1.3- understand how the results of experiments involving the dilution of coloured solutions and diffusion of gases can be explained

Diffusion is the spreading out of particles in a gas or liquid. The net movement of particles from an area of which they are of high concentration (many in a certain volume) to an area of low concentration. 

Diffusion in gases experiment:

When ammonia gas (NH₃) and hydrogen chloride (HCl) mix, they react together to form a white solid called ammonium chloride (NH₄Cl). Pieces of cotton wool are soaked in concentrated ammonia solution (a source of ammonia gas) and concentrated hydrochloric acid (a source of hydrogen chloride gas). These are placed at the end of a long glass tube with rubber bungs at the ends, in order to stop the poisonous gases from escaping.

The ammonia and hydrogen chloride particles diffuse along the tube, and a white ring of solid ammonium chloride forms where they meet. The white ring of ammonium chloride takes time to form (because the particles move freely and collide with air particles), and it appears closer to the hydrogen chloride side. This is because ammonia particles are lighter than hydrogen chloride particles and therefore travel faster.

Diffusion in liquids:

Diffusion in a liquid can be extremely slow through a liquid if the liquid is still. This is because liquid particles move more slowly than gas particles, and because there are only small gaps (less space) between the particles of liquid for the other particles to diffuse into, for example potassium manganate (VII) in water.

Varying the dilution of the solution, for example adding more water causes the potassium manganate particles to spread further apart from one another, making the solution less purple.