Water
Water
Water is a chemical compound (H2O) composed of a single oxygen (O) atom bonded to two hydrogen (H) atoms, which are separated by an angle of 105°. Because of this asymmetrical arrangement, water molecules have a tendency to orient themselves in an electric field, with the positively charged hydrogen toward the negative pole and the negatively charged oxygen toward the positive pole. This tendency results in water having a large dielectric constant, which is responsible for making water an excellent solvent. Water is, therefore, commonly referred to as the universal solvent. Water is the most abundant liquid on Earth. It covers more than 70% of Earth’s surface. Including the clouds (which are, of course, also water), it makes the entire planet look blue and white from space. About 97.5% of Earth’s water is contained in seawater. Approximately 1.875% is found in polar ice caps. The remaining 0.625% is drinkable water.
Since mineral salts and organic materials can dissolve in water, it is the ideal medium for transporting life-sustaining minerals and nutrients into and through animal and plant bodies. Brackish and ocean waters may contain large quantities of sodium chloride (NaCl) as well as many other soluble compounds leached from the crust of Earth. For example, the concentration of mineral salts in ocean water is about 35,000 parts per million. Water is considered to be drinkable if it contains less than 500 parts per million of salts. Water can be reused indefinitely as a solvent because it undergoes almost no modification in the process.
Hydrogen bonding
Hydrogen bonding, which joins water molecule to water molecule, is responsible for other properties that make water a unique substance. These properties include its large heat capacity, which causes water to act as a moderator of temperature fluctuations; its high surface tension (due to cohesion among water molecules); and its adherence to other substances, such as the walls of a vessel (due to adhesion between water molecules and the molecules of a second substance). The high surface tension makes it possible for surface-gliding insects and broad, flat objects to be supported on the surface of water. Adhesion of water molecules to soil particles is the primary mechanism by which water moves through unsaturated soils.
Three physical states
Water is the only common substance that occurs naturally on Earth in three different physical states. The solid state, ice, is characterized by a rigid crystalline structure occurring at or below 32° F(0° C) and occupying a definite volume (found as glaciers and ice caps, as snow, hail, and frost, and as clouds formed of ice crystals). The liquid state exists over a definite temperature range 32 to 212° F (0° C to 100° C), but is not rigid nor does it have a particular shape. In other words, it has a definite volume but assumes the shape of its container. Liquid water covers three-fourths of Earth’s surface in the form of swamps, lakes, rivers, and oceans as well as found as rain clouds, dew, and ground water. The gaseous state forms at temperatures above 212° F(100° C), and neither occupies a definite volume nor is rigid. In other words, it takes on the exact shape and volume of its container. It occurs naturally as fog, steam, and clouds. One phase does not suddenly replace its predecessor as the temperature changes, but for a time at the melting point or boiling point, two phases will coexist. As water changes from the gaseous form to the liquid form, it gives off heat at about 540 calories per gram, and as it changes from the liquid form to the solid form, it gives off about 80 calories per gram. The turbulence of thunderstorms is in large part due to the release of large amounts of energy into the atmosphere as water condenses into water droplets or into crystals of ice (i.e., hail). Pressure affects the transition temperature between phases. For example, at pressures below atmospheric pressure, water boils at temperatures under 212° F(100° C), so food will take longer to cook at higher elevations.
Water can pass directly from the solid phase to the gaseous phase without going through the liquid phase. This process occurs at low temperatures and greatly reduced pressures through a process called sublimation. Dehydrated foods are produced by sublimation, in which foods are quick-frozen and then placed in evacuation chambers. Dehydration by sublimation requires less energy than other methods, reduces physical deterioration that accompanies prolonged or excessive heating, and decreases the loss of volatile aromatic compounds responsible for flavor.
Essential to living things
Earth’s supply of water is constantly being recycled. It is evaporated from the oceans by the sun and is given off by the forests. The vapor condenses into clouds, which rain out onto the land. The land water runs off into the lakes and rivers, which then run back to the seas, and the cycle is complete. The total amount of water on Earth, in the form of oceans, lakes, rivers, clouds, polar ice, etc., is 1.5× 1018(one-and-a-half billion) tons, occupying a total volume of 8.7 million cubic miles.
It is impossible to overstate the importance of water to almost every process on Earth, from the life processes of the lowest bacteria to the shaping of continents. Water is the most familiar of all chemical compounds known to humans. It is essential to all living things, plant and animal. Humans drink it, wash with it, play in it, and cook in it. In fact, human bodies are more than one-half water.
Scientists never see absolutely pure water because it dissolves so many substances. If people want pure water, then scientists have to prepare it laboriously by such means as distillation, ion exchange, and reverse osmosis. Moving water even dissolves rock slightly, to form caves and to wear away mountains. All of the water on Earth, therefore, is in the form of solutions. The dissolved substances change the properties of water from what they would be in pure water. They affect its freezing point and its boiling point, among many other physical and chemical properties. The dissolved or suspended substances in water can be in the form of ions, molecules, or larger particles. For drinking water, bacteria must also be killed.
What is water?
