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

• A. The temperature of the substance is increased by heating.
• B. The substance is dissolved in water.
• C. The substance is crushed into smaller pieces.
• D. A color change is observed in the substance.

Answer: (D)

Verified step by step guidance

1. Understand the difference between physical changes and chemical changes. Physical changes affect the form of a substance but not its chemical composition, while chemical changes result in the formation of new substances with different properties.
2. Analyze each observation to determine if it indicates a chemical change or a physical change. For example, heating a substance to increase its temperature is a physical change because it only affects the energy of the particles, not their identity.
3. Consider dissolving a substance in water. This is typically a physical change because the substance's chemical identity remains the same, even though it is dispersed in the solvent.
4. Crushing a substance into smaller pieces is a physical change because it changes the size and shape but not the chemical composition of the substance.
5. Recognize that a color change often indicates a chemical reaction because it suggests that new substances with different properties have formed, which is a hallmark of a chemical change.