= Concentration
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<Concentration> quantifies how much of a named constituent is present relative to a specified sample measure. In chemical terminology, volumetric concentrations use the final mixture volume: <mass concentration> is mass per volume and <molar concentration> is amount of substance per volume. <Mass fraction> and <amount fraction> instead compare a constituent with the corresponding total quantity in the mixture.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Entries C01222 concentration, M03713 mass concentration, A00295 amount concentration and A00296 amount fraction]{ref}
The distinction matters in transport models. If an isolated sample expands without exchanging constituents, its volumetric concentration decreases even though its composition fractions stay fixed. A scalar called concentration in an advection model may represent a fraction instead, so its definition must be stated.
Conservation applies to the transported constituent amount. Reactions, diffusion and exchange then determine how the chosen concentration changes relative to its carrier and denominator.
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