Which is a sign of a chemical change?

• A. Change in shape only
• B. Dissolving a solute in water
• C. Temperature constant but no other changes
• D. Color change, energy release or absorption, gas production, precipitate formation

Answer: (D)

A chemical change shows evidence that new substances are forming. The most telling signs are color change, energy released or absorbed (temperature change or light), gas production, and the formation of a precipitate. When you see any of these together, it points to a chemical reaction taking place because the materials are reacting to form something different.

The other scenarios describe physical changes rather than chemical ones. A change in shape doesn’t create new substances; dissolving a solid in water is usually just a physical process of separation into ions or molecules in solution, not a new substance formed. If the temperature stays the same and nothing else changes, there’s no chemical reaction occurring.