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 term concentration can be applied to any kind of chemical mixture, but most frequently it refers to solutes and solvents in solutions. The molar (amount) concentration has variants such as normal concentration and osmotic concentration.

## Contents

## 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 *fully* 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

The SI unit is kg/m^{3} (equal to g/L).

## Molar concentration

The molar concentration

The SI unit is mol/m^{3}. However, more commonly the unit mol/L (= mol/dm^{3}) is used.

## Number concentration

The number concentration

The SI unit is 1/m^{3}.

## Volume concentration

The **volume concentration**

Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%; its unit is 1.

## 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

## Molality

(Not to be confused with Molarity)

The molality of a solution **not** the mass of the solution):

The SI unit for molality is mol/kg.

## Mole fraction

The mole fraction

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 *other* constituents in a mixture:

If

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

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 *other* constituents in a mixture:

If

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

## Dependence on volume

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