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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 Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
'', ''
number concentration A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...
'', 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 Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...
.


Etymology

The term concentration comes from the word concentrate, from the French , from con– + center, meaning “to put at the center”.


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 In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
(for example, alcohol), or reduce the amount of
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
(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 Phase separation is the creation of two distinct phases from a single homogeneous mixture. The most common type of phase separation is between two immiscible liquids, such as oil and water. Colloids are formed by phase separation, though ...
will occur, leading to coexisting phases, either completely separated or mixed as a
suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspende ...
. 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 The word schema comes from the Greek word ('), which means ''shape'', or more generally, ''plan''. The plural is ('). In English, both ''schemas'' and ''schemata'' are used as plural forms. Schema may refer to: Science and technology * SCHEMA ...
of levels on the vertical axis of a
graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discre ...
, 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 \rho_i is defined as the
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
of a constituent m_i divided by the volume of the mixture V: :\rho_i = \frac . The
SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
is kg/m3 (equal to g/L).


Molar concentration

The molar concentration c_i is defined as the
amount Quantity or amount is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value multiple of a unit ...
of a constituent n_i (in moles) divided by the volume of the mixture V: :c_i = \frac . 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. The SI unit is 1/m3.


Volume concentration

The volume concentration \sigma_i (not to be confused with
volume fraction In chemistry and fluid mechanics, the volume fraction φ''i'' is defined as the volume of a constituent ''V'i'' divided by the volume of all constituents of the mixture ''V'' prior to mixing: :\phi_i = \frac Being dimensionless, its unit is ...
) is defined as the volume of a constituent V_i divided by the volume of the mixture V: :\sigma_i = \frac . 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 \sigma_i 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 c_i divided by an equivalence factor f_\mathrm. Since the definition of the equivalence factor depends on context (which reaction is being studied), the
International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
and
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
discourage the use of normality.


Molality

Not to be confused with
molarity Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solu ...
. The molality of a solution b_i is defined as the amount of a constituent n_i (in moles) divided by the mass of the solvent m_\mathrm (not the mass of the solution): :b_i = \frac. The SI unit for molality is mol/kg.


Mole fraction

The mole fraction x_i is defined as the amount of a constituent n_i (in moles) divided by the total amount of all constituents in a mixture n_\mathrm: :x_i = \frac . The SI unit is mol/mol. However, the deprecated
parts-per notation In science and engineering, the parts-per 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 quantity-per-quantity measures, th ...
is often used to describe small mole fractions.


Mole ratio

The mole ratio r_i is defined as the amount of a constituent n_i divided by the total amount of all ''other'' constituents in a mixture: :r_i = \frac. If n_i is much smaller than n_\mathrm, 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 w_i is the fraction of one substance with mass m_i to the mass of the total mixture m_\mathrm, defined as: :w_i = \frac . 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 \zeta_i is defined as the mass of a constituent m_i divided by the total mass of all ''other'' constituents in a mixture: :\zeta_i = \frac. If m_i is much smaller than m_\mathrm, 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 Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
.


Table of concentrations and related quantities


See also

* * * *


References

{{authority control Analytical chemistry Chemical properties