water

water A colourless liquid, H₂O. In the gas phase water consists of single H₂O molecules in which the H–O–H angle is 105°. The structure of liquid water is still controversial; hydrogen bonding of the type H₂O…H–O–H imposes a high degree of structure (see hydrogen bond) and current models supported by X-ray scattering studies have short-range ordered regions, which are constantly disintegrating and re-forming. This ordering of the liquid state is sufficient to make the density of water at about 0°C higher than that of the relatively open-structured ice; the maximum density occurs at 3.98°C. This accounts for the well-known phenomenon of ice floating on water and the contraction of water below ice, a fact of enormous biological significance for all aquatic organisms. Water is a powerful solvent for both polar and ionic compounds; molecules or ions in solution are frequently strongly hydrated. Pure liquid water is very weakly dissociated into H₃O⁺ (hydroxonium) and OH^– (hydroxyl) ions by self-ionization and consequently any compound that increases the concentration of the positive ion, H₃O⁺, is acidic and compounds increasing the concentration of the negative ion, OH^–, are basic (see acid). The phenomena of ion transport in water and the division of materials into hydrophilic (water loving) and hydrophobic (water hating) substances are central features of almost all biological chemistry.