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

• A. Change in temperature without external heating or cooling
• B. Dissolving a solid in water
• C. Change in physical state due to melting
• D. Formation of a precipitate

Answer: (D)

Verified step by step guidance

1. Understand that a chemical reaction involves the rearrangement of atoms to form new substances, which often results in observable changes.
2. Recognize that common indicators of a chemical reaction include color change, gas production, temperature change without external influence, and formation of a precipitate.
3. Analyze each option: dissolving a solid in water and melting are physical changes because they do not produce new substances.
4. Note that a change in temperature without external heating or cooling can indicate a chemical reaction, but it is not always definitive on its own.
5. Identify that the formation of a precipitate (a solid that forms and settles out of a liquid mixture) is a clear and common indicator that a chemical reaction has occurred.