In a precipitation reaction, what forms and separates from the solution?

• A. An insoluble solid
• B. A gas
• C. A solution
• D. A catalyst

Answer: (A)

When ions in solution join to form a compound that cannot dissolve in water, it separates from the solution as a solid. This solid is the precipitate, which settles out or can be filtered off. The other options don’t describe what happens in a precipitation reaction: a gas would be released as bubbles in gas-evolving reactions, a solution would mean everything stays dissolved, and a catalyst is a substance that speeds up a reaction without being consumed and isn’t produced as a separate solid product. For example, mixing solutions that form lead(II) iodide yields a yellow solid that precipitates out.