Chemically, water is a compound of hydrogen and oxygen, having the formula H2O.
It is chemically active, reacting with certain metals and metal oxides
to form bases, and with certain oxides of nonmetals to form acids. It
reacts with certain organic compounds to form a variety of products,
e.g., alcohols from alkenes. Because water is a polar compound, it is a
good solvent. Although completely pure water is a poor conductor of
electricity, it is a much better conductor than most other pure liquids
because of its
self-ionization, i.e., the ability of two water molecules
to react to form a hydroxide ion, OH - , and a hydronium ion, H3O+. Its polarity and ionization are both due to the high dielectric constant of water.
Water
has interesting thermal properties. When heated from 0°C, its melting
point, to 4°C, it contracts and becomes more dense; most other
substances expand and become less dense when heated. Conversely, when
water is cooled in this temperature range, it expands. It expands
greatly as it freezes; as a consequence, ice is less dense than water
and floats on it. Because of hydrogen bonding between water molecules,
the latent heats of fusion and of evaporation and the heat capacity
of water are all unusually high. For these reasons, water serves both
as a heat-transfer medium (e.g., ice for cooling and steam for heating)
and as a temperature regulator (the water in lakes and oceans helps
regulate the climate).
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