Which of the following is the only sure sign that a chemical change is taking place?

• A. Change in state (e.g., melting or boiling)
• B. Change in temperature
• C. Formation of a new substance
• D. Change in size or shape

Answer: (C)

Verified step by step guidance

1. Understand that a chemical change involves the formation of new substances with different chemical properties and compositions from the original substances.
2. Recognize that physical changes, such as changes in state (melting, boiling), temperature, size, or shape, do not produce new substances; they only affect the physical form or energy of the material.
3. Identify that the formation of a new substance is indicated by evidence such as color change, gas production, precipitate formation, or energy change that cannot be explained by physical changes alone.
4. Conclude that among the options given, the only sure sign of a chemical change is the formation of a new substance, because it directly implies a chemical reaction has occurred.
5. Therefore, the correct answer is the formation of a new substance, as it distinguishes chemical changes from physical changes.