Page 82 - Basic physical chemistry for the atmospheric sciences

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S o lution chemistry and aqueous equilibria

4. 1 Definitions and types of solutions
A solution is a homogeneous mixture of substance . For example,
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when salt dissolves in water, a homogeneous mixture, or solution,
forms. The component of a mixture that is present in the greatest
quantity or that determines the state of matter (solid, liquid, or gas) of
the solution is called the solvent. The other component(s) is (are)
called the solute(s). If water is the solvent, the solution is said to be
aqueous (abbreviation: aq). If the quantity of solute is relatively large,
the solution is said to be concentrated; if it is relatively small, the
solution is dilute. Although we generally think of solutions as being
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liquid , they may also be gases or solid : Air is a gaseous solution;
alloys (e.g. , steel) are solid solutions.
As the amount of solid solute dissolved in a liquid solvent increase ,
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the reverse process, namel , the crystallization r precipitation f the
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o
solute from the solvent, becomes increasingly important. When the
solute dissolves and precipitates at the same rate, the amount of solute
in the solution will remain constant. The solution is then said to be
saturated, and the amount of solute present in a given quantity of the
saturated solution is called the solubility of the solute in the solvent. If
a solution is below its solubility limit, i t is said to be unsaturated; if it
is above the solubility limit, it is said to be supersaturated. Supersatu­
rated solutions are unstable, and may rapidly return to the equilibrium
(saturated) state through the crystallization of an appropriate quantity
of solute.

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