What characterizes precipitation reactions?

Precipitation reactions are characterized by the formation of an insoluble solid when two aqueous solutions are mixed. This solid, known as a precipitate, occurs when the product of the reaction has low solubility in the solvent, typically water. In this process, the ions in solution combine to form a compound that cannot remain dissolved, leading to the visible separation of the solid from the liquid.

This definition aligns with the understanding of precipitation reactions in aqueous chemistry, as they are often utilized in various applications, such as qualitative analysis, where distinguishing between different ions in a solution is critical. For example, when mixing solutions of barium nitrate and sodium sulfate, barium sulfate forms as a white precipitate, clearly demonstrating the characteristic of a precipitation reaction.

The other choices do not define precipitation reactions accurately: gas formation pertains to gas evolution reactions, the dissolution of solid reactants involves solubility and does not lead to the formation of a solid product, and changes in temperature can occur but are not indicative of a precipitation reaction specifically. Therefore, the most defining feature of precipitation reactions is indeed the formation of an insoluble solid.