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

• A. A new substance is formed with different properties from the original substances.
• B. The temperature of the substance increases due to heating, but no new substance appears.
• C. The substance changes from solid to liquid without any change in composition.
• D. The substance is crushed into smaller pieces.

Answer: (A)

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 changes in state (solid to liquid) or changes in size (crushing), do not produce new substances but only alter the form or appearance of the original substance.
3. Identify that an increase in temperature due to heating alone does not necessarily indicate a chemical reaction; it could be a physical change or simply energy being added to the system.
4. Focus on the key indicator of a chemical reaction: the formation of a new substance with properties different from the original substances.
5. Conclude that the reliable indicator of a chemical reaction is when a new substance is formed with different properties from the original substances.