What observable change indicates a chemical reaction occurred when fireworks are ignited?

The observable change that indicates a chemical reaction during the ignition of fireworks is a change in color. When fireworks explode, they produce vibrant colors due to the different chemical compounds present in the pyrotechnics. These compounds release energy in the form of light when they undergo combustion, leading to the striking visual displays associated with fireworks. This change in color is a clear indicator that a chemical reaction has occurred, as new substances are formed through the combustion of the fireworks' components.

The other options do not serve as definitive indicators of a chemical reaction. While sound changes occur, they may not reflect a conversion of substances or a transformation at the molecular level. A temperature decrease is generally not observed in the explosion of fireworks; rather, there is a significant increase in temperature due to the exothermic nature of the chemical reactions taking place. Changes in physical shape can occur, but they do not signify a chemical change since they may simply indicate a physical alteration rather than the formation of new chemical substances. Hence, the change in color stands out as the most evident sign of a chemical reaction during fireworks ignition.