Why is 18-karat gold considered a mixture rather than a pure substance?

Karat, when used with gold, is a unit of purity-- 24-karat gold is pure gold, but usually you mix gold with a metal like copper or silver to make jewelry (because pure gold is too soft). Each karat indicates 1/24th of the whole. So if a piece of jewelry is made of metal that is 18 parts gold and 6 parts copper, that is 18-karat gold.

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xo, Leafling

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