Which statement best describes a double replacement reaction?

• A. Two elements switch places
• B. A single element replaces another
• C. A compound forms from elements
• D. A substance changes phase

Answer: (A)

In a double replacement reaction, two ionic compounds in solution swap their ions to form two new compounds. This is like two partners in the compounds exchanging components, so the cation from one compound teams up with the anion from the other, and vice versa. That idea—that the ions switch partners between the two reactants—best matches the description of this type of reaction.

For example, when solutions of two salts react, you can end up with a new pair of salts, often with one product precipitating out or a gas forming, which signals the reaction has occurred. A common illustration is a solution reaction where sodium sulfate and barium chloride exchange ions to form barium sulfate (a precipitate) and sodium chloride.

This differs from the other ideas: a single element replacing another describes a single-displacement reaction, forming a different kind of substitution; forming a compound from elements describes synthesis; and a phase change is just a physical change, not a chemical one.