Diffusion

When Molecules are left alone, they will want to want to move randomly, the proper term is that the particles will move from a place with high concentration to a place with a lower concentration. 

This can happen in gasses and liquids, for example when you spray perfume it diffuses throughout the air so it is very fragrant, 
or smoke comes out of the car the gasses from the car's exhaust disperse into the air, and if you add food colouring to water.

Diffusion can also occur through materials like cell membranes, usually only really small things can fit through cell membranes, things like water, glucose and amino acids can pass through cell membranes. 

There are three types of diffusion:

Firstly is the Concentration gradient, the larger the concentration gradient the higher the rate of diffusion if we were hypothetically to have two boxes, one on top and one on the bottom, and we put more particles inside the top box and put very little in the bottom one. 

The top one would have a harder time passing through the cell membrane compared to the bottom one since they have a different amount of particles one having much more than others. 

The second is Temperature, the reason is that particles will move faster due to the heat and that the heat gives the particles more energy, a higher temperature helps with the rate of diffusion. 

Lastly is a Surface area, which the higher surface area the higher rate of diffusion,