# What is vapor pressure?

Jun 21, 2018

By way of example, chemists and physicists habitually use ${P}_{\text{saturated vapour pressure}}$...where ${P}_{\text{SVP}}$ is the vapour pressure exerted by liquid water. At $100$ ""^@C, ${P}_{\text{SVP}} = 1 \cdot a t m$. Why?
Well, because this is the normal boiling point of water: i.e. the conditions of pressure (i.e. here $1 \cdot a t m$) and temperature, here $100$ ""^@C, at which the VAPOUR PRESSURE of the liquid is ONE ATMOSPHERE...and bubbles of vapour form directly in the liquid. As an undergraduate you should commit this definition, or your text definition, to memory...
At lower temperatures, water exerts a much lower vapour pressure...but these should often be used in calculations...especially when a gas is collected by water displacement. Tables of $\text{saturated vapour pressure}$ are available.