it will be found that a point is reached at which the liquid will not
dissolve more of the solid at that temperature. The solid and the
solution remain in contact with each other unchanged. This condition may
be described by saying that they are in equilibrium with each other. A
solution is said to be _saturated_ when it remains unchanged in
concentration in contact with some of the solid. The weight of the solid
which will completely saturate a definite volume of a liquid at a given
temperature is called the _solubility_ of the substance at that
temperature.

~Supersaturated solutions.~ When a solution, saturated at a given
temperature, is allowed to cool it sometimes happens that no solid
crystallizes out. This is very likely to occur when the vessel used is
perfectly smooth and the solution is not disturbed in any way. Such a
solution is said to be _supersaturated_. That this condition is unstable
can be shown by adding a crystal of the solid to the solution. All of
the solid in excess of the quantity required to saturate the solution at
this temperature will at once crystallize out, leaving the solution
saturated. Supersaturation may also be overcome in many cases by
vigorously shaking or stirring the solution.

~General physical properties of solutions.~ A few general statements may
be made in reference to the physical properties of solutions.

1. _Distribution of the solid in the liquid._ A solid, when dissolved,
tends to distribute itself uniformly through the liquid, so that every
part of the solution has the same concentration. The process goes on
very slowly unless hastened by stirring or shaking the solution. Thus,
if a few crystals of a highly colored substance such as copper sulphate
are placed in the bottom of a tall vessel full of water, it will take
weeks for the solution to become uniformly colored.

2. _Boiling points of solutions._ The boiling point of a liquid is
raised by the presence of a substance dissolved in it. In general the
extent to which the boiling point of a solvent is raised by a given
substance is proportional to the concentration of the solution, that
is, to the weight of the substance dissolved in a definite weight of the
solvent.

3. _Freezing points of solutions._ A solution freezes at a lower
temperature than the pure solvent. The lowering of the freezing point
obeys the same law which holds for the raising of the boiling point: the
extent of lowering is proportional to the w