Water absorbs a great deal of solar heat without its temperature rising substantially. It is this high heat capacity that allows the oceans to have a mod­erating influence on climate, particularly of coastal areas. Another consequence of water's high heat capacity is that oceans do not experience the wide temperature fluctuations that are common on land.

    Water must absorb a lot of heat before it vapor­izes, or changes from liquid to vapor. When it does evaporate, it carries the heat (called its heat of va­porization) with it. Thus, evaporating water has a cooling effect. That is why your body is cooled when perspiration evaporates from your skin.

    Water is sometimes called the "universal sol­vent," and although this is an exaggeration, many materials do dissolve in water. In nature, water is never completely pure, because it contains dis­solved gases from the atmosphere and dissolved mineral salts from the Earth. Seawater, for exam­ple, contains a variety of dissolved salts including sodium chloride, magnesium chloride, magnesium sulfate, calcium sulfate, and potassium chloride. Water's dissolving ability has a major drawback, however: many of the substances that dissolve in water also pollute it.