Inchemistry,concentration is theabundance 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, andvolume concentration.^[1] The concentration can refer to any kind of chemical mixture, but most frequently refers to solutes andsolvents insolutions. The molar (amount) concentration has variants, such asnormal concentration andosmotic concentration.Dilution is reduction of concentration, e.g. by adding solvent to a solution. The verbto concentrate means to increase concentration, the opposite of dilute.

Concentration-,concentratio, action or an act of coming together at a single place, bringing to a common center, was used inpost-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).^[2]

Often in informal, non-technical language, concentration is described in aqualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. Toconcentrate a solution, one must add moresolute (for example, alcohol), or reduce the amount ofsolvent (for example, water). By contrast, todilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances aremiscible, there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to besaturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, whensupersaturation may occur. Instead,phase separation will occur, leading to coexisting phases, either completely separated or mixed as asuspension. 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 calledlevels, reflecting the mentalschema oflevels on the vertical axis of agraph, which can behigh or low (for example, "high serum levels of bilirubin" are concentrations ofbilirubin in theblood serum that are greaterthan normal).

There are four quantities that describe concentration:

The mass concentration${\displaystyle {\rho }_i}$  is defined as themass of a constituent${\displaystyle m_i}$  divided by the volume of the mixture${\displaystyle V}$ :

${\displaystyle {\rho }_i=\frac{m_i}{V}.}$ 

TheSI unit is kg/m³ (equal to g/L).

The molar concentration${\displaystyle c_i}$  is defined as theamount of a constituent${\displaystyle n_i}$  (in moles) divided by the volume of the mixture${\displaystyle V}$ :

${\displaystyle c_i=\frac{n_i}{V}.}$ 

The SI unit is mol/m³. However, more commonly the unit mol/L (= mol/dm³) is used.

The number concentration${\displaystyle C_i}$  is defined as the number of entities of a constituent${\displaystyle N_i}$  in a mixture divided by the volume of the mixture${\displaystyle V}$ :

${\displaystyle C_i=\frac{N_i}{V}.}$ 

The SI unit is 1/m³.

Thevolume concentration${\displaystyle {\sigma }_i}$  (not to be confused withvolume fraction^[3]) is defined as the volume of a constituent${\displaystyle V_i}$  divided by the volume of the mixture${\displaystyle V}$ :

${\displaystyle {\sigma }_i=\frac{V_i}{V}.}$ 

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${\displaystyle {\sigma }_i}$  used here is normative in German literature (seeVolumenkonzentration).

Several other quantities can be used to describe the composition of a mixture. These shouldnot be called concentrations.^[1]

Normality is defined as the molar concentration${\displaystyle c_i}$  divided by an equivalence factor${\displaystyle f_{\mathrm{e}\mathrm{q}}}$ . Since the definition of the equivalence factor depends on context (which reaction is being studied), theInternational Union of Pure and Applied Chemistry andNational Institute of Standards and Technology discourage the use of normality.

The molality of a solution${\displaystyle b_i}$  is defined as the amount of a constituent${\displaystyle n_i}$  (in moles) divided by the mass of the solvent${\displaystyle m_{\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}}$  (not the mass of the solution):

${\displaystyle b_i=\frac{n_i}{m_{\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}}}.}$ 

The SI unit for molality is mol/kg.

The mole fraction${\displaystyle x_i}$  is defined as the amount of a constituent${\displaystyle n_i}$  (in moles) divided by the total amount of all constituents in a mixture${\displaystyle n_{\mathrm{t}\mathrm{o}\mathrm{t}}}$ :

${\displaystyle x_i=\frac{n_i}{n_{\mathrm{t}\mathrm{o}\mathrm{t}}}.}$ 

The SI unit is mol/mol. However, the deprecatedparts-per notation is often used to describe small mole fractions.

The mole ratio${\displaystyle r_i}$  is defined as the amount of a constituent${\displaystyle n_i}$  divided by the total amount of allother constituents in a mixture:

${\displaystyle r_i=\frac{n_i}{n_{\mathrm{t}\mathrm{o}\mathrm{t}}-n_i}.}$ 

If${\displaystyle n_i}$  is much smaller than${\displaystyle n_{\mathrm{t}\mathrm{o}\mathrm{t}}}$ , 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.

The mass fraction${\displaystyle w_i}$  is the fraction of one substance with mass${\displaystyle m_i}$  to the mass of the total mixture${\displaystyle m_{\mathrm{t}\mathrm{o}\mathrm{t}}}$ , defined as:

${\displaystyle w_i=\frac{m_i}{m_{\mathrm{t}\mathrm{o}\mathrm{t}}}.}$ 

The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions.

The mass ratio${\displaystyle {\zeta }_i}$  is defined as the mass of a constituent${\displaystyle m_i}$  divided by the total mass of allother constituents in a mixture:

${\displaystyle {\zeta }_i=\frac{m_i}{m_{\mathrm{t}\mathrm{o}\mathrm{t}}-m_i}.}$ 

If${\displaystyle m_i}$  is much smaller than${\displaystyle m_{\mathrm{t}\mathrm{o}\mathrm{t}}}$ , 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.

Concentration depends on the variation of the volume of the solution with temperature, due mainly tothermal expansion.

• Dilution ratio – Change in concentration when mixing two liquids
• Dose concentration – Ratio of part of a mixture to the wholePages displaying short descriptions of redirect targets
• Serial dilution – Step-wise dilution of a substance in solution
• Wine/water mixing problem
• Standard state § Solutes

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