# 6.2A: Overview of Boiling Point

The boiling point of a compound is the temperature where the liquid-gas phase change occurs. In more technical terms, it is when a liquid's vapor pressure equals its applied pressure (typically the atmospheric pressure). Boiling points are very sensitive to changes in applied pressure, so all boiling points should be reported with the measured pressure. A compound's "normal boiling point" refers to its boiling point at a pressure of $$760 \: \text{mm} \: \ce{Hg}$$.

A compound's boiling point is a physical constant just like melting point, and so can be used to support the identification of a compound. Unlike melting points however, boiling points are not generally used as a gauge of purity. Impure liquids do boil over a range of temperatures (similar to how melting points have breadth), but the temperature span does not correlate well to purity. Thus, measurement of a compound's boiling point is mainly used to support its identification.

An experimental boiling point is often compared to the literature boiling point, which are typically reported for 1 atmosphere of pressure. If a boiling point is determined at any pressure significantly different than 1 atmosphere, the pressure should be corrected. A general rule of thumb is that for pressures within $$10\%$$ of one atmosphere, a $$10 \: \text{mm} \: \ce{Hg}$$ drop in pressure will account for a $$0.3$$-$$0.5^\text{o} \text{C}$$ drop in boiling point.$$^4$$ Another rule of thumb is that for every halving of pressure, the boiling point drops by about $$10^\text{o} \text{C}$$.

$$^4$$This generality is based on tables in the Handbook of Chemistry and Physics, CRC Press, 84$$^\text{th}$$ edition, 2003-2004, 15-19.