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.
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.
IMPORTANCE OF SOLUTION TO THE FOOD SCIENTIST
Solution is very important in the study of foods and human nutrition. Only substances which can be dissolved can be assimilated. Many substances which Relation of solution to assimilation will not dissolve in pure water will dissolve in water which contains something else in solution. The blood is water containing many things in solution. The salts of the blood keep the other food elements in solution, many of which would not dissolve if the blood did not contain these salts. The chief work of the digestive juices is to reduce foods to a soluble form so that they can be taken into the circulation by absorption; otherwise they would pass through the alimentary canal practically unchanged.
Milk as an example of both "Solution" and "Mixture"
We must learn to distinguish carefully between chemical solution and merely mixing things with water. A good example is milk. In addition to water, milk contains principally fat, sugar, and casein. The sugar is truly dissolved in the water. The fat and the casein are fine particles held in suspension. If the milk stands for a while, the fat particles rise to the top as cream. If it stands long enough, the casein particles adhere to each other and settle to the bottom, leaving the water with the dissolved sugar or whey in the middle.
IMPORTANCE OF WATER IN THE HUMAN BODY
Proportion of water and solids in the human body