What is Water?
Water is one of the most abundant chemical compounds on earth. In the form of oceans, seas, lakes, rivers, and the polar caps, it covers about three-fourths of the surface of the globe. Living organisms are composed of at least 50 percent water by weight. The human body is almost 70 percent water. In addition, most minerals contain water in the form of water of crystallization. Great quantities of water in the form of steam are liberated from beneath the earth's surface during volcanic eruptions.
Water is one of the most important chemical compounds. Many chemical reactions, particularly the life processes, take place only in aqueous, or water, solution. Therefore, life, even in its simplest forms, would be impossible without water.
Water in all three states (solid, liquid, and gaseous) is extremely important as a heat-transfer agent. Ice is widely used for cooling, particularly in connection with food preservation. Water is used in both heating and air-conditioning systems for buildings and for automobile engines, industrial machinery, and electronic equipment. Steam is a very efficient heat-transfer agent and is used in heating large buildings and building complexes and for providing heat for industrial processes. In steam engines and steam turbines, steam is used to convert heat energy to mechanical energy.
The Water Cycle
The major use of water is the nourishment of living things, which, in nature, is usually taken care of adequately by the water cycle. In all but the most primitive societies, however, special arrangements must be made to provide an adequate supply of water for men, animals, and crops.
Water in nature undergoes continual movement. It falls to earth as rain, snow, or other forms of precipitation. Most of the water that falls on land drains off into some large body of water. This drainage may take place by means of rivers, both on the surface and underground, or the water may seep through porous rock, in which case it is called groundwater.
The heat of the sun causes water to evaporate from the surfaces of water bodies, leaving behind dissolved minerals. Water that remains on land also evaporates, but some of it is absorbed by living organisms, both plants and animals. These organisms use the water and then eliminate it, and this water, too, evaporates. The water vapor in the air condenses to form clouds, and the water once again falls as precipitation.
The water cycle is, in effect, a natural distillation process that purifies water and redistributes it over the surface of the earth. By this process fresh water is made available to land organisms, making life outside the sea possible. A side effect of the water cycle is the erosion caused by water drainage. This erosion includes erosion by glaciers, as well as by rivers. Thus, water has played a major role in determining the present topography of the earth's surface.