Water is an odorless, tasteless, transparent liquid that appears colorless but is actually very pale blue. The color is obvious in large quantities of water such as lakes and oceans, but it can even be seen in a full bathtub. It is a single chemical compound whose molecules consist of two hydrogen atoms attached to one oxygen atom. The chemical formula of this compound is therefore H2O. The two hydrogen atoms are attached to the oxygen atom in such a way as to make an angle-shaped molecule. The angle is not 90°, however, but 104.5°—close to a right angle, but a little wider.
The formula H2O means that no matter how much water is talked about, it always contains exactly twice as many hydrogen atoms as oxygen atoms. Considering that a hydrogen atom weighs only about one-sixteenth as much as an oxygen atom, most of the weight in water is due to oxygen: 88.8% of the weight is oxygen and 11.2% is hydrogen. That goes for everything from a single molecule to a lake.
Water can be made (synthesized) from hydrogen and oxygen, both of which are gases. When these two gases are mixed, however, they do not react unless the reaction is started with a flame or spark. Then, they react with explosive violence. The tremendous energy that is released is a signal that hydrogen and oxygen are very eager to become water. Another way of saying that is that water is an extremely stable compound, compared with loose molecules of hydrogen and oxygen. It is hard to break water molecules apart into its components.
Not only that, but water molecules are stuck quite tightly to each other—at least when compared with similar compounds. The molecule-to-molecule stickiness is caused mainly by the fact that the water molecule is a dipole, because the oxygen atom pulls electrons away from the hydrogen atoms, giving the oxygen corner of the molecule a slight negative charge and the two hydrogen ends a slight positive charge. The negative part of one water molecule then attracts the positive parts of others like a magnet, although they can still slide around over each other as molecules do in any liquid. Water molecules stick to each other also by hydrogen bonds.
An unusual liquid
The strong attractions that water molecules have for each other are responsible for many of water’s highly unusual properties, as compared with other liquids of about the same molecular weight. Among these are:
- Its unusually high boiling point (if it were similar to the other liquids, it would be a gas at room temperature).
- Its high heat of vaporization (the amount of heat it takes to change the liquid to a gas).
- Its high heat of fusion (the amount of heat it takes to melt solid ice).
- Its high heat capacity (the amount of heat it takes to raise its temperature by a certain amount).
- Its world-champion rank among liquids as a solvent (it has been called the universal solvent because it dissolves so many different substances).
- The low density (the lightness) of ice, which makes it float on the surface of liquid water. As water is cooled to make ice, it gets slightly denser, like all liquids. But at 39.2° F(4° C) it reaches its maximum density; when cooled below that temperature, it gets less dense until it reaches 32° F (0° C), at which time it freezes and suddenly decreases to 91.7% of the density of the water. Being less dense than the water, the ice floats.
The normal boiling point of water is 212° F (100° C), and its freezing point is 32° F (0° C). In fact, zero and 100 degrees on the Celsius scale are defined as the freezing and boiling points of water. Water is also the standard by which many other quantities are measured. For example, the density of a material is often expressed as its specific gravity or specific weight: how many times denser it is than water.
In pure water, one out of every 555 million molecules is broken down—dissociated—into a hydrogen ion and a hydroxide ion:
H2OH+ +OH-
These ions are enough to make water a slight conductor of electricity. That is why water is dangerous when there is electricity around. The slight dissociation of water is responsible for the acid and base balances of all of the chemical reactions that take place in water, and that includes almost all the chemical reactions that take place anywhere, including those in the human body. Acid-base balance is probably the most important single factor that affects chemical reactions.
Today and tomorrow
Water is a major geologic agent of change for modifying Earth’s surface through erosion by water and ice. Water is also an important recreational medium, supporting fishing, swimming, and boating, and is a major factor in the tourism industry.
Water is scarce in many parts of the world, while it is plentiful in others. Many water resource experts state that water will become the next major commodity traded internationally. It has been called such phrases as liquid gold and the oil of the twenty-first century because of the increasing demands made by humans on water.
See also Groundwater; Hydrologic cycle; Hydrology; Irrigation; Precipitation; States of matter; Water conservation; Water pollution.
Resources
BOOKS
Black, Maggie. Water, Life Force. Toronto, Canada: Between the Lines, 2004.
Editors of I.B. Tauris. A History of Water. London, UK, and New York: I.B. Tauris, 2004.
Hancock P.L., and Skinner B.J., eds. The Oxford Companion to the Earth. Oxford: Oxford University Press, 2000.
Herschy, Reginald, and Rhodes Fairbridge, eds. Encyclopedia of Hydrology and Water Resources. Boston: Kluwer Academic Publishing, 1998.
Kandel, Robert S. Water from Heaven: The Story of Water from the Big Bang to the Rise of Civilization and Beyond. New York: Columbia University Press, 2003.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics Boca Raton: CRC Press, 2001.
McConnell, Robert, and Daniel Abel. Environmental Issues: Measuring, Analyzing, Evaluating. 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 2002.
Oxtoby, David W., et al. The Principles of Modern Chemistry. 5th ed. Pacific Grove, CA: Brooks/Cole, 2002.
OTHER
United States Environmental Protection Agency. “Water.” <http://www.epa.gov/water/> (accessed November 7, 2006).
United States Geological Survey, U.S. Department of the Interior. “Water Science for Schools.” <http://ga.water.usgs.gov/edu/> (accessed November 7, 2006).
Robert L. Wolke