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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: [[Concentration#Mass concentration|mass concentration]], [[Concentration#Molar concentration|molar concentration]], [[Concentration#Number concentration|number concentration]], and [[Concentration#Volume concentration|volume concentration]].<ref name="goldbook">{{GoldBookRef | file = C01222 | title = concentration}}</ref> The term concentration can be applied to any kind of chemical mixture, but most frequently it refers to [[solute]]s and solvents in [[solution]]s. The molar (amount) concentration has variants such as [[normal concentration]] and [[osmotic concentration]].
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| ==Qualitative description==
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| [[Image:Dilution-concentration simple example.jpg|frame|right|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.]]
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| Often in informal, non-technical language, concentration is described in a [[qualitative data|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 [[solution|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 [[Saturation (chemistry)|saturated]]. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when [[supersaturation]] may occur. Instead, [[Phase (matter)#Phase separation|phase separation]] will occur, leading to coexisting phases, either completely separated or mixed as a [[suspension (chemistry)|suspension]]. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.
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| ==Quantitative notation==
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| There are four quantities that describe concentration:
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| ===Mass concentration===
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| {{main|Mass concentration (chemistry)}}
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| The mass concentration <math>\rho_i</math> is defined as the [[mass]] of a constituent <math>m_i</math> divided by the volume of the mixture <math>V</math>:
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| :<math>\rho_i = \frac {m_i}{V}.</math>
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| The [[International System of Units|SI unit]] is kg/m<sup>3</sup> (equal to g/L).
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| ===Molar concentration===
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| {{main|Molar concentration}}
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| The molar concentration <math>c_i</math> is defined as the [[amount of substance|amount]] of a constituent <math>n_i</math> divided by the volume of the mixture <math>V</math>:
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| :<math>c_i = \frac {n_i}{V}.</math>
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| The [[International System of Units|SI unit]] is mol/m<sup>3</sup>. However, more commonly the unit mol/L (= mol/dm<sup>3</sup>) is used.
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| ===Number concentration=== | |
| {{main|Number concentration}}
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| The number concentration <math>C_i</math> is defined as the number of entities of a constituent <math>N_i</math> in a mixture divided by the volume of the mixture <math>V</math>:
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| :<math>C_i = \frac{N_i}{V}.</math>
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| The [[International System of Units|SI unit]] is 1/m<sup>3</sup>.
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| ===Volume concentration===
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| {{main|Volume concentration}}
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| The '''volume concentration''' <math>\phi_i</math> (also called '''volume fraction''') is defined as the volume of a constituent <math>V_i</math> divided by the volume of all constituents of the mixture <math>V</math> prior to mixing:
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| :<math>\phi_i = \frac {V_i}{V}.</math>
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| Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%; its unit is 1.
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| ==Related quantities==
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| Several other quantities can be used to describe the composition of a mixture. Note that these should '''not''' be called concentrations.<ref name="goldbook" />
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| ===Normality===
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| {{main|Normality (chemistry)}}
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| Normality is defined as the molar concentration <math>c_i</math> divided by an equivalence factor <math>f_\mathrm{eq}</math>. Since the definition of the equivalence factor may not be unequivocal, [[IUPAC]] and [[NIST]] discourage the use of normality.
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| ===Molality=== | |
| {{main|Molality}}
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| (Not to be confused with [[Molarity]])
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| The molality of a solution <math>b_i</math> is defined as the [[amount of substance|amount]] of a constituent <math>n_i</math> divided by the mass of the solvent <math>m_\mathrm{solvent}</math> ('''not''' the mass of the solution):
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| :<math>b_i = \frac{n_i}{m_\mathrm{solvent}}.</math>
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| The [[International System of Units|SI unit]] for molality is mol/kg.
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| ===Mole fraction===
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| {{main|Mole fraction}}
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| The mole fraction <math>x_i</math> is defined as the [[amount of substance|amount]] of a constituent <math>n_i</math> divided by the total amount of all constituents in a mixture <math>n_\mathrm{tot}</math>:
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| :<math>x_i = \frac {n_i}{n_\mathrm{tot}}.</math> | |
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| The [[International System of Units|SI unit]] is mol/mol. However, the deprecated [[parts-per notation]] is often used to describe small mole fractions.
