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Particles in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher concentration, they will therefore move from this region to where they are in a lower concentration. This is called diffusion.

It is important to remember that the particles:

• will move in both directions, but there will be a net movement from high to low concentration
• will end up evenly spread throughout the liquid or gas but will continue to move

Diffusion of coloured chemicals

If a crystal of a coloured chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles. The process is:

1. the potassium manganate(VII) has dissolved
2. the potassium manganate(VII) is the solute
3. the water is the solvent
4. the mixture that results is the solution

The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

Some examples of diffusion in biological systems

Some substances move into and out of living cells by diffusion.

In a leaf:

In the lungs:

In liver cells: