A solution is a mixture containing at least two kinds of pure
substances. Although we usually think of solutions as being liquid, a
solution can exist in any of the three states of matter. We can have
solutions of gas in gas (e.g. air), gas in liquid (e.g. soda pop), gas
in solid (e.g. ice), liquid in liquid (e.g. ethanol and water), solid in
liquid (e.g. brine), or solid in solid (e.g. alloys).
In most solutions,
one material predominates and this is called the solvent, with the other
compounds being called solutes, although this convention is arbitrary
and only used for convenience. When the components of a solution are in
different states of matter, the solvent is considered to be the one
which does not undergo a change of state upon mixing.
Aqueous solutions are important for cryobiology since the freezing of
biological systems always involves solutions containing electrolytes,
non-electrolytes, polymers, and gases. During the phase change that
occurs with freezing, the concentration and distribution of the solutes
is altered, sometimes accompanied by irreversible chemical reactions.
The composition of a solution is described by the concentration of its
constituents. There are two primary ways of expressing concentration:
Molarity is based on the volume of solution whereas molality is based on
the weight of solvent. The difference becomes most noticeable when
temperature effects are considered. Because the volume of liquids can
expand or contract with changes in temperature, molarity can change with
changing temperature. The weight of solvent, on the other hand, is
constant with temperature, so molality gives a measure of concentration
that is independent of temperature.
- Molarity (M) - the number of moles of solute in 1 litre of solution;
- Molality (m) - the number of moles of solute associated with 1000g
A more fundamental expression of the concentration of a solution uses
the mole fraction (that is, the fraction of molecules on the interval 0-
1). The mole fraction of solute is given by: