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

• A. It happens without any energy change
• B. The mixture remains completely transparent
• C. The mass disappears
• D. Color change

Answer: (D)

A color change is evidence of a chemical reaction because the substances involved actually transform into new ones that have different properties, including different colors. When bonds break and new bonds form, the way the material absorbs or reflects light changes, so you see a different color. That visible shift is a practical clue that a chemical change has occurred.

The other ideas aren’t reliable signs. An energy change can happen in many reactions, but saying there’s no energy change isn’t a safe indication of whether a reaction has occurred. A mixture staying transparent can happen with or without a chemical reaction, especially if the products or reactants are both colorless. Mass appearing to disappear isn’t a valid sign either, since mass is conserved in a closed system; if gas escapes, the observed mass may drop, but that’s about containment, not the reaction itself.