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

• A. The substance changes from solid to liquid
• B. The temperature of the substance increases without any other change
• C. A new substance with different properties is formed
• D. The substance is broken into smaller pieces

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction involves the transformation of substances into new substances with different chemical properties and compositions.
2. Recognize that physical changes, such as a change in state from solid to liquid or breaking a substance into smaller pieces, do not produce new substances; they only alter the form or appearance.
3. Note that a temperature change alone, without other evidence, may be due to physical processes or energy changes but does not necessarily indicate a chemical reaction.
4. Identify that the formation of a new substance with different properties (such as color change, gas production, precipitate formation, or energy change) is a reliable indicator of a chemical reaction.
5. Conclude that among the options, the statement 'A new substance with different properties is formed' best indicates that a chemical reaction has occurred.