Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: In this reaction, and are positively-charged cations, while and are negatively-charged anions. Double-replacement reactions generally occur between substances in aqueous solution. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water. Answer: aqueous
Output:
Double-replacement reactions generally occur between substances in what kind of solution?