A gravel can be broken down into different kinds of substances by physical processes. No chemical reactions are needed to separate different parts of gravel into pure substances. This is because gravel is a(n)

A mixture contains substances that are physically combined, so its parts can be separated without chemical reactions. Gravel has visibly different components, making it a heterogeneous mixture rather than a uniform homogeneous mixture. Therefore, the correct choice is heterogeneous mixture.

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