Physical Properties of Water

Pure water is a tasteless, odorless liquid. It boils at 100 C. (212 F.), and freezes at 0 C. (32 F.). Water, like other liquids, cannot be compressed. This property makes it useful in hydraulic presses and other hydraulic devices.

The density of water is greatest at a temperature of 4 C. (39.2 F.). At this temperature, one gallon of water weighs about 8.3 pounds, one liter of water, 1 kilogram; one cubic foot of water, about 62.4 pounds; and one cubic centimeter of water, 1 gram. The maximum density of water is used as a standard of comparison for expressing the density of other liquids and solids.

Unlike most other liquids, water expands when it freezes. When water turns to ice its volume increases by a factor of 112. Since a given weight of ice occupies a greater space than an equal weight of water, ice floats in water. This physical property of water has important consequences. If water contracted on changing into a solid, ice would be heavier than an equal volume of water and would sink. The bottoms of lakes and oceans would then fill with ice, out of reach of the sun's warmth. Gradually the earth would become colder, more and more ice would form, and in time there would be little, if any, life on earth.

Specific Heat

The specific heat of water in the metric system is 1 calorie—the amount of heat required to raise the temperature of one gram one degree Celsius. (In the traditional English system it is 1 British Thermal Unit—the amount of heat required to raise the temperature of one pound one degree Fahrenheit.) Water has a higher specific heat than almost any other substance. For example, it takes almost 33 times as much heat to raise the temperature of one gram of water 1 C. as it takes to raise the temperature of one gram of gold by an equal amount. As it becomes warmer, water stores 33 times as much heat as gold; when it cools, water gives off 33 times as much heat as gold.

The high specific heat of water protects living things, which consist chiefly of water, from drastic temperature changes. It also has an important influence on weather and climate. Oceans and large lakes are cooler than nearby land in summer, and help to cool the land. In winter they slowly give off the heat absorbed in summer and help to warm the land. Currents of warm and cool ocean water greatly modify temperatures in some parts of the earth.

Evaporation

Like other substances, water takes heat from its surroundings when it evaporates (changes to a vapor). A great amount of heat is required to evaporate water—about 540 calories are needed to evaporate one gram at the boiling point (100 C.). At lower temperatures even more calories are needed. At normal body temperature (98.6 F. [37 C.]), for example, about 577 calories are required to evaporate one gram of water. Thus every gram of perspiration that evaporates takes away 577 calories of heat, cooling the body. Similarly, evaporating rain and snow take vast amounts of heat away from their surroundings, and thus cool the air.

Because water absorbs so much heat in changing to a vapor, water-vapor molecules have extremely high energy. It is this property that makes steam a practical source of power.

Water vapor greatly influences climate. It absorbs part of the sun's heat by day and hinders the escape of heat from the earth by night. In deserts, where water vapor is scarce, there are extreme variations of temperature between day and night.

Condensation

The condensation (changing to liquid) of water vapor greatly affects weather and climate. In condensing, water vapor gives off the same amount of heat that was required to produce it. Heat is given off when fog, dew, and frost form, and when steam condenses in a radiator.

Freezing and Melting

When water at 0 C. freezes, each gram of water gives off 80 calories. The temperature of the water stays at 0 C. until every drop has frozen.

When ice at 0 C. melts, it absorbs an equal amount of heat—80 calories for each gram of ice—and its temperature remains unchanged until it has completely melted. It is by absorbing heat that ice keeps an iced drink cool.

The huge quantities of heat that are given off or absorbed when a large lake freezes or thaws have a noticeable influence on the climate of the land on its shores.

Surface Tension,

caused by surface molecules clinging together and being pulled inward by other molecules, forms an apparent film on water. A needle will float when carefully placed on the surface. This tension enables water spiders and certain insects to walk on water. In using water for washing, it is necessary to reduce the surface tension to permit the water to fully wet the particles of dirt and the surface being washed; this is done with soaps and other detergents.