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

• A. Change in the physical state without new substances
• B. Formation of a precipitate
• C. Melting of a solid
• D. Evaporation of water

Answer: (B)

Verified step by step guidance

1. Understand that a chemical reaction involves the formation of new substances with different chemical properties, not just physical changes.
2. Recognize that changes in physical state, such as melting or evaporation, are physical changes and do not necessarily indicate a chemical reaction.
3. Identify that the formation of a precipitate means a solid forms from a solution during a reaction, indicating a new substance has been created.
4. Recall that the formation of a precipitate is a classic sign of a chemical reaction because it shows a chemical change has occurred.
5. Conclude that among the options, the formation of a precipitate is the indicator that a chemical reaction has taken place.