Diffusion Definition in Chemistry

Chemical diffusion in three beakers
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Diffusion is the movement of a fluid from an area of higher concentration to an area of lower concentration. Diffusion is a result of the kinetic properties of particles of matter. The particles will mix until they are evenly distributed. Diffusion may also be thought of as the movement of particles down a concentration gradient.

The term "diffusion" comes from the Latin word diffundere, which means "to spread out."

Diffusion Examples

  • H2S(g) in a test tube will slowly diffuse into the air of a lab until equilibrium is reached.
  • Food coloring in water diffuses until it's evenly distributed throughout the liquid.
  • Perfume diffuses throughout an entire room.
  • Adding a dot of dye to gelatin is a good example. The color will slowly diffuse throughout the gel.

Note, however, most of the common examples of diffusion also illustrate other mass transport processes. For example, when perfume is smelled across a room, air currents or convection are more of a factor than diffusion. Convection also plays a large role in the dispersion of food coloring in water.

How Diffusion Works

In diffusion, particles move down a concentration gradient. Diffusion is different from other transport processes in that it results in mixing without bulk matter flow. How it works is that molecules in motion from thermal energy randomly move about. Over time, this "random walk" leads to uniform distribution of different particles. In reality, atoms and molecules only appear to move randomly. Most of their motion results from collisions with other particles.

Increasing temperature or pressure increases the rate of diffusion.