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In chemistry, a substance is pure if it has a homogeneous chemical composition. At the nanoscale, this only applies to a substance made up of one type of atom, molecule, or compound.

In the more general sense, a pure substance is any homogeneous mixture. That is, it is matter that appears uniform in appearance and composition, no matter how small the sample size.

Examples of pure substances include iron, steel, and water. Air is a homogeneous mixture that is often considered to be a pure substance.

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