Which of the following observations indicates a chemical reaction has occurred?
I. Change in state
II. Change in color
III. Evolution of gas
IV. Formation of precipitate
Correct: B
While a change in state can occur in both physical and chemical changes (e.g., melting ice is physical), specific indicators for chemical reactions include: a change in color, the evolution of a gas (seen as bubbles), and the formation of a precipitate (an insoluble solid). These observations suggest that new substances with different properties have been formed. Change in state alone is not a definitive indicator of a chemical reaction.