What is a precipitation reaction?

A precipitation reaction is defined by the formation of a solid from a solution when two soluble compounds react to form an insoluble product, known as a precipitate. In this type of reaction, the solubility rules guide us in predicting whether a reaction between two ionic compounds in solution will produce a precipitate. When the ions in solution combine to form a compound that is not soluble in water, that compound will precipitate out of the solution and can often be observed as a solid forming at the bottom of the reaction container.

This process is significant in various fields such as analytical chemistry, where precipitation reactions are used for qualitative and quantitative analyses. The other options describe different reactions; for instance, the formation of a gas or a liquid from a gas does not involve solid formation, which is the fundamental aspect of precipitation reactions. Hence, the correct characterization of a precipitation reaction is highlighted by the solid formation from a solution.