1.4 describe and explain experiments to investigate the small size of particles and their movement including: i dilution of coloured solutions ii diffusion experiments

Potassium manganate (VII) and water can be used for both of these experiments.
Diffusion experiments

Potassium manganate (VII) works very well for diffusion experiments as its bright purple.

Take a beaker of water and add some potassium manganate (VII) to the bottom. OBSERVATION: the purple colour will slowly spread out to fill the beaker. 

This is the particles of potassium manganate (VII) diffusing out among the particles of water. It's the random motion of particles in a liquid that cause the potassium manganate (VII) to eventually be evenly spread out throughout the water.


Dilution of coloured substances

If you were to add even more water to the final purple solution, the potassium manganate (VII) particles would spread out even further and the solution would be less purple.