Boiling, the process of changing a liquid into a vapor by the application of heat. During this change the liquid remains at a constant temperature known as the boiling point. At the boiling point the atoms or molecules of the liquid gain enough energy to change the liquid into vapor both within the liquid and at its surface. Thus boiling is a rapid evaporation.

The boiling points of liquids vary widely. At normal atmospheric pressure, for example, liquid oxygen boils at -297.4 F. (-183 C.), ether at 94.1 F. (34.5 C.), water at 212 F. (100 C.), and mercury at 674.4 F. (356.9 C.). Because of this variation in boiling points, two or more liquids can often be separated by a process called fractional distillation.

Pressure affects boiling points. An increase of pressure retards the formation of vapor so that greater heating of the liquid becomes necessary to bring about boiling. A decrease of pressure, such as that which accompanies an increase in altitude, lowers the boiling point. For example, the boiling point of water drops about 1.8 F. with each increase of 1,000 feet (about 1 C. per 300 m) in altitude. The following table shows the approximate boiling point of pure water at various altitudes:

Place Altitude Boiling Point
(feet) (meters) (F.) (C.)
Dead Sea -1,312 -400 215 101.7
Sea level 0 0 212 100.0
Chicago, Illinois 580 177 211 99.4
Denver, Colorado 5,280 1,609 202 94.4
Pikes Peak 14,110 4,301 187 86.1
Mount McKinley 20,320 6,194 176 80.0
Mount Everest 29,028 8,848 157 69.4

Dissolving a solid in a liquid raises the liquid's boiling point. In general, the more of a given solid is added, the more the boiling point is raised, but the increase is rarely more than a few degrees.