Evaporation is the conversion of a liquid or solid to a gas. Energy in the form of heat causes evaporation. For example, evaporation occurs when water in an open pan in a warm room disappears, leaving a dry pan. Sometimes solids may change directly into vapor (a gaseous form) without first becoming a liquid. This process is called sublimation. The vaporization of dry ice (solid carbon dioxide) is an example of sublimation.
How evaporation occurs.
The molecules of all substances have a certain amount of kinetic energy (energy of motion). This energy is provided by heat from the surroundings, including other nearby molecules. The more energy molecules have, the faster they move, making it possible for them to break the bonds that hold them together. Evaporation occurs when the molecules of a substance have enough kinetic energy to escape from the substance’s surface as vapor. The escaping molecules absorb heat energy from the remaining molecules and cause the substance to become cooler.
Evaporation occurs more rapidly when temperature is increased. An increase in temperature increases the energy of a substance’s molecules, enabling them to escape at a faster rate. Different substances vary greatly in the speed at which they evaporate at a given temperature. For example, alcohol, ammonia, and the anesthetic diethyl ether evaporate quickly when poured onto an open surface at room temperature. Such substances are said to be volatile. Other substances, such as table salt (sodium chloride), evaporate extremely slowly, if at all, at room temperature. They are said to be nonvolatile.
A substance that is nonvolatile at one temperature may become volatile if it is heated. In this sense, the boiling point of a substance is a measure of its volatility. The lower the boiling point of a substance, the more easily it evaporates. See Boiling point.
The vapor pressure of a substance also helps determine how readily the substance evaporates. Vapor pressure is the pressure produced by vapor molecules escaping from the surface of a liquid or solid. The greater a substance’s vapor pressure, the faster the substance evaporates. As a substance gains heat, more and more molecules escape. Vapor pressure increases until the number of molecules escaping from the liquid or solid equals the number returning to it. When these numbers are equal, the amount of the liquid or solid does not change. For example, water evaporates slowly, if at all, when exposed to humid air because the vaporization occurring at the surface of the water is almost matched by the condensation of moisture in the air. But water evaporates rapidly in dry air because dry air has only a fraction of the maximum vapor pressure of water.
Uses of evaporation.
Evaporation plays an important part in the earth’s water cycle. Water evaporated from the earth by the heat of the sun later falls as precipitation. Because evaporating molecules absorb heat from their surroundings, evaporation functions as a cooling process. Air-conditioning and refrigeration systems contain evaporators that remove heat and moisture from the surrounding air and so reduce the temperature. The evaporation of perspiration cools the skin.
Evaporation provides a way to separate mixtures. In distillation, mixtures of liquids that differ in volatility (ease of evaporation) are heated to form a vapor that is then condensed. The resulting liquid contains a greater proportion of the more volatile substance than does the original mixture. Such products as alcoholic beverages and gasoline are manufactured by distillation.
Evaporation also serves as a means of concentrating nonvolatile substances. For example, the juice from sugar cane is heated until nearly all the liquid evaporates. This process produces a concentrated sugar solution from which sugar crystals form.
See also Cloud; Steam; Sublimation; Vapor.
