Which of the following is a reliable indicator that a chemical change has occurred during a reaction?

• A. Breaking a substance into smaller pieces
• B. Change in the physical state without altering composition
• C. Formation of a new substance with different properties
• D. Dissolving a solid in water

Answer: (C)

Verified step by step guidance

1. Understand the difference between physical and chemical changes: A physical change affects the form of a substance but not its chemical composition, while a chemical change results in the formation of one or more new substances with different properties.
2. Analyze each option to determine if it indicates a chemical change: Breaking a substance into smaller pieces is a physical change because the composition remains the same.
3. Consider the option 'Change in the physical state without altering composition': This describes a physical change, such as melting or boiling, where the substance's identity does not change.
4. Evaluate 'Formation of a new substance with different properties': This is a hallmark of a chemical change, as new substances with new chemical properties are produced.
5. Review 'Dissolving a solid in water': This is generally a physical change because the solid dissolves but does not chemically transform into a new substance.