What Is Water? - Chemical Composition

Water is a molecule made up of two hydrogen atoms and one oxygen atom: H₂O. (See Figure 1.1.) (While its composition is actually more complex, a simple description serves for this discussion.) Atoms in a molecule of any substance are joined together by a process known as chemical bonding, where two or more atoms mutually share one or more electrons. This bonding is particularly strong in water. Water is also both a weak acid and a weak base—chemical properties that allow it to function as a universal solvent capable of dissolving many substances.

THREE STATES OF WATER

Water exists naturally in three states: a liquid (its most common form), a solid (ice), and a gas (water vapor). It is the only substance on earth in which all three of its natural states occur within the normal range of climatic conditions, sometimes at the same time. Familiar examples of water in its three natural states are rain, snow or hail, and steam.

Compared to other liquids, water has some unusual properties. For example, most liquids contract as they freeze. Water contracts only until it reaches 4°C. Then it expands until it reaches its freezing point of 0°C (32°F). This expansion can exert a tremendous force on surrounding objects, enough to crack an unprotected automobile engine, burst a basement water pipe, or even shatter a boulder. Expansion makes ice lighter than water, which is why ice floats. This phenomenon causes rivers and lakes to freeze from the top down, a necessity for the survival of aquatic life. If freezing occurred from the bottom up, bodies of water might freeze solid in winter, killing aquatic plants and animals, and never thaw completely, even in summer.

When ice is warmed to 0°C, it melts, becoming liquid. As a liquid, its molecules are more loosely bound FIGURE 1.1
H₂O: the water molecule
SOURCE: Created by Information Plus Staff for Thomson Gale to graphically depict a water molecule together and can move around each other rather freely. The molecules' ability to slip and slide around gives water and other liquids their fluid properties.

In the gaseous or vapor state, water molecules move rapidly about and have very little attraction for each other, creating the diffuse appearance of steam or mist, or the haze of a humid day (humidity is the measure of the amount of water vapor in the air). Evaporation is the general term used to describe the process by which water in its liquid form is changed to its gaseous state. Evaporation can occur under a wide variety of conditions. Examples include water vaporizing off of wet pavement following rainfall, boiling water to produce steam, or heating materials under a wide range of temperatures to dry them.