Encyclopedia

Concentration

Ratio of part of a mixture to the whole

Wikipedia contributorsAdapted for Fathomly · View authors

4 min read
Source updated September 5, 2026

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.

01Etymology

Concentration-, concentratio, action or an act of coming together at a house on a farm in a single place, was used in post-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).

These glasses containing red dye demonstrate qualitative changes in concentration. The solutions on the left are more dilute, compared to the more concentrated solutions on the right.

02Qualitative 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).

03Formulation

There are four quantities that describe concentration:

Mass concentration

The mass concentration $\rho _{i}$ is defined as the mass of a constituent $m_{i}$ divided by the volume of the mixture $V$ :

$\rho _{i}={\frac {m_{i}}{V}}.$

The SI unit is kg/m³ (equal to g/L).

Molar concentration

The molar concentration $c_{i}$ is defined as the amount of a constituent $n_{i}$ (in moles) divided by the volume of the mixture $V$ :

$c_{i}={\frac {n_{i}}{V}}.$

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

Number concentration

The number concentration $C_{i}$ is defined as the number of entities of a constituent $N_{i}$ in a mixture divided by the volume of the mixture $V$ :

$C_{i}={\frac {N_{i}}{V}}.$

The SI unit is 1/m³.

Volume concentration

The volume concentration $\sigma _{i}$ (not to be confused with volume fraction) is defined as the volume of a constituent $V_{i}$ divided by the volume of the mixture $V$ :

$\sigma _{i}={\frac {V_{i}}{V}}.$

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 $\sigma _{i}$ used here is normative in German literature ().

05Dependence on volume and temperature

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

06Table of concentrations and related quantities

Concentration type	Symbol	Definition	SI unit	other unit(s)
mass concentration	$\rho _{i}$ or $\gamma _{i}$	$m_{i}/V$	kg/m³	g/100mL (= g/dL)
molar concentration	$c_{i}$	$n_{i}/V$	mol/m³	M (= mol/L)
number concentration	$C_{i}$	$N_{i}/V$	1/m³	1/cm³
volume concentration	$\sigma _{i}$	$V_{i}/V$	m³/m³
Related quantities	Symbol	Definition	SI unit	other unit(s)
normality		$c_{i}/f_{\mathrm {eq} }$	mol/m³	N (= mol/L)
molality	$b_{i}$	$n_{i}/m_{\mathrm {solvent} }$	mol/kg	m
mole fraction	$x_{i}$	$n_{i}/n_{\mathrm {tot} }$	mol/mol	ppm, ppb, ppt
mole ratio	$r_{i}$	$n_{i}/(n_{\mathrm {tot} }-n_{i})$	mol/mol	ppm, ppb, ppt
mass fraction	$w_{i}$	$m_{i}/m_{\mathrm {tot} }$	kg/kg	ppm, ppb, ppt
mass ratio	$\zeta _{i}$	$m_{i}/(m_{\mathrm {tot} }-m_{i})$	kg/kg	ppm, ppb, ppt
volume fraction	$\phi _{i}$	$V_{i}/\sum _{j}V_{j}$	m³/m³	ppm, ppb, ppt

Watch videos about ConcentrationExplainers and documentaries on YouTube (opens in a new tab)

Sources and credits

This article is adapted from the Wikipedia article “Concentration”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

Images, from Wikimedia Commons:

Dilution-concentration simple example.jpg by en:User:FirstPrinciples, Public domain

Fathomly is not affiliated with or endorsed by the Wikimedia Foundation. Spotted a problem? Tell us.

Concentration

01Etymology

02Qualitative description