What is a precipitate in a chemical reaction?

A precipitate is defined as an insoluble solid that emerges from a liquid solution due to a chemical reaction. This occurs when the concentration of certain ions in a solution exceeds their solubility product, leading to the formation of a solid that separates from the liquid phase. This often happens in double displacement reactions, where two aqueous solutions mix and lead to the formation of a compound that is not soluble in water, resulting in the solid precipitate.

In contrast, the other choices describe different phenomena. The release of a gas during combustion reactions highlights a different type of reaction outcome and does not pertain to the formation of solid materials from liquid solutions. The separation of a liquid after distillation relates to the physical separation of components in a mixture based on differences in boiling points, which is not relevant to the formation of a precipitate. Lastly, while an aqueous substance containing dissolved ions may be a precursor to precipitation, it does not itself define a precipitate, as it lacks the solid phase that characterizes precipitates.