Diffusion is one of the key concepts you need to learn for GCSE. With diffusion you will know why if some sprays some deodorant or perfume in one corner of the room why it spreads everywhere else.
You will need to know this text-book definition.
Diffusion is the movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient.
High concentration – Lots of particles in one area
Low concentration – Not many particles in an area
Concentration gradient – The difference created when there are different amounts of particles in an area
If in doubt just remember that they go from high to low.
NOTE: Diffusion can happen in both solids and liquids!!!