A substance is soluble in a fluid if it dissolves in the fluid. The dissolved substance is called the solute and the dissolving fluid (usually present in excess) is called the solvent, which together form a solution. The process of dissolving is called solvation, or hydration if the solvent is water.
A solution at equilibrium cannot hold any more solute and is said to be saturated. Solutions may, under special conditions, hold more solute than the solvent can normally dissolve. This is called supersaturation. The maximum equilibrium amount of solute which can normally dissolve per amount of solvent (or solution) is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. The solubility of one substance dissolving in another is determined by the intermolecular forces between the solvent and solute, temperature, the entropy change that accompanies the solvation, the presence and amount of other substances, and sometimes pressure or partial pressure of a solute gas.
For inorganic salts, solubility in aqueous solutions is often dependent on a solubility constant. The solubility constant is a special case of an equilibrium constant for the reaction of dissolving the salt in question, with the concentration of undissolved compound not in the expression because it is not in the aqueous phase. The solubility constant is also "applicable" (i. e. useful) to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant.
Solvents are normally characterized as polar or nonpolar. Polar solvents will dissolve ionic compounds and covalent compounds which ionize, while nonpolar solvents will dissolve nonpolar covalent compounds. For example, ordinary table salt, an ionic compound, will dissolve in water, but not in ethanol.
Common solvents used in organic chemistry include acetone, ethanol, water, and benzene.
Water and nonpolar solvents are immiscible; they do not form homogenous mixtures but separate into two distinct phases or form milky emulsions.
While solutions are typically thought of as solids being mixed into liquids, any two states of matter can be mixed and be called a solution. Carbonated water is a solution of a gas in a liquid, hydrogen (a gas) can dissolve in palladium (a solid), and stainless steel is a solution of a solid in a solid (called an alloy).
Last updated: 06-02-2005 14:04:33