The evidence that a chemical change has happened in the case described—when two different liquids are mixed together and a powdery substance forms that settles to the bottom of the container—is the formation of a precipitate.
A precipitate is a solid that forms in a solution during a chemical reaction and separates out from the liquid. In this scenario, the appearance of the powdery substance indicates that a new chemical compound has been produced as a result of the mixing of the two liquids, which is indicative of a chemical change.