What is diffusion?

  • Diffusion is the movement of a substance from an area of high concentration to an area of low concentration.

  • Diffusion happens in liquids and gases because their particles move randomly from place to place.

  • Diffusion is an important process for living things; it is how substances move in and out of cells.

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

What is diffusion?

You walk into a coffee shop and immediately smell coffee. Why?

One reason is convection. Moving currents of air carry coffee smell particles all around the coffee shop.

But even without convection, the smell would still reach you eventually - not by convection, but by diffusion. In diffusion, coffee particles move from the coffee machine (an area of high concentration) to the rest of the coffee shop (an area of low concentration).

Making a cup of coffee also involves the diffusion of coffee particles through hot water:

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

What causes diffusion?

In gases and liquids, particles move randomly from place to place. The particles collide with each other or with their container. This makes them change direction. Eventually, the particles are spread through the whole container.

Diffusion happens on its own, without stirring, shaking or wafting.

Why is diffusion useful?

In living things, substances move in and out of cells by diffusion. For example:

  • Respiration produces waste carbon dioxide, causing the amount of carbon dioxide to increase in the cell. Eventually, the carbon dioxide concentration in the cell is higher than that in the surrounding blood. The carbon dioxide then diffuses out through the cell membrane and into the blood.

  • Water diffuses into plants through their root hair cells. The water moves from an area of high concentration (in the soil) to an area of lower concentration (in the root hair cell). This is because root hair cells are partially permeable. The diffusion of water like this, is called osmosis.

Diffusion quiz

Do you now feel knowledgeable about diffusion? Have a go at the quiz!

Where next?

Discover more about this topic from around Bitesize.

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