Cooling Reduces Molecular Kinetic Energy

Steps:

• Temperature measures the average kinetic energy of gas molecules.
• Cooling the gas decreases its temperature.
• With constant volume, the number of molecules and container space remain unchanged.
• Lower kinetic energy means molecules have reduced speed.

Why D is correct:

• Kinetic molecular theory states average kinetic energy is proportional to absolute temperature (KE = (3/2)kT); cooling lowers T, so molecules move more slowly.

Why the others are wrong:

• A: Slower molecules collide less often with walls.
• B: Gas molecules are point masses and do not contract.
• C: Constant volume keeps molecular density unchanged.

Final answer: D