Diffusion

Diffusion is the movement of particles from areas of higher concentration to areas of lower concentration. 

This happens because particles move about randomly, so over time will spread out, moving from where there are lots of them (higher concentration), to where there are fewer of them (lower concentration).

Diffusion of bromine gas in air

Bromine gas is brown, so we can easily see it spreading out by looking at where the brown colour spreads.

This diagram shows two test tubes that are initially separated by a glass plate. Once the glass plate is removed, the bromine gas diffuses from the bottom tube to the top tube, until in the end it is evenly distributed. 

Diffusion of potassium manganate (VII) in water

Potassium manganate(VII) is a bright purple colour. If you place a lump of it in water it will initially sink to the bottom. As some of the potassium manganate(VII) particles start to diffuse throughout the beaker of water, the colour will slowly spread out.

Eventually, the particles will have diffused evenly throughout the beaker and it will all be the same colour. 

Diffusion and reaction of ammonia and hydrogen chloride:

1. This experiment uses one piece of cotton wool soaked in aqueous hydrogen chloride (HCl), and another piece soaked in aqueous ammonia (NH₃). These pieces are placed at either end of a sealed glass tube.
2. As the NH₃ and HCl evaporate into gas, they begin to diffuse along the glass tube towards each other.
3. When these gases meet, they'll react together to form ammonium chloride (NH₄Cl), which will be visible as a white ring around the tube.
4. Importantly, the white ring will be closer to the end with the HCl cotton wool. This is because NH₃ particles are smaller than HCl particles and so diffuse more quickly. This means the NH₃ particles will travel further before the NH₃ and HCl react.