Composition of Mixtures

A pure compound is built from atoms locked together in a fixed ratio, so its mass percent composition never changes from sample to sample — that's the law of definite proportions. A mixture has no such law to obey. Two scoops from the same jar of mixed powders can have different ratios of the ingredients, because the components are just physically combined, not chemically bonded in fixed amounts. So if you analyze several samples from one source and the percent composition shifts around, you're not looking at one compound — you're looking at a mixture.

Elemental analysis exposes this by heating, dissolving, or reacting a sample and measuring how much of some product forms. If the sample is a mixture of a reactive compound and an inert one, only part of the sample reacts, and the amount of product depends on exactly how much of the reactive compound happened to be in that scoop. Run the same procedure on another sample from the same source, get a different answer, and you've proven it's a mixture, not a pure substance.

This sits right after mole conversions because every elemental analysis problem is really a mole ratio problem: you convert a measured mass into moles, use a chemical equation's ratio to find moles of the species you actually care about, then convert back to mass. The next step in the unit builds on this by using percent composition to pin down empirical formulas, so getting comfortable with the grams-to-moles-to-grams pipeline here pays off immediately.