Evaporation

III

Dynamic Equilibrium

Dynamic equilibrium occurs when the rate that particles leave a liquid in a closed container equals the rate that particles reenter the liquid. When liquid evaporates in a closed container, gas molecules collect above the liquid as vapor. As these gas molecules fly around inside the container, they hit the container walls and the liquid’s surface. Gas molecules that collide with the liquid’s surface lose kinetic energy by transferring it to the molecules in the liquid. If the gas molecules lose enough energy, they are reabsorbed by the liquid. This process of changing from a gas to liquid is called condensation.

When a liquid first begins evaporating in a closed container, the rate of evaporation typically exceeds the rate of condensation. As evaporation proceeds and more gas molecules collect above the liquid, higher numbers of gas molecules strike the liquid’s surface, speeding the rate of condensation. In a closed container, the liquid and vapor reach a state of dynamic equilibrium when the rate of evaporation of molecules equals the rate of condensation of molecules.

IV

Role and Uses of Evaporation

Evaporation is an important part of the earth’s water cycle, the continual movement of fresh water between the earth’s surface and its atmosphere. Evaporation also plays a key role in the function of plants and animals.

A

Earth’s Water Cycle

Evaporation occurs in the earth’s water cycle when energy from the sun causes water to heat up and evaporate from the earth’s surface. The water rises into the atmosphere, condenses in clouds, and falls back to the earth as precipitation. About 505,000 cubic km (about 121,000 cubic mi) of water evaporates from the earth’s surface each year—about 86 percent of which evaporates from the oceans. After forming, water vapor drifts over land masses where the vapor cools, slowing the kinetic energy of the water molecules until attractive intermolecular forces cause the molecules to condense into rain, sleet, snow, or hail. This precipitation replenishes streams, rivers, lakes, groundwater reservoirs, and other freshwater supplies.

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B

Secretion and Evaporation in Living Organisms

Many living organisms depend on evaporation to regulate their internal body temperature. For example, exercising causes a person’s muscles to contract, producing heat. If this heat is not transferred from the body to the surrounding environment, the individual’s internal temperature can rise to life-threatening levels. During vigorous activity, humans excrete perspiration through pores in the skin. These water secretions absorb body heat and use this energy to evaporate into the environment, carrying the heat energy with them.

Plants use evaporation to transport water up from the soil into the leaves. When leaves excrete water through surface openings (called stomata), the water molecules evaporate, pulling (by attractive intermolecular forces) other water molecules up through the plant vessels behind them. Evaporation of water from leaf surfaces is called transpiration. Through transpiration, an average-sized maple tree can lose more than 200 L (200 kg) of water per hour on a summer day.

C

Chemical and Industrial Processes

Evaporation is used to separate and purify substances in many chemical and industrial processes. One of the most important industrial applications is separation of crude petroleum into gasoline, kerosene, and gas oil. In this process, called fractional distillation, crude petroleum is boiled, and the evaporated materials are cooled until they condense. These condensed vapors contain higher percentages of the more volatile (most easily vaporized) compounds necessary for producing gasoline, kerosene, and gas oil. As a result, repeating the cycle of evaporation and condensation can isolate these compounds.

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