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Glossary - Vapour pressure

Vapour pressure, or equilibrium vapour pressure, is the pressure of a vapour in thermodynamic equilibrium with its condensed phases in a closed container.  All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.  The equilibrium vapour pressure is an indication of a liquid's evaporation rate -- it relates to the tendency of particles to escape from the liquid (or a solid).

A substance with a high vapour pressure at normal temperatures is often referred to as volatile.  The vapour pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation.  The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature at which the vapour pressure equals the ambient atmospheric pressure.  With any incremental increase in that temperature, the vapour pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance.  Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

   
 

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