration at the bottom to low concentration at the top / net movement down the concentration gradient [1]

(c) The brown colour would fill the jar faster / rate of diffusion increases [1]; because particles have more kinetic energy / move faster at higher temperature [1]

Common exam mistakes

• Writing “particles move from high to low concentration” without the word net. Some particles move in all directions; it is the overall net movement that goes down the gradient.
• Claiming diffusion occurs in solids. It does not — solid particles vibrate in fixed positions.
• Confusing diffusion with osmosis. Osmosis is the net movement of water molecules through a partially permeable membrane — that is a biology term, not required for 0620 Chemistry.