Solutes, or substances capable of solution, may be divided into two classes—substances which are capable of crystallization, or crystalloids; and those which are incapable of crystallization, the colloids. Crystalloids may be divided again into two classes, those whose solutions are ionizable and therefore conduct electricity, chiefly salts, acids, and bases; and those whose solutions are non-ionizable and are therefore non-conductors. These latter are for the most part crystallizable substances of organic origin, such as sugars, urea, etc.
Avogadro's law asserts that under similar conditions of temperature and pressure, equal volumes of various gases
contain an equal number of molecules. Under similar conditions, the molecular weights of different substances have therefore the same ratio as the weights of equal volumes of their vapours. Hence if we fix arbitrarily the molecular weight of any one substance, the molecular weight of all other substances is thereby determined. The molecular weight of hydrogen has been arbitrarily fixed as two, and hence the molecular weight of any substance will be double its gaseous density when compared with that of hydrogen.
Gramme-Molecule.—A gramme-molecule is the molecular weight of a body expressed in grammes. Occasionally for brevity a gramme-molecule is spoken of as a "molecule." Thus we may say that the molecular weight of oxygen is 16 grammes, meaning thereby that there are the same number of molecules in 16 grammes of oxygen as there are atoms in 1 gramme of hydrogen.
Concentration.—The concentration of a solution is the ratio between the quantity of the solute and the quantity of the solvent. The concentration of a solution is expressed in various ways. (a) The weight of solute dissolved in 100 grammes of the solvent. (b) The weight of solute present in 100 grammes of the solution. (c) The weight of solute dissolved in a litre of the solvent. (d) The weight of solute in a litre of the solution. The most usual method is to give the concentration as the weight of solute dissolved in 100 grammes or in one litre of the solvent.
Molecular Concentration.—Many of the physical and biological properties of a solution are proportional, not to its mass or weight concentration, but to its molecular concentration, i.e. to the number of gramme-molecules of the solute contained in a litre of the solution. Many physical properties are quite independent of the nature of the solute, depending only on its degree of molecular concentration.
Normal Solution.—A normal solution is one which contains one gramme-molecule of the solute per litre. A decinormal solution contains one-tenth of a gramme-molecule of the solute per litre, and a centinormal solution one-hundredth of a gramme-molecule. A normal solution of urea, for example,
contains 60 grammes of urea per litre, while a normal solution of sugar contains 342 grammes of sugar per litre.
The Dissolved Substance is a Gas.—Van t' Hoff, using the data obtained by the botanist Pfeffer, showed that the dissolved matter in a solution behaved exactly as if it were a gas. The analogy is complete in every respect. Like the gaseous molecules, the molecules of a solute are mobile with respect to one another. Like those of a gas, the molecules of a solute tend to spread themselves equally, and to fill the whole space at their disposal, i.e. the whole volume of the solution. The surface of the solution represents the vessel containing the gas, which confines it within definite limits and prevents further expansion.
Osmotic Pressure.—Like the molecules of a gas, the molecules of a solute exercise pressure on the boundaries of the space containing it. This osmotic pressure follows exactly the same laws as gaseous pressure. It has the same constants, and all the notions acquired by the study of gaseous pressure are applicable to osmotic pressure. Osmotic pressure is in fact the gaseous pressure of the molecules of the solute.