A Level Chemistry OCR Practice Exam

What phenomenon describes the formation of a solid when two liquids are mixed?

1. Evaporation

2. Crystallization

3. Precipitation

4. Filtration

The correct answer is: Precipitation

The formation of a solid when two liquids are mixed is referred to as precipitation. This phenomenon occurs when the combined solutions contain ions or molecules that react to form an insoluble solid. When the conditions are such that the solubility product of the resulting compound is exceeded, this compound precipitates out of the solution. Precipitation is a common occurrence in various chemical reactions, particularly in the field of analytical chemistry and in processes such as wastewater treatment. For instance, mixing two clear solutions that contain different ions may yield a solid that is visible, indicating the formation of a precipitate. Other processes like crystallization involve the formation of solid crystals from a liquid solution, usually under different conditions and with the intent of purifying a substance, while evaporation refers to the transition of a substance from liquid to gas. Filtration, in the context of separating solids from liquids, does not describe the process of solid formation but rather the means by which a solid precipitate can be collected once it has formed. Thus, precipitation specifically denotes the production of a solid from liquid reactants.