 Binary mixture

A binary mixture is a mixture with two specified chemical components. Water mixed with ethanol is a familiar example. “Binary” counts the components used to describe composition, rather than the number of thermodynamic phases: two components can form one homogeneous mixture or coexist in distinct thermodynamic phases.^[ref]

For two nonreacting components with amounts of substance $n_A,n_B$, the amount fractions satisfyOnly one composition coordinate is independent. In an original preparation using1mole of ethanol and3moles of water, $x_{\mathrm{ethanol}}=1/4$. Using rounded molar masses46.07and18.015grams per mole gives the ethanol mass fractionThus25percent by amount of substance and about46percent by mass describe the same preparation. A composition percentage without its basis is ambiguous. The calculation uses component masses, without assuming additive liquid volume.

Hexane and water illustrate why component count and phase count differ. Their limited mutual solubility permits two liquid thermodynamic phases, each containing some of the other component. A primary compiled experiment equilibrated the liquids by overnight shaking, sampled the aqueous portion with a microsyringe, and measured dissolved hydrocarbon by gas chromatography with flame-ionization detection.^[ref] The liquid immiscibility article develops a lever rule calculation distinguishing each phase's composition from the overall composition.

A binary mixture is not automatically an ideal solution. Interactions between unlike molecules can change activities, mixing enthalpy and phase stability. Stating temperature, pressure and the composition basis is therefore part of specifying the system, rather than optional experimental detail.