Open a perfume bottle in one corner and the whole room smells it — why, and what's fastest?
Gas molecules are in constant rapid random motion, so they diffuse and spread until evenly mixed, even against still air.
Gas molecules are in constant rapid random motion, so they diffuse and spread until evenly mixed, even against still air. By Graham's law, lighter molecules diffuse faster (rate ∝ 1/√molar mass), so lighter, more volatile scent molecules reach you sooner. The same principle lets us separate gases of different masses by their diffusion rates.
Key point
Gases diffuse due to constant random motion; lighter molecules diffuse faster (Graham's law).
Common mistake
thinking the smell needs a breeze or draft to travel.
Memory tip
random molecular motion spreads a gas; lighter = faster diffusion.
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