Volume fraction

Volume percent is the concentration of a certain solute, measured by volume, in a solution. It has as a denominator the volume of the mixture itself, as usual for expressions of concentration,[2] rather than the total of all the individual components’ volumes prior to mixing:

${\displaystyle {\text{volume percent}}={\frac {\text{volume of solute}}{\text{volume of solution}}}\times 100\%={\text{volume concentration}}\times 100\%}$

Volume percent is usually used when the solution is made by mixing two fluids, such as liquids or gases. However, percentages are only additive for ideal gases.[3]

The percentage by volume (vol%) is one way of expressing the composition of a mixture with a dimensionless quantity; mass fraction (percentage by weight, wt%) and mole fraction (percentage by moles, mol%) are others.

In the case of a mixture of ethanol and water, which are miscible in all proportions, the designation of solvent and solute is arbitrary. The volume of such a mixture is slightly less than the sum of the volumes of the components. Thus, by the above definition, the term "40% alcohol by volume" refers to a mixture of 40 volume units of ethanol with enough water to make a final volume of 100 units, rather than a mixture of 40 units of ethanol with 60 units of water. The "enough water" is actually slightly more than 60 volume units, since water-ethanol mixture loses volume due to intermolecular attraction.

Volume fraction is related to mass fraction,

${\displaystyle Y_{i}\equiv {\frac {m_{i}}{\sum _{j}m_{j}}}={\frac {m_{i}}{m_{tot}}}}$

by

${\displaystyle Y_{i}={\frac {\rho _{i}\varphi _{i}}{\rho _{m}}},\rho _{i}\equiv {\frac {m_{i}}{V_{i}}},\rho _{m}\equiv \sum _{j}{\rho _{j}\varphi _{j}}}$

where ${\displaystyle \rho _{i}\ }$is the constituent density, and ${\displaystyle \rho _{m}}$ is the mixture density.

• Alcohol by volume
• Breathalyzer
• Alcohol proof
• Apparent molar property
• For non-ideal mixtures, see Partial molar volume and Excess molar quantity
• Percentage
• Mass fraction (chemistry)