WATER

a substance composed of the chemical elements hydrogen and oxygen and existing in gaseous, liquid, and solid states. Water is one of the most plentiful and essential of compounds. It is vital to life, participating in virtually every process that occurs in plants and animals. Although the molecules of water are simple in structure (H2O), the physical and chemical properties of the compound are extraordinarily complicated. See also ice; steam; water resource; precipitation. Water is a colourless, tasteless, and odourless liquid at room temperature. One of its most important properties is its ability to dissolve many other substances. The versatility of water as a solvent is essential to living organisms. Life is believed to have originated in the world's oceans, which are complicated solutions. Living organisms use aqueous solutions-e.g., blood and digestive juices-as mediums for carrying out biological processes. The water molecule is composed of two hydrogen atoms, each linked by a single chemical bond to an oxygen atom. Most hydrogen atoms have a nucleus consisting solely of a proton. Two isotopic forms, deuterium and tritium, in which the atomic nuclei also contain one and two neutrons, respectively, are found to a small degree in water. Deuterium oxide (D2O), called heavy water, is important in chemical research and is also used as a neutron moderator in some nuclear reactors. Although its formula (H2O) seems simple, water exhibits very complex chemical and physical properties that are incompletely understood. For example, its melting point, 0 C (32 F), and boiling point, 100 C (212 F), are much higher than would be expected by comparison with analogous compounds, such as hydrogen sulfide and ammonia. In its solid form, ice, water is less dense than when it is liquid, another unusual property. The root of these anomalies lies in the electronic structure of the water molecule. An oxygen atom has six electrons in its outer (valence) shell, which can hold a total of eight. When an oxygen atom forms a single chemical bond, it shares one of its own electrons with the nucleus of another atom and receives in return a share of an electron from that atom. When bonded to two hydrogen atoms, the outer electron shell of the oxygen atom is filled. The water molecule is not linear but bent in a special way. As a result, part of the molecule is negatively charged and part positively charged. It is thus a highly polar molecule. Hydrogen atoms in water molecules are attracted to regions of high electron density and can form weak linkages, called hydrogen bonds, with those regions. This means that the hydrogen atoms in one water molecule are attracted to the non-bonding electron pairs of the oxygen atom on an adjacent water molecule. As a result, water molecules associate strongly. In an ice crystal, the association is a highly ordered but loose structure. When the ice melts, this orderly arrangement breaks down partially and the molecules pack more closely together. This makes the liquid denser than the solid, which is why ice forms on top of liquid water. The associative force, however, is still strong enough to prevent water molecules from separating completely even at room temperature. This continued association in the liquid state accounts for the high boiling point of water. The structure of liquid water is believed to consist of aggregates of water molecules that form and re-form continually. This short-range order, as it is called, accounts for other unusual properties of water, such as its high viscosity and surface tension. The polarity of the water molecule plays a major part in the formation of aqueous solutions. If an ionic compound such as sodium chloride is placed in water, the polar water molecules reduce the electrostatic attraction between the positively charged sodium and negatively charged chloride ions. This helps to pull the ions apart. They then become hydrated-i.e., surrounded by water molecules. As a result, their charge is effectively dispersed over a larger structure, and this keeps the sodium and chloride from recombining. To a limited extent, water dissociates into hydrogen (H+) ions, which make a solution acidic, and hydroxyl (OH-) ions, which make it alkaline (basic). Consequently, water can sometimes act as an acid or a base.