Which of the following must occur when a substance undergoes a chemical change?

• A. The substance changes from a solid to a liquid.
• B. The temperature of the substance must decrease.
• C. The chemical composition of the substance changes.
• D. The mass of the substance increases.

Answer: (C)

Verified step by step guidance

1. Understand the definition of a chemical change: it involves the transformation of substances into new substances with different chemical compositions and properties.
2. Recognize that a change in physical state, such as from solid to liquid, is a physical change, not a chemical change, because the chemical composition remains the same.
3. Note that temperature changes (increase or decrease) can occur during both physical and chemical changes, so a temperature decrease is not a requirement for a chemical change.
4. Recall the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction, so the mass of the substance does not increase during a chemical change.
5. Conclude that the key characteristic of a chemical change is that the chemical composition of the substance changes, meaning new substances are formed.