Diffusion in gases
When chemicals, like the smell of perfume or burning toast, are let loose in a room, the particles mix with the air particles. The particles of smelly gas are free to move quickly in all directions. They eventually spread throughout the whole room. This is called diffusion.
Diffusion in gases is quick because the particles in a gas move quickly. It happens even faster in hot gases.
Mixture of two gases
Before diffusion, particles of one gas are concentrated in one area. After diffusion, the particles are spread more evenly throughout the other gas. Diffusion happens quickly in gases.
Diffusion in liquids
Diffusion can also happen in liquids. This is because the particles in liquids can move around each other, which means that eventually they are evenly mixed. For example, if you drop a little bit of paint into a jar of water the colour will spread slowly through the water by diffusion.
Diffusion in liquids is slower than in gases because the particles in a liquid move more slowly.
Solids
Diffusion does not happen at all in solids because the particles in a solid can only vibrate on the spot, rather than being able to move from place to place.