Diffusion

Key Definition Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

It occurs in gases and liquids because particles possess kinetic energy and move randomly in all directions. Where concentration is higher, more particles move outward than inward, so the substance spreads until its concentration is uniform. Diffusion in gases is faster than in liquids because gas particles travel at higher speeds and have larger gaps between them.

Rate of diffusion

• The rate of diffusion of a gas also depends on the relative molecular mass (Mᵣ) of its particles.
• At the same temperature, lighter molecules (lower Mᵣ) have higher average speeds, so they diffuse more quickly.
• This is demonstrated by the reaction between ammonia and hydrogen chloride in a glass tube: ammonia, with its lower Mᵣ, travels further in the same time, so the white ring of ammonium chloride forms closer to the hydrogen chloride end.

Understanding both the definition and the factors affecting the rate of diffusion is essential for answering CIE exam questions accurately.