As molecules are heated, which of the following best describes what happens to them?

• A. Their average kinetic energy increases.
• B. They stop moving completely.
• C. They lose energy and move more slowly.
• D. Their mass increases.

Answer: (A)

Verified step by step guidance

1. Understand that temperature is a measure of the average kinetic energy of molecules in a substance.
2. Recall that when molecules are heated, energy is transferred to them, increasing their motion.
3. Recognize that increased molecular motion means an increase in the average kinetic energy of the molecules.
4. Note that molecules do not stop moving completely when heated; rather, they move faster.
5. Also, heating does not cause molecules to lose energy or change their mass; mass remains constant.