When you transfer energy into a substance, the molecules' kinetic energy:

• A. decreases
• B. is converted entirely into potential energy
• C. remains unchanged
• D. increases

Answer: (D)

Verified step by step guidance

1. Understand that kinetic energy of molecules is related to their motion, such as translation, rotation, and vibration.
2. Recognize that when energy is transferred into a substance as heat, it generally increases the motion of the molecules, thus increasing their kinetic energy.
3. Recall that potential energy in molecules is related to their position or arrangement, such as in chemical bonds or phase changes, but the question focuses on kinetic energy.
4. Eliminate options that suggest kinetic energy decreases or remains unchanged, since adding energy typically causes molecules to move faster.
5. Conclude that the correct understanding is that the molecules' kinetic energy increases when energy is transferred into the substance.