How diffusion spreads particles without a plan
Diffusion is the net spreading of particles from a region of higher concentration towards lower concentration through random motion. Individual particles do not know which way to go. More leave the crowded region than return from the less crowded region, producing an overall redistribution.
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Random motion can produce a directional result
Particles in liquids and gases are constantly moving. When one region contains more of a substance than another, that random movement can produce a net flow down the concentration gradient.
There are more particles available to leave the concentrated region than to return from the dilute region. Over time, the difference tends to decrease.
The overall trend does not mean every particle moves in the same direction. Particles still travel both ways, and random motion continues even after concentration becomes uniform.
- A concentration difference creates net diffusion.
- Mixing reduces that difference.
- Uniform concentration does not mean molecular motion has stopped.
In everyday liquids and air, currents often move material much faster than diffusion alone. A rapidly spreading drop in a stirred glass is therefore not a pure demonstration of diffusion.