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A diagram of diffusion happening. The first diagram shows particles in a liquid. The second shows the same liquid a few seconds later after the particles have spread out.

Diffusion is the chemical & biological process when molecules from a material move from an area of high concentration (where there are lots of molecules) to an area of low concentration (where there are fewer molecules).[1]

Diffusion usually happens in a gas or in a liquid. It is possible to see diffusion happening when two liquids are mixed in a transparent container. It describes the constant movement of particles in all liquids and gases.These particles move in all directions bumping into each other. Diffusion can only work with gases and liquids. Here are some examples of diffusion:

  • A sugar cube is left in a beaker of water for a while.
  • The smell of ammonia spreads from the front of the classroom to the back of the room.
  • Fumes of perfume rise from the bottle when the top is removed.
  • Food coloring dropped on the beaker spreads out.

Molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. For example, there is more oxygen in a lung than there is oxygen in the blood so oxygen molecules will tend to move into the blood. Similarly, there is more carbon dioxide molecules in the blood than in the lung so carbon dioxide molecules will tend to move into the lung.

Areas of high density are, due to the random movement of fluid molecules, likely to spread out within their boundary until they can do so no longer.

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