A pure substances has only one components. Elements and compounds are both pure substances. It is collection of dissimilar particles that will not undergo a chemical reaction while Mixture has variable combinations. It can be either homogenous or heterogeneous.
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Pure substance - a substance made up of one kind of material w/ unique set of chemical and physical properties.

Mixture - consist of one or more components that are not chemically combined and are not in fixed proportion.
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