Vapor Pressure And Evaporation

Type of physical science: Vapor Pressure and Evaporation, Phase changes, Gases; behavior of, Hydrology, Chemistry

Field of study: Chemical processes

Evaporation is a process in which a liquid is converted into a gaseous vapor, while vapor pressure is the pressure that the vapor above a liquid exerts. Water evaporates continuously from oceans, lakes, and streams. The water that enters the atmosphere through this process later returns to Earth in the form of rain and snow.

Overview

Some of the molecules of a liquid left in an open container will escape from the surface of the liquid and become molecules of gaseous vapor. This process is called "evaporation." If the container is closed, the number of gas molecules will build and exert a pressure on the walls of the container. This pressure is called the "vapor pressure" of the liquid.

The process of evaporation and the vapor pressure of liquids are best understood in terms of kinetic molecular theory, which was slowly developed between the sixteenth and eighteenth centuries. The theory holds that molecules are in constant motion and that the speed of their motion depends upon the temperature. If the temperature is raised, the molecules will move with greater speed. Cooling will result in the slowing of the molecules. Motion is a manifestation of heat; when all molecular motion ceases, a substance is as cold as it can possibly be. This occurs at a temperature of -273 degrees Celsius, or absolute zero.

Liquids differ from gases in that a liquid has a definite volume, while a gas expands to fill any container. The fact that a gas expands to fill a container means that any forces of attraction between the molecules must be very weak. In liquids, the attractive forces between molecules must be considerably stronger, since they do hold the molecules together in a definite volume. A molecule of a liquid must overcome the attractive forces of other molecules holding it in the liquid if it is to escape and become a molecule of gas.

As heat is added to a liquid, the molecules will gain energy and move faster. At any given temperature, all the molecules will not have the same energy and so will not be moving at the same speed. Most of the molecules will have energies clustered fairly closely around the average energy, but a small percentage will have energies significantly above or below average. If one of the faster-moving gas molecules is near the surface of the liquid, it might have enough energy to escape into the area above the liquid. When that happens, evaporation has occurred.

Vapor pressure and evaporation are closely related. The pressure that is exerted by a gas above a liquid will depend upon the number of gas molecules. If a liquid evaporates rapidly, there will be a large number of gas molecules above the liquid's surface, and it will have a high vapor pressure. Imagine that you have a container with some liquid in it, but also with some empty space above the liquid surface. As molecules begin to escape from the liquid by evaporation, the number of molecules of gas above the liquid will increase. The gas molecules that have escaped from the attraction of their neighbors thus will be able to spread throughout the space above the liquid.

As the gas molecules randomly race around the empty space, some of them will strike the surface of the liquid and reenter it. As more and more molecules enter into the gas, the number striking the liquid surface will steadily increase. Eventually, a point will be reached at which the number of molecules entering the gas and returning to the liquid are equal. Once that point is reached, the relative amounts of liquid and gas will remain constant. Molecules will continue to move from gas to liquid and from liquid to gas, but at exactly the same rate. Movement between the phases continues, but there is no net change in the amounts of liquid and gas. This is an example of a dynamic equilibrium, as contrasted to the static equilibrium achieved when the two sides of a seesaw are balanced, for example.

The pressure that a gas exerts is caused by the collisions of gas molecules with the walls of their container. As noted previously, as the temperature of a liquid is increased, the number of molecules that have enough energy to escape from the liquid will increase. The more molecules in the gas phase, the greater the number of collisions with the container's walls, and the greater the vapor pressure. As the temperature of a liquid increases, so will its vapor pressure.

Imagine now that the liquid is contained in an open container. The pressure acting upon the surface of the liquid will be the pressure of the atmosphere. As the temperature of the liquid increases, more and more molecules will be able to escape from the surface of the liquid, and its vapor pressure will increase. When the vapor pressure becomes equal to the atmospheric pressure pushing down on the surface of the liquid, something else happens. Now molecules throughout the liquid will have enough energy to vaporize. Gas does not form only at the surface of the liquid; pockets of gas form throughout it. Since these bubbles contain gas with a vapor pressure equal to that of the atmosphere pressing on the liquid's surface, they can force their way up through the liquid and push their way out. This results in the bubbling that indicates that a liquid has reached its boiling point.

