The bonds in water molecules joining hydrogen and oxygen atoms together are, as previously discussed, polar covalent bonds. Although these bonds are strong, a small proportion of them break as the electron from the hydrogen atom is completely transferred to oxygen. When this occurs, the water molecule ionizes to form a hydroxyl ion (OH-) and a hydrogen ion (H+), which is simply a free proton (see fig. 2.4). A proton released in this way does not remain free for long, however, because it is attracted to the electrons of oxygen atoms in water molecules. This forms a hydronium ion, shown by the formula H3O+. For the sake of clarity in the following discussion, however, H+ will be used to represent the ion resulting from the ionization of water.
Ionization of water molecules produces equal amounts of OH- and H+. Since only a small proportion of water molecules ionize, the concentrations of H+ and OH- are each equal to only 10-7 molar (the term molar is a unit of concentration, described in chapter 6; for hydrogen, one molar equals one gram per liter). A solution with 10-7 molar hydrogen ion, which is produced by the ionization of water molecules in which the H+ and OH- concentrations are equal, is said to be neutral.
A solution that has a higher H+ concentration than that of water is called acidic; one with a lower H+ concentration is called basic, or alkaline. An acid is defined as a molecule that can release protons (H+) into a solution; it is a "proton donor." A base can be a molecule such as ammonia (NH3) that can combine with H+ (to form NH4+, ammonium ion). More commonly, it is a molecule such as NaOH that can ionize to produce a negatively charged ion (hydroxyl, OH-), which in turn can combine with H+ (to form H2O, water). A base thus removes H+ from solution; it is a "proton acceptor," thus lowering the H+ concentration of the solution. Examples of common acids and bases are shown in table 2.2.
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