= 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.<Solution in IUPAC Gold Book>[EntryS05746, definition and solvent/solute convention]{ref}
For an invented sampling example, dissolve 10 millimoles of a conserved <solute> and bring the final <volume (thermodynamics)> 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.
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