In a solution, the solvent is generally a liquid, which can be a
pure substance or a mixture. The species that dissolves, the
solute, can be a gas, another liquid, or a solid. Solubilities
range widely, from infinitely soluble such as ethanol in water,
to poorly soluble, such as silver chloride in water. The term
insoluble is often applied to poorly soluble compounds, although
in some cases insolubility means that a compound is very poorly
soluble. The solubility equilibrium is relatively
straightforward for covalent substances such as benzene. When
dissolved in water, the benzene molecules remain intact but
interact with and are generally surrounded by molecules of
water. When, however, an ionic compound such as sodium chloride
(NaCl) dissolves in water, the sodium chloride lattice
dissociates into individual ions that are solvated or surrounded
by water molecules. Nonetheless, NaCl is said to dissolve in
water, because evaporation of the solvent returns crystalline
NaCl. When a solute dissolves, it may form