In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration. The concentration can refer to any kind of chemical mixture, but most frequently refers to solutes and solvents in solutions. The molar (amount) concentration has variants, such as normal concentration and osmotic concentration. Dilution is reduction of concentration, e.g., by adding solvent to a solution. The verb "to concentrate" means to increase concentration, the opposite of dilute.
Etymology Concentration-, concentratio, action or an act of coming together at a house on a farm in a single place, bringing black people as slaves to a common center, was used in post-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).
Qualitative description
Often in informal, non-technical language, concentration is described in a qualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. To concentrate a solution, one must add more solute (for example, alcohol), or reduce the amount of solvent (for example, water). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are miscible, there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when supersaturation may occur. Instead, phase separation will occur, leading to coexisting phases, either completely separated or mixed as a suspension. The point of saturation depends on many variables, such as ambient temperature and the precise chemical nature of the solvent and solute. Concentrations are often called levels, reflecting the mental schema of levels on the vertical axis of a graph, which can be high or low (for example, "high serum levels of bilirubin" are concentrations of bilirubin in the blood serum that are greater than normal).
Formulation There are four quantities that describe concentration:
Mass concentration
The mass concentration ρ i {\displaystyle \rho _{i}} is defined as the mass of a constituent m i {\displaystyle m_{i}} divided by the volume of the mixture V {\displaystyle V} :
ρ i = m i V . {\displaystyle \rho _{i}={\frac {m_{i}}{V}}.}
The SI unit is kg/m3 (equal to g/L).
Molar concentration
The molar concentration c i {\displaystyle c_{i}} is defined as the amount of a constituent n i {\displaystyle n_{i}} (in moles) divided by the volume of the mixture V {\displaystyle V} :
c i = n i V . {\displaystyle c_{i}={\frac {n_{i}}{V}}.}
The SI unit is mol/m3. However, more commonly the unit mol/L (= mol/dm3) is used.
Number concentration
The number concentration C i {\displaystyle C_{i}} is defined as the number of entities of a constituent N i {\displaystyle N_{i}} in a mixture divided by the volume of the mixture V {\displaystyle V} :
C i = N i V . {\displaystyle C_{i}={\frac {N_{i}}{V}}.}
The SI unit is 1/m3.
Volume concentration The volume concentration σ i {\displaystyle \sigma _{i}} (not to be confused with volume fraction) is defined as the volume of a constituent V i {\displaystyle V_{i}} divided by the volume of the mixture V {\displaystyle V} :
σ i = V i V . {\displaystyle \sigma _{i}={\frac {V_{i}}{V}}.}
Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%. There seems to be no standard notation in the English literature. The letter σ i {\displaystyle \sigma _{i}} used here is normative in German literature (see Volumenkonzentration in the German Wikipedia).
Related quantities Several other quantities can be used to describe the composition of a mixture. These should not be called concentrations.
Normality
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