Vapor Pressure and Boiling Point:

A liquid boils when the vapor pressure equals the external pressure acting on the surface of the liquid − in other words, boiling occurs when the vapor pressure of a liquid equals atmospheric pressure. At this point bubbles of vapor are able to form anywhere within the liquid.

The temperature at which a given liquid boils increases with increasing external pressure.

The boiling point of a liquid at 1 atm pressure is called its normal boiling point.
 

Determining Boiling Point