Which observation indicates that a chemical reaction was occurring in this experiment?

• A. The mass of the reactants stayed the same.
• B. The temperature of the room remained constant.
• C. A color change was observed in the solution.
• D. The solution was stirred with a glass rod.

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances, often indicated by observable changes such as color change, gas production, temperature change, or formation of a precipitate.
2. Evaluate each observation to determine if it suggests a chemical change: the mass of reactants staying the same indicates conservation of mass, not necessarily a reaction occurring.
3. Note that the temperature of the room remaining constant is an external condition and does not directly indicate a chemical reaction.
4. Recognize that stirring the solution with a glass rod is a physical action and does not by itself indicate a chemical reaction.
5. Identify that a color change in the solution is a common sign of a chemical reaction because it suggests new substances with different properties are being formed.