Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion.

Why diffusion happens (kinetic particle theory)

Particles in gases and liquids move randomly and collide. Over time, more particles drift away from crowded areas to less crowded areas, so the overall movement is from high to low concentration.

Where and how fast

• State: Fastest in gases, slower in liquids, extremely slow in solids.
• Temperature: Higher temperature gives particles more energy, so diffusion is faster.
• Relative molecular mass: Lighter gas particles diffuse faster than heavier ones.
• Concentration difference: A bigger difference makes diffusion faster.
• Distance: Shorter distances diffuse faster.

Examples

Perfume smell spreading across a room. Purple potassium manganate(VII) spreading through water. In a tube with cotton wool soaked in ammonia and hydrochloric acid, a white ring forms closer to HCl because ammonia is lighter: $M_r(\mathrm{NH_3})=17$, $M_r(\mathrm{HCl})=36.5$.

Common misconceptions

• Diffusion does not need wind; it happens by random particle motion.
• It occurs in gases and liquids, not only in gases.
• Osmosis is a special case of diffusion of water through a membrane, not the same as general diffusion.