The attractive forces that operate between the molecules of substance are called "intermolecular" forces. These have to be distinguished from the intramolecular forces that operate within a molecule to hold the atoms together. The stronger the intermolecular attractive forces, the more strongly the liquid molecules are held together. This means that it will be difficult for the molecules to escape into the gas, so vapor pressure and the rate of evaporation will be low.

A polar molecule is one that has a center of positive charge in one part of the molecule and a center of negative charge in another part. The oppositely charged centers in neighboring molecules will be attracted to each other, producing intermolecular attraction.

A particularly strong kind of intermolecular force that results from polarity in a molecule is called hydrogen bonding. It operates between water molecules and results in some of water's unusual properties. The oxygen in a water molecule carries a negative charge, and the hydrogen carries a positive one. The hydrogen bond occurs between the oxygen atom in one molecule and the hydrogen atom in another. Most small molecules such as water exist as gases at ordinary Earth temperatures. The fact that water exists mainly as a liquid on Earth is the result of its strong hydrogen bonding. Substances with weaker intermolecular forces than water will have higher vapor pressures at the same temperature. Ether is an example of this. At ordinary room temperature (about 70 degrees Fahrenheit), the vapor pressure of ether is about twenty times greater than that of water. This is one of the properties that made ether an effective anesthetic as a liquid. The ether produces a substantial amount of vapor that can be inhaled by a patient.

The attractive forces between molecules must be overcome before liquid molecules can escape to the gas. This takes energy, but in evaporation, there is no external source of energy. This means that the energy to overcome the attraction must come from the other molecules in the liquid. If the hottest molecules leave, the temperature of those that remain will be lower. This means that evaporation of a liquid is accompanied by cooling and a drop in temperature.

Applications

Evaporation plays a key role in homeothermy, the maintenance of normal body temperature in animals and humans. Normal human body temperature is around 98 degrees Fahrenheit. However, the temperature of the air around the body is frequently well above or well below that temperature. If the temperature of the body gets too low, the rates of its metabolic processes will fall below levels necessary to sustain life. At high body temperatures, the rates of the metabolic processes increase, producing more heat. As the temperature of the body increases, the cooling center in the hypothalamus of the brain that controls the body's cooling mechanisms becomes ineffective. With no controls on the heat production and dissipation, the temperature of the body continues to rise. The result can be heat stroke.

If normal body temperature is to be maintained, the body then must have mechanisms both to produce heat and to dissipate it. The main way heat is produced is through metabolic reactions that release heat. The body loses heat through a number of mechanisms. At normal room temperature, 50 percent of the heat lost by the body is through radiation. Any object at a temperature above absolute zero will radiate heat to its environment. The heat is given off as infrared radiation. Another 40 percent is given off by convection, in which air flowing over an object removes heat from its surface. This is the basis for the windchill factor. Finally, 9 percent of the heat loss occurs through evaporation of sweat. An important aspect of sweating is that the rate of evaporation--and thus of cooling--can be increased.

The cooling effect of evaporation is most readily recognized when one fans oneself on a hot day or steps out of a shower and feels a sudden chill. Evaporation is an equilibrium process. Water molecules that stay close to the surface of the liquid can return. When they do, they will release the heat to the body that they absorbed when they left the liquid. If the water-vapor molecules are removed from the region above the liquid surface, they cannot return, and other molecules will be able to leave the liquid water to replace them. Steadily removing the gaseous molecules from above the surface will increase the rate of evaporation and the cooling of the body.

At any temperature, there is a limit to how much water air can hold before condensation occurs. The air in a bathroom frequently becomes saturated with water vapor, and a fog forms on the bathroom mirror. Relative humidity is defined as the percentage of the maximum value that the air actually contains at a given temperature. A relative humidity of 50 percent means that the air has half the maximum amount that it can hold.

