Kids Research Express

. A. Boiling Point The boiling point of a liquid is the temperature at which molecules escape from the liquid and enter the gaseous state. Heat causes a liquid to boil by adding energy to the liquid’s molecules. As the molecules gain energy, they move about more quickly and range farther from each other. When the molecules are far enough apart, intermolecular forces are too weak to pull them back together, so the molecules form a vapor. Boiling starts when bubbles of vapor form within the liquid. These bubbles rise to the top of the liquid and release the gaseous molecules to the atmosphere above the liquid’s surface. It takes 2,260 Joules (540 calories) of heat energy to evaporate 1 gram of water at 100° C (212° F) at sea level. At the boiling point, the vapor pressure of a liquid must equal the pressure of the atmosphere above the liquid. For a liquid boiling in an open container, the atmosphere above the liquid is simply Earth’s atmosphere. The pressure in the bubbles of vapor must

. 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