Diffusion is the spreading out of particles, moving from a high concentration to a low concentration.

For example, look at the diagram below, in which a drop of pink ink is dropped into a beaker of water.

Due to the motion of molecules in water, the ink particles slowly mix with the water. The overall process is diffusion between the pink ink particles and the water particles.

Ink Diffusion Steps

1. When the ink is dropped in the beaker, the ink particles are bundled together at the top right. This means that there is a high concentration of ink at the top right of the beaker (there are a lot of ink particles per volume of water).

2. Due to diffusion, the ink in the second beaker is travelling from an area of high concentration to an area of low concentration. This is why the ink has spread out, but it is still less concentrated at the bottom and left of the beaker.

3. The ink in the third beaker has completely diffused throughout the water, which means that diffusion has occurred.

For diffusion to take place, there needs to be a difference in concentration, which is called a concentration gradient.

In the ink example, the ink particles spread throughout the beaker to even out areas where there were initially uneven concentrations of ink relative to water. As long as a concentration gradient exists, diffusion will continue to take place.

Diffusion in gases

This process tends to happen in gases, allowing substances to mix together. As gas particles are very spread out, they can move and vibrate quickly.

For example, a spray of perfume in one corner of the room will quickly disperse throughout the entire room. This is the result of the perfume particles diffusing through the air until the fragrance becomes evenly distributed.

Diffusion in Root Hair Cells

In plants, root hair cells have a lower concentration of water inside the cell than in the soil (where there is a high concentration of water). Water molecules will move by diffusion, from the high concentration outside the cell, to the low concentration inside the cell.

Factors Affecting the Rate of Diffusion

FactorImpact on the Rate of Diffusion
TemperatureAs the temperature increases, the rate of diffusion also increases. This is because the particles have more energy and therefore move faster.
Size of particlesThe smaller the particles, the faster the rate of diffusion
State of MatterDiffusion doesn’t occur in solids, and it is faster in gases than in liquids.