Water | Research & Encyclopedia Articles

Water

Water is a chemical compound composed of a single oxygen atom bonded to two hydrogen atoms (H₂O) which are separated by an angle of 105°. Because of this asymmetrical 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." Since mineral salts and organic materials can dissolve in water, it is the ideal medium for transporting 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 the crust of the earth. For example, the concentration of mineral salts in ocean water is about 35,000 parts per million. Water is considered to be potable is it contains less than 500 parts per million of salts. Water can be reused indefinitely as a solvent because it undergoes almost no modification in the process.

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; 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 to -24.8°F (4°C), the tendency of water to contract as its molecular motion decreases is overcome by the strength of hydrogen bonding between molecules. At 4°C, water molecules start to structure themselves directionally along the lines of the hydrogen bonds, at angles of 105° As the temperature drops toward -32°F (0°C), 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 -32°F (0°C) and occupying a definite volume (found as glaciers and ice caps, as snow, hail, and frost, and as clouds formed of ice crystals). The liquid state exists over a definite temperature range -32 to 148°F (0°C to 100°C), but is not rigid nor does it have a particular shape. In other words, it has a definite volume but assumes the shape of its container. Liquid water covers three-fourths of the 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 forms at temperatures above 148°F (100°C), and neither occupies a definite volume nor is rigid. In other words, it takes on the exact shape and volume of its container. It occurs naturally as fog, steam, and clouds. 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 of large amounts of energy into the atmosphere 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, so food will take longer to cook at higher elevations.

Water can pass directly from the solid phase to the gaseous phase without going through the liquid phase. This process occurs at low temperatures and greatly reduced pressures through a process called sublimation. Dehydrated foods are produced by sublimation, in which foods are quick-frozen and then placed in evacuation chambers.Dehydration by sublimation requires less energy than other methods, reduces physical deterioration that accompanies prolonged or excessive heating, and decreases the loss of volatile aromatic compounds responsible for flavor.

Liquid water is critical to sustain life. Without water to drink, a human will die in less than a week. Water in the form of perspiration is important in maintaining thermal stability in the body by dispersing large quantities of heat from the surface of the skin into the atmosphere. As the principal constituent of blood, water is the medium by which red blood cells transport oxygen (O₂) through the body, and by which carbon dioxide (CO₂) and other wastes are removed from the body. As a solvent, water in blood carries sugars and proteins, mineral salts, metabolites such as urea, hormones, and compounds that cause blood to clot.

The principal use of water in agriculture is for irrigation of crops, while lesser amounts are used for watering of livestock and cleaning of produce. Most industrial and manufacturing processes use large quantities of water to provide energy, remove unwanted heat, serve as a solvent, wash away impurities, function as a transport mechanism, and serve as a raw material. Water is used to keep individuals, homes, and communities clean, thus improving public health. Water is used to flush, wash, and dilute wastes from both households and industries. Because water is an excellent solvent and a convenient repository for wastes, it may contain many impurities that, if present in sufficient concentrations, constitute pollution and may require reclamation and treatment.

Water is a major geologic agent of change for modifying the earth's surface through erosion by water and ice. Water is also an important recreational medium, supporting fishing, swimming, and boating, and is a major factor in the tourism industry.