# How is the boiling point relate to vapor pressure?

Jun 11, 2018

An old question....and an old problem....

#### Explanation:

The boiling point of a liquid are those conditions of temperature and pressure, when the VAPOUR pressure of a liquid is equal to the ambient pressure, and bubbles of vapour form directly in the liquid. The $\text{normal boiling point}$ is specified when the AMBIENT pressure, and ALSO the vapour pressure of the BOILING LIQUID is $\text{ONE ATMOSPHERE}$.

And this underlies the principle of vacuum distillation. A vacuum pump can SUBSTANTIALLY reduce the ambient pressure...and at an accessible temperature, the vapour pressure of the liquid may be $1 \cdot m m \cdot H g$ or even lower...and so the liquid comes across...

AS a first approx. we can sometimes use a vapour pressure nomograph to approximate the boiling point of a liquid AT REDUCED pressure...

I know there is such a nomograph on the front of the $\text{Chemists Companion}$...and also instructions on how to use it....