When heat is added to a substance, what happens to its molecules?

• A. The molecules decrease in size and lose energy.
• B. The molecules combine to form larger molecules.
• C. The molecules stop moving completely.
• D. The molecules move faster and their kinetic energy increases.

Answer: (D)

Verified step by step guidance

1. Understand that when heat is added to a substance, energy is transferred to its molecules.
2. Recall that molecules in a substance are always in motion, and their motion is related to their kinetic energy.
3. Recognize that adding heat increases the kinetic energy of the molecules, causing them to move faster.
4. Note that molecules do not decrease in size or stop moving; instead, their increased motion can lead to changes in state (e.g., melting or boiling).
5. Conclude that the correct description is that the molecules move faster and their kinetic energy increases when heat is added.