proximity of the component atoms,
ions, or molecules and the strength
of the forces between them. True
solids are crystalline (see also amor-
phous). If a crystalline solid is
heated, the kinetic energy of the
components increases. At a speciÜc
temperature, called the melting
point, the forces between the compo-
nents become unable to contain
them within the crystal structure. At
this temperature, the lattice breaks
down and the solid becomes a liquid.
solid solutionA crystalline ma-
terial that is a mixture of two or
more components, with ions, atoms,
or molecules of one component re-
placing some of the ions, atoms, or
molecules of the other component in
its normal crystal lattice. Solid solu-
tions are found in certain alloys. For
example, gold and copper form solid
solutions in which some of the cop-
per atoms in the lattice are replaced
by gold atoms. In general, the gold
atoms are distributed at random, and
a range of gold–copper compositions
is possible. At a certain composition,
the gold and copper atoms can each
form regular individual lattices (re-
ferred to as superlattices). Mixed crys-
tals of double salts (such as alums)
are also examples of solid solutions.
Compounds can form solid solutions
if they are isomorphous (see isomor-
phism).
solidusA line on a phase diagram
below which a substance is solid.
solubilityThe quantity of solute
that dissolves in a given quantity of
solvent to form a saturated solution.
Solubility is measured in kilograms
per metre cubed, moles per kilogram
of solvent, etc. The solubility of a
substance in a given solvent depends
on the temperature. Generally, for a
solid in a liquid, solubility increases
with temperature; for a gas, solubil-
ity decreases. See also concentration.solubility product Symbol Ks. The
product of the concentrations of ions
in a saturated solution. For instance,
if a compound AxByis in equilibrium
with its solution
AxBy(s) ˆxA+(aq) + yB–(aq)
the equilibrium constant is
Kc= [A+]x[B–]y/[AxBy]
Since the concentration of the undis-
solved solid can be put equal to 1,
the solubility product is given by
Ks= [A+]x[B–]y
The expression is only true for spar-
ingly soluble salts. If the product of
ionic concentrations in a solution ex-
ceeds the solubility product, then
precipitation occurs.soluteThe substance dissolved in a
solvent in forming a *solution.solutionA homogeneous mixture
of a liquid (the *solvent) with a gas
or solid (the solute). In a solution, the
molecules of the solute are discrete
and mixed with the molecules of sol-
vent. There is usually some interac-
tion between the solvent and solute
molecules (see solvation). Two liq-
uids that can mix on the molecular
level are said to be miscible. In this
case, the solvent is the major compo-
nent and the solute the minor com-
ponent. See also solid solution.solvationThe interaction of ions of
a solute with the molecules of sol-
vent. For instance, when sodium
chloride is dissolved in water the
sodium ions attract polar water mol-
ecules, with the negative oxygen
atoms pointing towards the positive
Na+ion. Solvation of transition-metal
ions can also occur by formation of
coordinate bonds, as in the hexaquo-
copper(II) ion [Cu(H 2 O) 6 ]2+. Solvation
is the process that causes ionic solids
to dissolve, because the energy re-
leased compensates for the energy
necessary to break down the crystalsolid solution 494s