Heat increases molecular kinetic energy in fixed-volume gases

Steps:

• Container has fixed volume, so gas cannot expand overall.
• Added heat raises the gas's internal energy.
• Per kinetic molecular theory, internal energy equals average translational kinetic energy of molecules.
• Thus, molecules' average speed increases, raising temperature.

Why B is correct:

• Kinetic molecular theory states temperature is proportional to average kinetic energy (KE = 3/2 kT), so heating boosts molecular speeds.

Why the others are wrong:

• A: Gas molecules are point masses and do not expand; only kinetic energy changes.
• C: Fixed volume prevents molecules from moving further apart; intermolecular distances remain similar, increasing pressure instead.
• D: Gas molecules primarily translate, not vibrate; vibration is negligible at typical temperatures.

Final answer: B