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Diffusion

Particles spread because their thermal motion constantly changes where they are.

What happens

Diffusion is the net spreading of particles from regions of higher concentration toward regions of lower concentration, even though individual particles move in many directions.

The process is usually faster over short distances and at higher temperatures. In liquids and gases, molecular motion can gradually erase a concentration difference.

What changes the process

A drop of dye spreading through still water is a useful picture, but real mixing can also involve convection. Diffusion itself does not require a bulk flow of the fluid.

Why it matters

Diffusion matters in cells, membranes, gases, alloys and countless chemical processes. At microscopic scales it is one of the basic ways matter redistributes itself.

Go one step further

A useful way to understand this phenomenon is to separate the driving force from the visible result. Temperature, pressure, concentration, surface area and the nature of the materials can each change the rate or the final state. In real systems, several of these factors often act at the same time.

That is why the same phenomenon can look different in a laboratory, in the kitchen or outdoors. The underlying chemistry or physics stays the same, but the conditions change the balance.