Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Boiling Point

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.