Which change indicates a chemical reaction has taken place?

The formation of a precipitate is a strong indicator that a chemical reaction has occurred. When two solutions are mixed and a solid forms that was not present in either of the original solutions, it signifies that new substances are being produced through a chemical reaction. This change is not just a physical alteration but involves the rearrangement of atoms and the formation of new chemical bonds, hence suggesting that a chemical change has taken place.

In contrast, other changes mentioned do not necessarily indicate a chemical reaction. A change of state from solid to liquid is a physical process involving the transition of matter without altering its chemical composition, meaning the substance remains the same on a molecular level. Mixing two liquids without reaction implies that no new substances have formed, as the original components maintain their chemical identities. Freezing a liquid also represents a physical change, where the liquid's state is altered, but the chemical structure remains uncharged. Thus, the formation of a precipitate is a clear and defining characteristic of a chemical reaction.