If the air surrounding a body has a high water content (high relative humidity), the rate at which sweat can evaporate will be reduced. Because of this, sweating is a more effective cooling mechanism in very dry climates, and much less effective in humid climates. (This phenomenon has given rise to the expression, "It's not the heat, it's the humidity.")

Plants also lose water by evaporation through the process of transpiration. The leaves of plants provide a large surface area from which evaporation can occur. A leaf can lose its own weight in water every day through transpiration. A plant with such a high rate of water loss cannot survive in arid desert regions. The survival of desert plants depends on adaptations that reduce water loss.

The most familiar of the desert plants are the cacti. In a cactus plant, the leaves have been replaced by spines that greatly reduce the surface area for evaporation. "Succulent" is the general term applied to water-storing and water-preserving plants. Some succulents do have leaves, but they are usually small and thick to reduce surface area. The leaves also have a waxy surface that further reduces water loss.

Evaporation of sweat is an effective cooling mechanism for humans because humans have so little body hair. It is not effective with other mammals, since their body hair will inhibit evaporation. Another way in which cooling can occur is through evaporation of water from the lungs. When dogs pant in hot weather, they are speeding up evaporation of water from the lungs.

There are home appliances that also utilize principles of evaporation. A clothes dryer is an evaporator. Warm air is blown into the dryer to provide heat to speed up the evaporation and to remove water vapor as it forms. The tumbling action promotes evaporation by maximizing the exposed surface area of the wet clothing.

Hair dryers operate similarly. The heat from the warm air increases the number of water molecules that have sufficient energy to escape from the liquid, and the flow of air removes them to speed further evaporation. The blowing action tends to separate the hairs and maximize the exposed surface area for evaporation.

Another natural phenomenon in which evaporation plays a key role is the weather. About three quarters of the surface of the earth is covered with water. The water that evaporates from this huge surface area later falls as precipitation. On average, water molecules will float through the atmosphere for about ten days before returning to the earth. This interchange of water between the atmosphere and the earth is called the "hydrologic cycle" or "water cycle."

The part of the atmosphere that is closest to the earth, the troposphere, extends for seven miles above the surface. The temperature drops by about 10 degrees Celsius for every kilometer rise through the troposphere. When a cold weather front collides with a warm one, the warm air will rise; as it does, it will carry up moisture with it. At colder temperatures, air can hold less water. As the water vapor rises, it will steadily be cooled, until the air is saturated. At that point, the water vapor will condense, forming tiny droplets. The aggregation of these tiny droplets results in cloud formation.

As warm air continues rising through the cold cloud, turbulence will be created. This will result in water molecules colliding and merging until they form drops that are large enough to fall as rain. At sufficiently low temperatures, the water drops will freeze. As these ice crystals fall through warmer parts of the troposphere, they can melt to form rain. If they stay sufficiently cold, they will remain as ice crystals and reach the earth as snow.

Context

Evaporation is such a common phenomenon that it has certainly been recognized for thousands of years. However, the mechanics of the process could not be understood until the development of the kinetic molecular theory. Its slow development took almost two centuries. In 1679, Robert Hooke expressed the idea that the entire universe and all the particles in it were in constant motion. He proposed that lighter particles of gas moved more quickly than heavier particles. Hooke cited little evidence for his ideas, which were not widely accepted.

A more comprehensive theory of gas kinetics was presented by Daniel Bernoulli in his Hydrodynamica (1738). While Hooke's treatment of gas kinetics was entirely qualitative, Bernoulli's was supported by quantitative work. Bernoulli advanced a number of ideas describing how gas molecules behave. Most are still accepted today, but for some, the acceptance took more than a century. Bernoulli reaffirmed the idea that gas molecules constantly move in rapid random motion. He developed a mathematical equation that described how the pressure of a gas could be accounted for by collisions of gas molecules with the sides of their container. He made an assumption that when a gas is heated, its molecules move more quickly.

