What term best describes a substance that cannot be broken down by ordinary chemical means?

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A substance that cannot be broken down by ordinary chemical means is an element. This describes a pure kind of matter made of one type of atom and cannot be decomposed into simpler substances through typical chemical reactions. Using the term "chemical element" specifically reinforces that we're talking about a pure substance defined by its atoms. By contrast, a compound is made from two or more elements bonded together and can be broken down chemically, while a mixture is a physical blend of substances that can be separated without chemical changes. So the best fit is chemical element because it precisely names the pure, indivisible-at-the-chemical-reaction level substance. Examples include hydrogen, oxygen, and iron.