Saturation (chemistry)

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In chemistry, saturation has four different meanings:

1. In physical chemistry, saturation is the point at which a solution of a substance can dissolve no more of that substance and additional amounts of that substance will appear as a precipitate. This point of maximum concentration, the saturation point, depends on the temperature of the liquid as well as the chemical nature of the substances involved. This can be used in the process of recrystallisation to purify a chemical: it is dissolved to the point of saturation in hot solvent, then as the solvent cools and the solubility decreases, excess solute precipitates. Impurities, being present in much lower concentration, do not saturate the solvent and so remain dissolved in the liquid. If a change in conditions (e.g. cooling) means that the concentration is actually higher than the saturation point, the solution has become supersaturated.
2. In physical chemistry, when referring to surface processes, saturation denotes the degree of which a surface is full of something. For example, base saturation refers to the fraction of exchangeable cations that are base cations. Similarly, in environmental soil science, nitrogen saturation means that an ecosystem, such as a soil, can not store any more nitrogen.
3. In organic chemistry, a saturated compound has the maximum amount of hydrogens possible: i.e., no double or triple bonds. In a saturated hydrocarbon chain, every carbon atom is attached to two hydrogen atoms, except carbon atoms at the ends of the chain, which are attached to three hydrogen atoms. In the case of saturated methane, four hydrogen atoms are attached to the single, central carbon atom. Of simple hydrocarbons, alkanes are saturated, and alkenes are unsaturated. The degree of unsaturation is a method of specifying the amount that a compound is partially saturated. In the modern treatment of electronic structure, unsaturated compounds are characterized by pi electron systems. The term is applied similarly to the fatty acid constituents of lipids, where the fat is described as saturated or unsaturated, depending on whether the constituent fatty acids contain carbon-carbon double bonds. Unsaturated is used when any carbon structure contains double or occasionally triple bonds. Many vegetable oils contain fatty acids with one (monounsaturated) or more (polyunsaturated) double bonds in them.
4. In biochemistry, the term saturation refers to the fraction of total protein binding sites that are occupied at any given time.