Which of the following is a common indicator that a chemical reaction has occurred?

• A. Evolution of a gas
• B. Change in temperature without any external heating or cooling
• C. Formation of a precipitate
• D. Change in color

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction is often indicated by observable changes that suggest new substances are formed.
2. Recognize that the evolution of a gas during a reaction can be seen as bubbles or fumes, indicating a chemical change.
3. Note that a change in temperature without external heating or cooling (an exothermic or endothermic reaction) is another sign of a chemical reaction.
4. Identify that the formation of a precipitate means a solid forms and separates from a solution, which is a clear indicator of a chemical reaction.
5. Observe that a change in color can also signal a chemical reaction, as it often reflects a change in the chemical composition of the substances involved.