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, rather than the total of all the individual components’ volumes prior to mixing: :\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. The
percentage by volume (
vol%,
% v/v) 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. ==Relation to mass fraction==