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In chemistry and physics, the dimensionless **mixing ratio** is the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio (see concentration) or mass ratio (see stoichiometry).^{[1]}

In atmospheric chemistry, mixing ratio usually refers to the **mole ratio** *r _{i}*, which is defined as the amount of a constituent

The mole ratio is also called **amount ratio**.^{[2]}
If *n _{i}* is much smaller than

In meteorology, mixing ratio usually refers to the **mass ratio** of water , which is defined as the mass of water divided by the mass of dry air () in a given air parcel:^{[3]}

The unit is typically given in . The definition is similar to that of specific humidity.

Two binary solutions of different compositions or even two pure components can be mixed with various mixing ratios by masses, moles, or volumes.

The mass fraction of the resulting solution from mixing solutions with masses *m*_{1} and *m*_{2} and mass fractions *w*_{1} and *w*_{2} is given by:

where *m*_{1} can be simplified from numerator and denominator

and

is the mass mixing ratio of the two solutions.

By substituting the densities *ρ _{i}*(

Considering a volume mixing ratio *r*_{V(21)}

The formula can be extended to more than two solutions with mass mixing ratios

to be mixed giving:

The condition to get a partially ideal solution on mixing is that the volume of the resulting mixture *V* to equal double the volume *V*_{s} of each solution mixed in equal volumes due to the additivity of volumes. The resulting volume can be found from the mass balance equation involving densities of the mixed and resulting solutions and equalising it to 2:

implies

Of course for real solutions inequalities appear instead of the last equality.

Mixtures of different solvents can have interesting features like anomalous conductivity (electrolytic) of particular lyonium ions and lyate ions generated by molecular autoionization of protic and aprotic solvents due to Grotthuss mechanism of ion hopping depending on the mixing ratios. Examples may include hydronium and hydroxide ions in water and water alcohol mixtures, alkoxonium and alkoxide ions in the same mixtures, ammonium and amide ions in liquid and supercritical ammonia, alkylammonium and alkylamide ions in ammines mixtures, etc....

**^**IUPAC,*Compendium of Chemical Terminology*, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "mixing ratio". doi:10.1351/goldbook.M03948**^**"Pure and Applied Chemistry, 2008, Volume 80, No. 2, pp. 233-276".*Iupac.org*. 2016-06-14. Retrieved 2016-06-30.**^**Whiteman, D.N. (2015).*Encyclopedia of Atmospheric Sciences*(Second Edition, Volume 3 ed.). Elsevier Ltd. p. 298. ISBN 978-0-12-382225-3.