Homogeneous mixture

A homogeneous mixture has the same macroscopic composition throughout the region being described, at the scale of representative sampling. It contains more than one substance, unlike a pure chemical compound. A well-mixed chemical solution is a familiar example. The IUPAC definition of a chemical solution describes a liquid or solid thermodynamic phase containing more than one substance, with the solvent and solute assigned different roles.^[ref]

For an invented sampling example, dissolve 10 millimoles of a conserved solute and bring the final volume to 100 millilitres. After complete mixing, a representative 10-millilitre aliquot contains 1 millimole: $n_{\mathrm{aliquot}}=n_{\mathrm{total}}{V}_{\mathrm{aliquot}}/V_{\mathrm{total}}$. Its molar concentration equals that of the whole chemical solution, although its total amount of substance is smaller. Sampling from different heights provides a practical test for incomplete mixing.

Uniformity refers to the chosen physical description. A tiny region containing only a few molecules can fluctuate in composition; this does not contradict macroscopic homogeneity. Conversely, a vessel containing clear salt chemical solution above undissolved sodium chloride is heterogeneous as a whole, even if the liquid alone is homogeneous. The boundary of the described sample matters as much as its appearance.