Vapor pressure

Definition

Vapor pressure in plain English

Any liquid is secretly losing a few of its fastest molecules to the air all the time — that's evaporation. Vapor pressure is a measure of how eager a liquid is to do this: it's the push exerted by those escaped molecules once, in a closed container, they're leaving and returning at the same rate (a balance called equilibrium). Think of a capped water bottle on a hot day — the air above the water fills with vapor that presses on everything, and that pressure is higher when the liquid is hot or naturally evaporates easily (like nail polish remover vs. water). When a liquid's vapor pressure climbs all the way up to the surrounding air pressure, it boils.

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