The solute particles in a solution are generally of molecular size or smaller, much smaller than those in a colloid or a suspension. The solute particles cannot be observed even with an ultramicroscope. They do not settle out from the solvent on standing, and they cannot be separated from the solvent by physical means, such as filtration or centrifugation. On the other hand, a solution differs from a compound in that its components can occur in continuously varying proportions, within certain limits (although within a given solution they are present in the same proportions throughout the solution), while the components of a compound can occur only in certain fixed proportions.
The addition of solute affects the boiling point, freezing point, and vapor pressure of the solution, in general raising the boiling point, depressing the freezing point, and lowering the vapor pressure (see Raoult's law). A number of substances (acids, bases, and salts) exhibit characteristic behavior in aqueous solution. These substances dissociate in water to form positive and negative ions that enable the solution to conduct electricity. Such solutions are called electrolytic (see electrolyte).
The proportion of solute to solvent in a given solution is expressed by the concentration of the solution. Concentrations may be stated in a number of ways, such as giving the amount of solute contained in a given volume of solution or the amount dissolved in a given mass of solvent. A solution having a relatively high concentration is said to be concentrated, and a solution having a low concentration is said to be dilute.
In many solutions the concentration has a maximum limit that depends on various factors, such as temperature, pressure, and the nature of the solvent. The maximum concentration is called the solubility of the solute under those conditions. When a solution contains the maximum amount of solute, it is said to be saturated; if it contains less than that amount, it is unsaturated.
The most obvious factor affecting solubility is the nature of the solvent. Ordinary table salt (sodium chloride) is soluble in water, but only slightly soluble in ethanol, and insoluble in diethyl ether. Temperature is also important in determining solubility. Solids are usually more soluble at higher temperatures; more salt will dissolve in warm water than in an equal amount of cold water. Graphs showing the solubility of different solids as a function of temperature are called solubility curves and are very useful in chemical analysis. Solubility also depends on pressure, especially in the case of gases, which are more soluble at higher pressures.
Under certain conditions a solution may be made to contain more solute than a saturated solution at the same temperature and pressure; such a solution is called supersaturated. If even a single crystal of undissolved solute is added to a supersaturated solution, all the excess solute above the normal solubility concentration will immediately crystallize out of the solution.