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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 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] This happens through otherwise random movement. Diffusion usually happens in a gas although it can happen 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 spreading from the front of the classroom to the back of the room.
  • Fumes of perfume rises from the bottle when the top is removed.
  • Food coloring dropped on the beaker causing to spread out.
  • Adding of sugar in water

One of the most important things about diffusion is that molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. Therefore, this is not the most effective way of moving in and out of cells, however there are many examples where diffusion takes place but is adapted so it is efficient. 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.

Diffusion can be considered to arise from probability alone - 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. Diffusion is also connected to Entropy.