What is diffusion? (GSCE: OCR, Edexcel, AQA)

Definition: diffusion is the spreading out of particles from an area of high concentration to an area of low concentration

The theory:

Diffusion, is the scientific description for the gradual movement of particles from an area of many particles (high concentration) to an area with only a few (low concentration).

In practice this theory can be illustrated in two ways.

In a gas: if someone sprays perfume in one corner of the room the smell spreads out and can be eventually smelt throughout the room.

In a liquid: if you put a drop of blue dye into a cup of water, you can see the dye spreading out until the whole cup is a blue colour.

It is based on 3 rules:

Particles move about randomly

This random movements results in them to spreading away from each other 

Eventually they are evenly distributed

Diffusion in a cell (a common exam question)

All cells need to exchange materials with their environment, whether this is to take in things they need like glucose and oxygen for respiration or get rid out waste products like carbon dioxide and urea.

The movement of these substance happens because they are following a concentration gradient, oxygen for example is moving from an area of high concentration outside the cell to low concentration inside the cell (as it has been used up for respiration). It is important to note that the oxygen particles will move both ways across the membrane as they are moving randomly just most of them will move into the cell.  

To allow the movement of these small molecules cells need to have small holes in the cell membrane to allow these molecules in and out. These holes in the membrane are very small so large molecules like starch and proteins are not lost from the cell. 

Further reading:

What factors do you think affect the rate of diffusion?

Osmosis is a special type of diffusion, how would you describe osmosis?