Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: When two or more pure substances are combined together, a mixture is formed. Unlike pure substances, mixtures have a variable composition. Variable composition indicates that the relative proportions of the mixtures components may vary, and they can be separated by physical methods. There are two main types of mixtures. Answer: variable composition
Output:
Unlike pure substances, mixtures have a what?