[Sidenote: Action of water in physiological chemistry]
Water is termed by the chemist a stable compound. This means that it is
difficult to get it to act chemically. Being thus inactive chemically,
we find that water does not combine with most substances. There are
exceptions to this, however, especially in physiological chemistry, an
instance being that starch combines with water when it is changed to
sugar in the process of digestion.
[Sidenote: Water as a solvent]
Water is the universal solvent. A greater number of substances dissolve
in it than in any other liquid. Chemical operations are frequently
carried on in solution, that is to say, the substances which are to act
chemically upon each other are first dissolved in water. The object of
this is to get the substances into as close contact as possible. If
we rub two solids together, the particles remain slightly separated,
no matter how finely the mixture may be powdered. If, however, the
substances are dissolved and the solutions poured together, the
particles of the liquid move so freely among each other that they come
in direct contact, thus aiding chemical action. In some cases substances
which do not act on each other at all when brought together in dry
condition, act readily when brought together in solution.
There is a limit to the amount of any substance which can be held in
solution at a given temperature.
[Sidenote: Chemical meaning of solution]
The question will probably arise in the mind of the student as to
whether a substance dissolved in water has chemically united with the
water, or is merely mixed. Solution is in reality a process about half
way between mixing dry substances and forming chemical combinations.
The chemist considers that the water does not form a compound with the
substance dissolved, when he can, by evaporating the water, get the
substance back into its original form.