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

• A. Dissolving a solid in water
• B. Change in temperature
• C. Change in physical state without new substances
• D. Formation of a gas

Answer: (D)

Verified step by step guidance

1. Understand that a chemical reaction involves the formation of new substances with different chemical properties from the reactants.
2. Recognize that physical changes, such as dissolving a solid in water or a change in physical state (like melting or boiling), do not produce new substances; these are physical changes, not chemical reactions.
3. Identify that a change in temperature alone can be due to either physical or chemical processes, so it is not a definitive sign of a chemical reaction by itself.
4. Note that the formation of a gas during a process often indicates a chemical reaction because it suggests that new substances are being produced, such as when bubbles form from a reaction.
5. Conclude that among the options, the formation of a gas is a clear sign that a chemical reaction has occurred.