Concentration
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. Etymology. The term concentration comes from the word concentrate, from the French concentrer, from con– + center, meaning “to put at the center”. 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). Quantitative  notation. There are four quantities that describe concentration: Mass concentration. The mass concentration formula_1 is defined as the mass of a constituent formula_2 divided by the volume of the mixture formula_3: The SI unit is kg/m (equal to g/L). Molar concentration. The molar concentration formula_5 is defined as the amount of a constituent formula_6 (in moles) divided by the volume of the mixture formula_3: The SI unit is mol/m. However, more commonly the unit mol/L (= mol/dm) is used. Number concentration. The number concentration formula_9 is defined as the number of entities of a constituent  formula_10 in a mixture divided by the volume of the mixture formula_3: The SI unit is 1/m. Volume concentration. The volume concentration formula_13 (not to be confused with volume fraction) is defined as the volume of a constituent formula_14 divided by the volume of the mixture formula_3: Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%; its unit is 1. There seems to be no standard notation in the English literature. The letter formula_13 used here is normative in German literature (see ). Related quantities. Several other quantities can be used to describe the composition of a  mixture. Note that these should not be called concentrations. Normality. Normality is defined as the molar concentration formula_5 divided by an equivalence factor formula_19. Since the definition of the equivalence factor depends on context (which reaction is being studied), IUPAC and NIST discourage the use of normality. Molality. The molality of a solution formula_20 is defined as the amount of a constituent formula_6 (in moles) divided by the mass of the solvent formula_22 (not the mass of the solution): The SI unit for molality is mol/kg. Mole fraction. The mole fraction formula_24 is defined as the amount of a  constituent formula_6 (in moles) divided by the total amount of all constituents in a mixture formula_26: The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole fractions. Mole ratio. The mole ratio formula_28 is defined as the amount of a constituent formula_6 divided by the total amount of all "other" constituents in a mixture: If formula_6 is much smaller than formula_26, the mole ratio is almost identical to the mole fraction. The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios . Mass fraction. The mass fraction formula_33 is the fraction of one substance with mass formula_2 to the mass of the total mixture formula_35, defined as: The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions. Mass ratio. The mass ratio formula_37 is defined as the mass of a constituent formula_2 divided by the total mass of all "other" constituents in a mixture: If formula_2 is much smaller than formula_35, the mass ratio is almost identical to the mass fraction. The SI unit is kg/kg. However, the deprecated parts-per  notation is often used to describe small mass ratios. Dependence on volume and temperature. Concentration depends on the variation of the volume of the solution with temperature due mainly to thermal expansion.