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

## 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 is defined as the mass of a constituent divided by the volume of the mixture :

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

### Molar concentration

The molar concentration is defined as the amount of a constituent (in moles) divided by the volume of the mixture :

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

### Number concentration

The number concentration is defined as the number of entities of a constituent in a mixture divided by the volume of the mixture :

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

### Volume concentration

The **volume concentration** (not to be confused with volume fraction) is defined as the volume of a constituent divided by the volume of the mixture :

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 used here is normative in German literature (see Volumenkonzentration).

## 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 divided by an equivalence factor . Since the definition of the equivalence factor depends on context (which reaction is being studied), IUPAC and NIST discourage the use of normality.

### Molality

(Not to be confused with Molarity)

The molality of a solution is defined as the amount of a constituent (in moles) divided by the mass of the solvent (**not** the mass of the solution):

The SI unit for molality is mol/kg.

### Mole fraction

The mole fraction is defined as the amount of a constituent (in moles) divided by the total amount of all constituents in a mixture :

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 is defined as the amount of a constituent divided by the total amount of all *other* constituents in a mixture:

If is much smaller than , 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 is the fraction of one substance with mass to the mass of the total mixture , 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 is defined as the mass of a constituent divided by the total mass of all *other* constituents in a mixture:

If is much smaller than , 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.