A Level Chemistry OCR Practice Exam

Which statement best describes a precipitation reaction?

1. The production of gas when two liquids mix

2. The formation of a solid from a solution during a chemical reaction

3. The conversion of a solid to a gas directly

4. The mixing of two solid substances

The correct answer is: The formation of a solid from a solution during a chemical reaction

A precipitation reaction is characterized by the formation of an insoluble solid, known as a precipitate, that occurs when two aqueous solutions are mixed. This process takes place when specific ions in the solutions react and create a compound that does not dissolve in water, leading to the appearance of solid material that can often be observed as cloudiness or sediment in the solution. In this context, the option that accurately captures this concept is the formation of a solid from a solution during a chemical reaction. This aligns with the fundamental nature of precipitation reactions, where the reactants, both in liquid form, interact to produce a solid phase as one of the products. Other options describe different phenomena: for instance, the production of gas from liquid mixtures pertains to gas evolution reactions, while the conversion of a solid to a gas directly describes sublimation, which is not related to precipitation. Finally, the mixing of two solid substances does not involve a reaction that results in a change of state or formation of a new product, which is a key characteristic of precipitation reactions.