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| ===Mole ratio===
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| {{main|Mixing ratio}}
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| The mole ratio <math>r_i</math> is defined as the [[amount of substance|amount]] of a constituent <math>n_i</math> divided by the total amount of all ''other'' constituents in a mixture:
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| :<math>r_i = \frac{n_i}{n_\mathrm{tot}-n_i}.</math>
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| If <math>n_i</math> is much smaller than <math>n_\mathrm{tot}</math>, the mole ratio is almost identical to the mole fraction.
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| The [[International System of Units|SI unit]] is mol/mol. However, the deprecated [[parts-per notation]] is often used to describe small mole ratios.
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| ===Mass fraction=== | |
| {{main|Mass fraction (chemistry)}}
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| The mass fraction <math>w_i</math> is the fraction of one substance with mass <math>m_i</math> to the mass of the total mixture <math>m_\mathrm{tot}</math>, defined as:
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| :<math>w_i = \frac {m_i}{m_\mathrm{tot}}.</math> | |
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| The [[International System of Units|SI unit]] is kg/kg. However, the deprecated [[parts-per notation]] is often used to describe small mass fractions.
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| ===Mass ratio===
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| {{main|Mixing ratio}}
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| The mass ratio <math>\zeta_i</math> is defined as the mass of a constituent <math>m_i</math> divided by the total mass of all ''other'' constituents in a mixture:
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| :<math>\zeta_i = \frac{m_i}{m_\mathrm{tot}-m_i}.</math>
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| If <math>m_i</math> is much smaller than <math>m_\mathrm{tot}</math>, the mass ratio is almost identical to the mass fraction.
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| The [[International System of Units|SI unit]] is kg/kg. However, the deprecated [[parts-per notation]] is often used to describe small mass ratios.
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| ==Dependence on volume==
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| Concentration depends on the variation of the volume of the solution due mainly to thermal expansion.
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| == Table of concentrations and related quantities ==
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| {| class="wikitable"
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| |-
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| ! Concentration type
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| ! Symbol
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| ! Definition
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| ! SI unit
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| ! other unit(s)
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| |-
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| | mass concentration
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| | <math>\rho_i</math> or <math>\gamma_i</math>
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| | <math>m_i/V</math>
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| | kg/m<sup>3</sup>
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| | g/100mL (= g/dL)
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| |-
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| | molar concentration
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| | <math>c_i</math>
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| | <math>n_i/V</math>
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| | mol/m<sup>3</sup>
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| | M (= mol/L)
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| |-
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| | number concentration
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| | <math>C_i</math>
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| | <math>N_i/V</math>
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| | 1/m<sup>3</sup>
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| | 1/cm<sup>3</sup>
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| |-
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| | volume concentration
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| | <math>\phi_i</math>
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| | <math>V_i/V</math>
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| | m<sup>3</sup>/m<sup>3</sup>
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| |-
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| ! Related quantities
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| ! Symbol
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| ! Definition
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| ! SI unit
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| ! other unit(s)
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| |-
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| | normality
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| | <math>c_i/f_\mathrm{eq}</math>
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| | mol/m<sup>3</sup>
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| | N (= mol/L)
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| |-
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| | molality
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| | <math>b_i</math>
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| | <math>n_i/m_\mathrm{solvent}</math>
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| | mol/kg
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| |-
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| | mole fraction
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| | <math>x_i</math>
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| | <math>n_i/n_\mathrm{tot}</math>
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| | mol/mol
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| | ppm, ppb, ppt
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| |-
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| | mole ratio
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| | <math>r_i</math>
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| | <math>n_i/(n_\mathrm{tot}-n_i)</math>
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| | mol/mol
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| | ppm, ppb, ppt
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| |-
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| | mass fraction
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| | <math>w_i</math>
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| | <math>m_i/m_\mathrm{tot}</math>
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| | kg/kg
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| | ppm, ppb, ppt
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| |-
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| | mass ratio
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| | <math>\zeta_i</math>
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| | <math>m_i/(m_\mathrm{tot}-m_i)</math>
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| | kg/kg
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| | ppm, ppb, ppt
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| |}
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| ==See also==
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| * [[Dose concentration]]
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| * [[Serial dilution]]
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| * [[Wine/water mixing problem]]
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| ==References==
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| {{reflist}}
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| {{Chemical solutions}}
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| <!--Categories-->
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| [[Category:Analytical chemistry]]
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| [[Category:Chemical properties]]
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| [[el:Συγκέντρωση]]
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