According to the kinetic molecular theory, what happens to the speed of molecules in a substance when energy is transferred into it?

• A. The speed of the molecules remains unchanged.
• B. The speed of the molecules decreases.
• C. The molecules stop moving.
• D. The speed of the molecules increases.

Answer: (D)

Verified step by step guidance

1. Recall that according to the kinetic molecular theory, the temperature of a substance is directly related to the average kinetic energy of its molecules.
2. Understand that when energy is transferred into a substance, it usually increases the thermal energy, which raises the average kinetic energy of the molecules.
3. Recognize that kinetic energy of a molecule is given by the formula \(KE = \frac{1}{2} m v^2\), where \(m\) is the mass of the molecule and \(v\) is its speed.
4. Since the kinetic energy increases with added energy, and mass remains constant, the speed \(v\) of the molecules must increase to reflect the higher kinetic energy.
5. Therefore, when energy is transferred into a substance, the speed of its molecules increases, meaning they move faster on average.