vapour pressure

Leave a glass of water in a sealed jar and some molecules keep escaping the surface into the air above, while others fall back in. Vapour pressure is the push exerted by that escaped vapour once escaping and returning are perfectly balanced. A liquid that evaporates eagerly, like alcohol or a perfume, has a high vapour pressure; one that barely evaporates has a low one.

Vapour pressure rises steeply with temperature, and a liquid boils at the temperature where its vapour pressure equals the surrounding atmospheric pressure. Solids have a vapour pressure too, usually tiny but enough to drive the slow loss of subliming materials like camphor or menthol. Dissolving a non-volatile solute always lowers a solvent's vapour pressure, which is one of the colligative properties.

In pharmacy vapour pressure shapes several practical concerns. It governs the propellants in pressurised inhalers, which are liquids chosen for a vapour pressure that atomises the dose when the valve opens; it explains evaporation losses from creams and solutions on storage; and it sets limits on how volatile flavours, solvents and anaesthetics behave during manufacture and use.