Water is a chemical compound composed of a single oxygen atom bonded to two hydrogen atoms (H2O) that are separated by an angle of 105°. Because of their polar covalent bonds and this asymmetrical bent 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 referred to as the universal solvent. Water can be reused indefinitely as a solvent because it undergoes almost no modification in the process.
Because mineral salts and organic materials can dissolve in water, it is the ideal medium for transporting products of geochemical weathering as well as life-sustaining minerals and nutrients into and through animal and plant bodies. Brackish and ocean waters may contain large quantities of sodium chloride as well as many other soluble compounds leached from Earth's crust.
The concentration of mineral salts in ocean water is about 35,000 parts per million. Water is considered to be potable (drinkable) only if it contains less than 500 parts per million of salts.
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 due to variations in solar illumination, 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.
Hydrogen bonding is also responsible for ice being less dense than water. If ice did not float, all bodies of water would freeze from the bottom up, becoming solid masses of ice and destroying all life in them. In addition, from season to season, frozen water bodies would remain frozen, resulting in large changes in climate and weather, such as decreased precipitation due to reduced evaporation. Ice floats because as the temperature of water is lowered the tendency of water to contract as its molecular motion decreases is overcome by the strength of hydrogen bonding between molecules. At 4°C (39°F), water molecules start to structure themselves directionally along the lines of the hydrogen bonds, at angles of 105° As the temperature drops toward 0°C (32°F), spaces develop between the lines until the open, crystalline form characteristic of ice develops. Its openness produces a density slightly less than that of liquid water, and ice floats on the surface, with approximately nine-tenths submerged.
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 0°C (32°F) and occupying a definite volume (found as glaciers and ice caps, as snow, hail, and frost, and as clouds formed of ice crystals). At sea level atmospheric pressure, the liquid state exists over a definite temperature range 0°C to 100°C (32 to 212°F), but is not rigid nor does it have a particular shape. Liquid water 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 of water (water vapor) neither occupies a definite volume nor is rigid because it takes on the exact shape and volume of its container. Water vapor (liquid water molecules suspended in the air) occurs in steam, humidity, fog, and clouds.
During phase changes, one phase does not suddenly replace its predecessor as the temperature changes, but for a time at the melting 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 latent heat of water especially 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, water boils at temperatures under 100°C (212°F), therefore food takes longer to cook at higher elevations.
Water is a major geologic agent of change for modifying Earth's surface through erosion by water and ice.
See also Acid rain; Atmospheric chemistry; Chemical bonds and physical properties; Chemical elements; Clouds and cloud types; Condensation; El Nino and La Nina phenomena; Erosion; Evaporation; Freezing and melting; Freshwater; Rate factors in geologic processes
Source: World of Earth Science, ©2003 Gale Cengage. All Rights Reserved. Full copyright.
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