The old adage “A watched pot never boils” may seem counterintuitive, but in fact it’s true. A boiling point table is a handy tool for determining the normal boiling points of various liquids at different temperatures, depending on atmospheric pressure. The tables also show how the boiling points of certain compounds change as their concentration changes.
Boiling points reflect the strength of intermolecular forces that keep molecules of a liquid clumped together rather than blasting them into the atmosphere as gases. The more attractive the forces are, the higher the boiling point will be.
Several methods are used to determine the boiling points of liquids under atmospheric and other conditions. These include equations, estimations, nomographs and on-line calculators.
A simple way to find a liquid’s boiling point is to use a nomograph, which is essentially a line graph that shows the known relationship between temperature and pressure. Start with one of the known values (a particular temperature or a known barometric pressure at sea level, for example) and connect it to a second value on the graph using a ruler. Then read the unknown value from the point where the two lines intersect to get a boiling temperature at that barometric pressure.
A more sophisticated method is to use the Clausius-Clapeyron equation, which provides a straight line on a graph relating the temperature and pressure of a given substance. The equation is based on the natural log of the pressure, which makes it easy to plot. The temperature is in kelvins and the pressure in mmHg, bars or psi, but these units cancel out, so it isn’t important which is used.