In
chemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
,
concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types 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
soluteSolute may refer to:* Solute, UMIK or UBOOK desolving in a substance,forming INT/INTY* Solute , a group of Paleozoic echinoderms...
s in homogeneous
solutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving. Types of solutions :...
s.
Qualitative description
Often in informal, nontechnical 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
soluteIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving. Types of solutions :...
(for example, alcohol), or reduce the amount of
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
(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
saturatedIn chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
. If additional solute is added to a saturated solution, it will not dissolve (except in certain circumstances, when
supersaturationThe term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...
may occur). Instead, phase separation will occur, leading to coexisting phases (either completely separated or mixed as a
suspensionIn chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.
Mass concentration
The mass concentration
is defined as the
massMass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of a constituent
divided by the volume of the mixture
:
The
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is kg/m
^{3}.
Molar concentration
The molar concentration
is defined as the
amountAmount of substance is a standardsdefined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
of a constituent
divided by the volume of the mixture
:
The
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is mol/m
^{3}. However, more commonly the unit mol/L 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
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is 1/m
^{3}.
Volume concentration
The
volume concentration (also called
volume fraction) is defined as the volume of a constituent
divided by the volume of all consituents of the mixture
prior to mixing:
The
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is m
^{3}/m
^{3}.
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 may not be unequivocal, IUPAC and NIST discourage the use of normality.
Molality
The molality of a solution
is defined as the
amountAmount of substance is a standardsdefined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
of a constituent
divided by the mass of the solvent
(
not the mass of the solution):
The
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
for molality is mol/kg.
Mole fraction
The mole fraction
is defined as the
amountAmount of substance is a standardsdefined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
of a constituent
divided by the total amount of all constituents in a mixture
:
The
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is mol/mol. However, the deprecated
partsper notationIn science and engineering, the partsper notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantityperquantity measures, they are pure numbers with no associated units of measurement...
is often used to describe small mole fractions.
Mole ratio
The mole ratio
is defined as the
amountAmount of substance is a standardsdefined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
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
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is mol/mol. However, the deprecated
partsper notationIn science and engineering, the partsper notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantityperquantity measures, they are pure numbers with no associated units of measurement...
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
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is kg/kg. However, the deprecated
partsper notationIn science and engineering, the partsper notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantityperquantity measures, they are pure numbers with no associated units of measurement...
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
SIunitThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is kg/kg. However, the deprecated
partsper notationIn science and engineering, the partsper notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantityperquantity measures, they are pure numbers with no associated units of measurement...
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.
Table of concentrations and related quantities
Concentration type 
Symbol 
Definition 
SIunit 
other unit(s) 
mass concentration 
or 

kg/m^{3} 
g/100mL (=g/dL) 
molar concentration 


mol/m^{3} 
M (=mol/L) 
number concentration 


1/m^{3} 
1/cm^{3} 
volume concentration 


m^{3}/m^{3} 

Related quantities 
Symbol 
Definition 
SIunit 
other unit(s) 
normality 


mol/m^{3} 
M (=mol/L) 
molality 


mol/kg 

mole fraction 


mol/mol 
ppm, ppb, ppt 
mole ratio 


mol/mol 
ppm, ppb, ppt 
mass fraction 


kg/kg 
ppm, ppb, ppt 
mass ratio 


kg/kg 
ppm, ppb, ppt 
See also