# How do vapour pressures of substances like dry ice, and water, and mercury, relate to boiling point?

As always, the boiling point of a substance is the temperature at which the vapour pressure 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 thus also the vapour pressure of the BOILING LIQUID, is $\text{1 atmosphere}$.