Which statement best describes a single replacement reaction?

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

Answer: (B)

Single replacement reactions happen when one element takes the place of another element in a compound, creating a new compound and freeing the replaced element. For example, zinc metal reacts with copper(II) sulfate to form zinc sulfate and copper: Zn + CuSO4 → ZnSO4 + Cu. The key idea is that a single element pushes out the element that’s attached in the compound, provided it is more reactive in the activity series.

If two elements switch places in two compounds, that describes a double replacement, not a single one. If a compound forms directly from elements, that’s synthesis. If a substance changes its phase (solid to liquid or gas, etc.), that’s a physical change, not a chemical reaction.