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

• A. Melting of a solid
• B. Formation of a gas
• C. Change in shape without altering composition
• 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 compositions, unlike physical changes which only alter the state or appearance without changing the substance itself.
2. Analyze each option to determine if it indicates a chemical change or a physical change:
3. Melting of a solid is a physical change because it only changes the state from solid to liquid without altering the chemical composition.
4. Formation of a gas often indicates a chemical reaction because it suggests that new substances are being produced, such as when a gas bubbles out during a reaction.
5. Change in shape without altering composition and evaporation of water are physical changes since the substance's chemical identity remains the same.