evaporation Evaporation is the process by which a liquid is transformed into a gas. The most common example in the natural world is the evaporation of water from the oceans, lakes, and rivers into the atmosphere. The main difference between a liquid and a gas is that in a gas the molecules are free to move anywhere. Each molecule moves until it collides with another molecule, after which it moves off in another direction. In a liquid the forces of attraction between the molecules bind them together. The binding is not as strong as in a solid, such as ice, but it is strong enough to keep almost all the molecules together as a liquid. The molecules do not all have the same energy; some move faster than others.

A few of the molecules in a liquid have sufficient energy to escape from the binding forces that keep them joined to the other liquid molecules. When these molecules leave the liquid surface they have entered the gaseous state, and are then free to move anywhere. On the other hand, some molecules in the gas, while moving about randomly, will collide with a liquid surface. When this happens, they will become bound to the liquid by the attractive forces, and these molecules will have changed from the gaseous to the liquid state. The net effect of these two processes is evaporation if more molecules are leaving the liquid state to become gas than the other way round. If more molecules are leaving the gas and becoming liquid then the process is condensation.

A state of saturation is reached when the molecules are leaving the liquid state at the same rate as molecules are joining the liquid from the gas. Once the gas has become saturated, the only way to increase the number of molecules leaving the liquid is to supply more energy to increase the energy of the molecules. This can be done by heating the liquid, producing thermal energy. (Heat energy which is used to provide energy for evaporation rather than to increase the temperature is called the latent heat of evaporation.) After heating, a new equilibrium level can be reached in which the gas is saturated; that is, the rates at which molecules leave and rejoin the liquid are the same. However, the number of molecules in the gaseous state is now greater than before heating because they have more energy and more molecules have been able to escape the attractive bonds of the liquid. This shows that the state of saturation is temperature-dependent. At higher temperatures gases are capable of supporting more of the molecules from the liquid. This explains why condensation occurs, for example, on windows at night, as the temperature drops. Air, which initially may not have been saturated with water, cools and eventually reaches a temperature at which it is saturated with water. If the air cools below this point it is supersaturated with water, and condensation occurs to restore the balance.

Evaporation and condensation are major factors in the hydrological cycle. Although only about 0.001 per cent of all the water on Earth is in the atmosphere, there is a never-ending cycle of evaporation from the surface, condensation into cloud droplets, and eventually the fall of raindrops and snowflakes returning the water to the surface. Averaged over the entire globe, the annual amount of precipitation would be about a metre deep. By comparison, the total depth of all the atmospheric water vapour at any one time is equivalent to a liquid depth of about 25 mm. To produce the required amount of rain in one year, water must be recycled round the hydrological cycle approximately every nine days.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co., New York.
McIlveen, J. F. R. (1986) Basic meteorology. Van Nostrand Reinhold, New York.

Evaporation

Evaporation is the name given to the process in which a liquid is converted to the gaseous state. Everyone is familiar with the process of evaporation. Suppose that you spill a teaspoon of water on the kitchen table. If you come back a few hours later, the water will have disappeared. It has changed from liquid water into water vapor, or evaporated.

Molecular explanation

Evaporation occurs because all molecules of all substances are constantly in motion. Consider the molecules that make up a teaspoon of water, for example. Those molecules are constantly in motion, flying back and forth within the water, sometimes colliding with each other. When collisions occur, some molecules gain energy from other molecules.

Those changes make little difference for molecules deep within the water. But for molecules at the surface of the water, the situation is different. Molecules at the surface that pick up energy from other molecules begin to travel faster. Eventually, they may be able to travel fast enough to escape from the surface of the water or to evaporate from the water.

This process continues as long as water molecules remain. Molecules that were once inside the water eventually work their way to the surface. When they pick up enough energy by colliding with other water molecules, they too escape. Eventually, no water molecules remain. The liquid has completely evaporated.

The remaining liquid

This description explains an interesting fact about an evaporating liquid: its temperature decreases as evaporation occurs. Remember that surface molecules escape from the liquid as they pick up energy from other molecules. The molecules left behind, therefore, have less energy than they had before the collisions. Since they have less energy, they also have a lower temperature.

The human body uses this principle to remain cool. On a warm day, we perspire (sweat). Sweat evaporates from the skin, taking body heat with it. As a result, the body is cooled.

Commercial applications

Evaporation is an important commercial process by which liquids are removed from solids. In many instances, a product is formed as the result of a chemical reaction that takes place in water. One way to obtain the final product is to simply allow the water to evaporate leaving the solid product behind.

[See also Matter, states of ]

Evaporation

Evaporation is a geologic process that concentrates the ion solute residues in the ocean basins. At a fundamental level, evaporation is the transition of the molecule of a liquid from the liquid state to the gaseous state by diffusion from the surface of the liquid.

Driven by solar energy , the only significant loss of water from the ocean basin occurs via evaporation. As the ocean surface and atmospheric interface is small compared to the total volume of the ocean, estimates of the time a particular molecule remains in the liquid phase range in the order of thousands to tens of thousands of years before once again entering the atmosphere as part of the hydrologic cycle .

Because solutes (e.g., dissolved salts) from weathering and erosion are not as volatile (i.e., as easy to move into the gas or vapor phase as the water molecules, evaporation plays a significant role in the formation of many geologic features (e.g., Great Salt Lake, Dead Sea, etc.).

Evaporation is usually also responsible for the majority of the loss of water from precipitation and results in a high cycling of water molecules during the hydrologic cycle.

Evaporation may be driven by solar energy or be a directed process used to concentrate an aqueous solution of nonvolatile solutes and a volatile solvent. In evaporation, a portion of the solvent is vaporized or boiled away, leaving a thick liquid or solid precipitate as the final product. The vapor is condensed to recover the solvent or it can simply be discarded. A typical example is the evaporation of sea water to produce salt.

Evaporation may also be used as a method to produce a liquid or gaseous product obtained from the condensed vapor. For instance, in desalinization processes, sea water is vaporized and condensed in a water-cooled heat exchanger and forms the fresh water product.

Although evaporation can be driven by the random motion of molecules near the liquid-gas interface, the addition of heat to a system speeds the evaporative process.

See also Caliche; Condensation; Drainage calculations and engineering; Leaching; Oceans and seas; Phase state changes; Runoff

evaporation change of a liquid into vapor at any temperature below its boiling point . For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity of the air, and the temperature. Evaporation occurs because among the molecules near the surface of the liquid there are always some with enough heat energy to overcome the cohesion of their neighbors and escape (see adhesion and cohesion ; matter ). At higher temperatures the number of energetic molecules is greater, and evaporation is more rapid. Evaporation is also increased by increasing the surface area of the liquid or by increasing the air circulation, thus carrying away the energetic molecules leaving the liquid before they can be slowed enough by collisions with air molecules to be reabsorbed into the liquid. If the air is humid some water molecules from the air will pass back into the liquid, thus reducing the rate of evaporation. An increase in atmospheric pressure also reduces evaporation. The process of evaporation is always accompanied by a cooling effect. For example, when a liquid evaporates from the skin, a cooling sensation results. The reason for this is that only the most energetic molecules of liquid are lost by evaporation, so that the average energy of the remaining molecules decreases; the surface temperature, which is a measure of this average energy, decreases also. Many refrigeration processes are based on this principle.

evaporation Process by which a liquid or solid becomes a vapour. The reverse process is condensation. Solids and liquids cool when they evaporate because they give up energy (latent heat) to the escaping molecules. Evaporation of sweat on the skin helps control the human body's temperature.

evaporation See EVAPOTRANSPIRATION.

evaporation See evapotranspiration.