Evaporation

INTRODUCTION
Evaporation, gradual change of state from liquid to gas that occurs at a liquid’s surface. Examples of evaporation include rainwater evaporating from warm pavement after a thunderstorm and wet paint drying as solvents in the paint evaporate. Fingernail polish also hardens as acetone (CH3COCH3) evaporates from the liquid polish.
A liquid is made up of atoms or molecules (bound groups of atoms) that are in constant motion, traveling at different speeds. The average speed of these particles depends only on the liquid’s temperature. If the particles have enough energy, fast-moving particles striking other particles near the liquid’s surface may impart enough speed, and therefore enough kinetic energy (energy of motion) to cause the surface particle to leave the liquid and become gas atoms or molecules. The particle’s kinetic energy is directly related to its speed.
As particles with the most kinetic energy evaporate, the average kinetic energy of the remaining liquid decreases. In a similar way, if a basketball team loses its tallest players, the average height of the team is diminished. Because a liquid’s temperature is directly related to the average kinetic energy of its molecules, the liquid cools as it evaporates (see Heat Transfer).
RATE OF EVAPORATION
A liquid’s surface area and temperature affect its rate of evaporation. Evaporation rates also depend on the type of liquid, since liquids made up of different molecules differ in the amount of attraction that exists between the molecules .
Surface Area and Temperature
Because molecules or atoms evaporate from a liquid’s surface, a larger surface area allows more molecules or atoms to leave the liquid, and evaporation occurs more quickly. For example, the same amount of water will evaporate faster if spilled on a table than if it is left in a cup.
Higher temperatures also increase the rate of evaporation. At higher temperatures, molecules or atoms have a higher average speed, and more particles are able to break free of the liquid’s surface. For example, a wet street will dry faster in the hot sun than in the shade.
Intermolecular Forces
Most liquids are made up of molecules, and the levels of mutual attraction among different molecules help explain why some liquids evaporate faster than others. Attractions between molecules arise because molecules typically have regions that carry a slight negative charge, and other regions that carry a slight positive charge. These regions of electric charge are created because some atoms in the molecule are often more electronegative (electron-attracting) than others. The oxygen atom in a water (H2O) molecule is more electronegative than the hydrogen atoms, for example, enabling the oxygen atom to pull electrons away from both hydrogen atoms. As a result, the oxygen atom in the water molecule carries a partial negative charge, while the hydrogen atoms carry a partial positive charge. Water molecules share a mutual attraction—positively charged hydrogen atoms in one water molecule attract negatively charged oxygen atoms in nearby water molecules.
Intermolecular attractions affect the rate of evaporation of a liquid because strong intermolecular attractions hold the molecules in a liquid together more tightly. As a result, liquids with strong intermolecular attractions evaporate more slowly than liquids with weak intermolecular attractions. For example, because water molecules have stronger mutual attractions than gasoline molecules (the electric charges are more evenly distributed in gasoline molecules), gasoline evaporates more quickly than water.
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.
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.
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.
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.
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.