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

• A. The physical state of the reactants never changes.
• B. The substance is simply mixed but remains unchanged.
• C. A change in color is observed.
• D. The temperature of the surroundings always decreases.

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances into new substances with different properties.
2. Recognize common indications of a chemical reaction, such as color change, gas production, formation of a precipitate, or temperature change.
3. Evaluate each option: a change in physical state alone does not guarantee a chemical reaction; substances remaining unchanged means no reaction; a temperature change can occur but does not always decrease.
4. Identify that a change in color is a classic and reliable sign that a chemical reaction has taken place because it indicates new substances with different properties are formed.
5. Conclude that among the given options, 'A change in color is observed' is the correct indication of a chemical reaction.