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Key points

  • Diffusion is the movement of a substance from an area of to an area of .
  • Diffusion occurs in liquids and gases when their collide randomly and spread out.
  • Diffusion is an important process for living things - it is how move in and out of cells.

If someone sprays deodorant from the other side of the room, how does the smell reach your nose?

Through a process called diffusion. The deodorant particles bump into air particles and are spread throughout the room. We smell the deodorant particles that reach our noses.

Video - diffusion

Watch this video to find out how the movement of particles in gases and liquids causes .

While you are watching, think about why diffusion is different in solids, liquids and gases.

How does the movement of particles and gases cause diffusion?

Why does diffusion happen faster in gases than in liquids?

The particles in gases have more energy and are more widely spaced out than the particles in a liquid. This means that gas particles will bump into each other harder and more often. Which makes gas particles spread out more quickly than liquid particles can.

What is diffusion?

Diffusion is the process by which particles of one substance spread out through the particles of another substance. Diffusion is how smells spread out through the air and how concentrated liquids spread out when placed in water.

Diffusion happens on its own when the particles spread out from an area of high , where there are many of them, to areas of low concentration where there are fewer of them.

is not the same as diffusion. Dissolving is the term used to describe when a breaks up and mixes thoroughly with a solvent to produce a solution.


Click through the slideshow below to see an example of diffusion.

Red, orange and purple sweets on a plate.

In the video, the coloured sweets were arranged on a plate, and then some water was added. What happened next?

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How does diffusion happen?

Diffusion occurs in gases like air and liquids like water because their particles can move around and collide with each other randomly.

For example, if you mix two drinks, the liquids diffuse into each other. Blackcurrant squash has a high concentration level. When the squash is mixed with water, it becomes less concentrated and is diluted.

Watch the video below to find out how making a cup of coffee involves diffusion.


Find out how diffusion is involved in the coffee-making process


1. Highly concentrated coffee molecules enter the cup of hot water.
2. Coffee molecules begin to spread out in between the water molecules.
3. Coffee molecules are now in a lower concentration than they started in.

Why doesn’t diffusion happen in solids?

Diffusion doesn’t happen in solids because the particles in a solid cannot move around, instead they vibrate about a fixed position.

What affects diffusion?

Two factors affect diffusion:

  • The type of substance - substances diffuse more quickly through gases than through liquids. This is because the particles in a gas are more spread out and move faster than the particles in a liquid.

  • Temperature - the hotter the gas or liquid, the faster diffusion happens. This is because the particles have more energy and move faster at higher temperatures.

Working scientifically

Working safely in the lab

An investigation into diffusion could be conducted by adding \(7cm^3\) copper sulfate solution to \(50cm^3\) cold water and also to \(50cm^3\) hot water.

What safety precautions would you need to take before carrying out this investigation?

Make sure that you know the laboratory safety rules and take precautions to reduce risks.

Make sure you have researched any potentially hazardous chemicals. Hazard symbols give information about chemicals and materials.

You might need to find out more about copper sulfate from a textbook, data book or reliable website.

Find out more about working safely in science.

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