When a substance is heated, what happens to its molecules?

• A. The molecules lose energy and slow down.
• B. The molecules become larger in size.
• C. The molecules change into atoms.
• D. The molecules move faster and their average kinetic energy increases.

Answer: (D)

Verified step by step guidance

1. Understand that heating a substance means adding thermal energy to it, which affects the motion of its molecules.
2. Recall the kinetic molecular theory, which states that the temperature of a substance is directly related to the average kinetic energy of its molecules.
3. Recognize that when a substance is heated, the molecules gain energy, causing them to move faster rather than slower.
4. Note that molecules do not become larger in size or change into atoms simply by heating; their size and identity remain the same under normal heating conditions.
5. Conclude that the correct effect of heating is an increase in the molecules' average kinetic energy, leading to faster molecular motion.