= 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>.<chemistry-2e-by-paul-flowers-and-others/1.2 Phases and Classification of Matter>[Classification of Matter, homogeneous and heterogeneous mixtures]{ref}
For two nonreacting components with <amounts of substance> $n_A,n_B$, the <amount fractions> satisfy
$$
x_A=\frac{n_A}{n_A+n_B},\qquad x_B=1-x_A.
$$
Only one composition coordinate is independent. In an original preparation using1mole of <ethanol> and3moles of <water>, $x_{\rm ethanol}=1/4$. Using rounded <molar masses>46.07and18.015grams per mole gives the <ethanol> <mass fraction>
$$
w_{\rm ethanol}=\frac{46.07}{46.07+3(18.015)}\approx0.4602.
$$
Thus25percent 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 (thermodynamics)>.
<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.<Hexane with water in the IUPAC-NIST Solubility Database>[Record37_269, Experimental Data and Method/Apparatus/Procedure]{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.
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