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Given a scientific passage and an answer, generate a question for the given answer.

Passage: When studying the different states that matter exhibits, we have been looking at pure substances . Pure substances have a constant composition and can only be changed through chemical reactions. Constant composition indicates that a sample of a pure substance always contains the same elements in the same proportions. There are two main types of pure substances:. Answer: chemical reactions
Output:
Pure substances have a constant composition and can only be changed through what?