Boiling Point
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Boiling Point, temperature at which the vapor pressure of a liquid slightly exceeds the pressure of the atmosphere above the liquid. At temperatures below the boiling point (b.p.), evaporation takes place only from the surface of the liquid; during boiling, vapor forms within the body of the liquid; and as the vapor bubbles rise through the liquid, they cause the turbulence and seething associated with boiling. If the liquid is a single substance or an azeotropic solution (a mixture that has a constant b.p.), it will continue to boil as heat is added without any rise in temperature; that is, boiling occurs at constant temperature regardless of the amount of heat applied to the liquid.
When the pressure on a liquid is increased, the b.p. goes up. Water at 1 atmosphere pressure (760 torr, or about 14.7 lb/sq in) boils at 100° C (212° F), but when the pressure is 218 atmospheres (165,000 torr, or 3200 lb/sq in), the b.p. reaches its maximum, 374° C (705° F). Above this temperature (the critical temperature of water), liquid water is identical to saturated steam. See Pressure.
If the pressure on a liquid is reduced, the b.p. is lowered. At higher elevations, where air pressure is less, water boils below 100° C. In Denver, Colorado, which is 1.6 km (1 mi) above sea level, the b.p. of water averages 94° C (201° F). When the pressure on a sample of water falls to 4.55 torr (0.088 lb/sq in), boiling occurs at 0° C (32° F), which is the normal freezing point.
Boiling points cover a wide temperature range. The lowest b.p. is that of helium, -268.9° C (-452° F). The highest is probably that of tungsten, about 5900° C (10,650° F). The boiling points given in the separate articles on the various elements and compounds apply at normal pressure unless specifically stated otherwise.
When the pressure on a liquid is increased, the b.p. goes up. Water at 1 atmosphere pressure (760 torr, or about 14.7 lb/sq in) boils at 100° C (212° F), but when the pressure is 218 atmospheres (165,000 torr, or 3200 lb/sq in), the b.p. reaches its maximum, 374° C (705° F). Above this temperature (the critical temperature of water), liquid water is identical to saturated steam. See Pressure.
If the pressure on a liquid is reduced, the b.p. is lowered. At higher elevations, where air pressure is less, water boils below 100° C. In Denver, Colorado, which is 1.6 km (1 mi) above sea level, the b.p. of water averages 94° C (201° F). When the pressure on a sample of water falls to 4.55 torr (0.088 lb/sq in), boiling occurs at 0° C (32° F), which is the normal freezing point.
Boiling points cover a wide temperature range. The lowest b.p. is that of helium, -268.9° C (-452° F). The highest is probably that of tungsten, about 5900° C (10,650° F). The boiling points given in the separate articles on the various elements and compounds apply at normal pressure unless specifically stated otherwise.
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