Before evaporation could be understood, the idea of molecules in constant motion had to be extended from gases to liquids. Evidence for this came from the discovery of Brownian motion. Robert Brown suspended tiny pollen particles in water in 1827 and observed them under a microscope. The pollen particles underwent irregular motion through the liquid. Brown used freshly gathered pollen, and his first assumption was that some life force accounted for the motion. However, he observed the same kind of motion with dried pollen and with tiny particles of minerals. This left Brown with no explanation for the curious motion he had observed.

The suggestion that Brownian motion of tiny particles in water might be due to collisions with rapidly moving water molecules was made in 1863 by Christian Weiner and in 1888 by Louis-Georges Guoy. Guoy considered other possible causes for the motion, such as external vibrations transmitted into the liquid, thermal convection currents, or energy from light. He did experiments to eliminate these possibilities, leaving him with motion of the water molecules as the only reasonable cause. Jean Perrin elaborated on the ideas of Guoy in his book Brownian Motion and Molecular Reality (1908). Perrin determined that Brownian motion does not diminish over time but stays constant as long as the temperature remains constant.

This had to mean that whatever produced the movement did not lose energy. When large objects such as billiard balls collide, they do lose energy. The moving molecules have to be colliding with other molecules, but yet they do not lose energy. Perrin recognized that this could only be true if the collisions are perfectly elastic and so result in no loss of energy. This important hypothesis of Perrin has been incorporated into the kinetic molecular theory.

Chemistry deals with atoms and molecules that are far too small to see. To really understand chemistry, however, one must know what those particles are doing. One way of achieving this is through the development of models. The kinetic molecular theory provides a model that can be used to understand a number of important chemical phenomena.

Principal terms

BOILING POINT: The temperature at which the vapor pressure of a liquid is equal to the external pressure on the liquid

CONDENSATION: A process in which gaseous vapor is converted to a liquid

DYNAMIC EQUILIBRIUM: An equilibrium in which two opposing changes occur at equal rates so that no net change from one side of the equilibrium to the other takes place

HEAT OF VAPORIZATION: The quantity of heat that must be added to one mole of a liquid to convert it to a gas

MOLE: The mass of a substance that contains 6.02 × 102³ molecules

Bibliography

Boorse, Henry, and Lloyd Motz. The World of the Atom. Vol. 1. New York: Basic Books, 1966. Provides a historical account of the development of numerous theories of the structure and properties of atoms. Includes brief biographies of the scientists involved, descriptions of their work and its significance, and excerpts from original writings. The chronological development of the kinetic molecular theory is clearly presented.

Burroughs, William, et al. Weather. New York: Time-Life Books, 1996. Beautifully illustrated with photographs and drawings that help to make the forces that produce weather quite understandable. Covers all aspects of weather, including evaporation, cloud formation, and precipitation.

Goldberg, David. Fundamentals of Chemistry. Dubuque, Iowa: William C. Brown, 1994. Written for an audience with no previous experience with chemistry. Includes mathematical treatments of topics, but concepts are presented in a way that they can be understood without the mathematics. Topics covered include the kinetic molecular theory, vapor pressure, equilibrium, intermolecular forces, and phase changes.

Harrison, G. A., et al. Human Biology. Oxford, England: Oxford University Press, 1977. This book is not a standard biology text; rather, it bridges the fields of biology and physical anthropology. Its primary focus is on the relationship of biology to human adaptation and evolution. It contains a chapter on climatic adaptation that thoroughly discusses the responses of the human body to temperature changes.

Leopold, A. Starker. The Desert. New York: Time-Life Books, 1969. Describes the desert habitat and the survival problems it presents to living things. The adaptations of plants and animals to the arid environment is covered effectively. Illustrated with well-chosen photographs and drawings.

Leopold, Luna, and Kenneth Davis. Water. New York: Time-Life Books, 1970. Part of a series on science topics written for a general audience. Presents a simple picture of the structure of the water molecule and describes some of its unusual characteristics. The hydrologic cycle and its effects upon the weather are well presented. Nicely illustrated with photographs, drawings, and diagrams.