Final answer:
The formation of a yellow precipitate when two colorless aqueous solutions are mixed is evidence of a chemical reaction, specifically a double-replacement reaction that results in the formation of an insoluble compound.
Step-by-step explanation:
The question pertains to the experimental evidence that indicates a chemical reaction has occurred when two colorless aqueous solutions are mixed. The formation of a yellow precipitate following the combination of two colorless solutions is a clear sign of a chemical change, particularly indicative of a double-replacement reaction. In such a reaction, two ionic compounds in aqueous solutions exchange anions or cations and form new compounds, of which at least one is typically insoluble in water and precipitates out of the solution.
The absence of a reaction is noted when no visual changes occur, such as no precipitate or gas production or no color change. In contrast, a visible change like the emergence of a yellow precipitate provides conclusive evidence of a reaction, such as when silver nitrate reacts with a halide ion to form insoluble silver halides.