What defines a precipitation reaction?

• A. A reaction in which a gas is formed.
• B. A reaction in which two liquids mix to form a solution.
• C. A reaction where two aqueous solutions produce an insoluble solid.
• D. A reaction that occurs only in the presence of light.

Answer: (C)

Precipitation reactions are about forming a solid that comes out of a solution. When two solutions containing dissolved ions are mixed, the ions can combine to make an insoluble ionic compound. That solid, called a precipitate, separates from the liquid. This is the defining feature: an insoluble solid appearing from mixing two aqueous solutions. For example, mixing solutions of silver nitrate and sodium chloride produces solid silver chloride that settles out, while the remaining ions stay dissolved as sodium nitrate. The other scenarios describe different types of reactions—gas formation, simply forming a fully soluble solution, or reactions driven by light—so they don’t define a precipitation reaction.