Which of the following statements is an effect of compressing gas molecules at constant temperature?

Analyze the question: A gas is compressed (its volume is reduced) while its temperature stays constant. We need the effect on the molecules.

Concept: Boyle's law and the kinetic theory of gases.

• At constant temperature, PV = constant. If volume decreases, pressure increases.
• Temperature is a measure of the average kinetic energy of the molecules. If temperature is constant, the average kinetic energy does not change.
• Molecules do not change in size when a gas is compressed. Only the spaces between them shrink.

Solution, checking each option:

• A. The pressure decreases. Wrong. By Boyle's law, reducing the volume increases the pressure.
• B. The molecules gain more kinetic energy. Wrong. The temperature is constant, so the average kinetic energy stays the same.
• C. The molecules make more impact per second on the walls of the container. Correct. In a smaller volume, the molecules have a shorter distance to travel between collisions with the walls. The collision frequency rises, so the force per unit area (pressure) rises.
• D. The molecules increase in size. Wrong. Molecular size does not change with compression.

**Answer: C.

Tip: For "constant temperature" gas questions, remember that the speed and kinetic energy of the molecules stay the same. The only change is the collision rate, which is why the pressure goes up. Think "squeeze, so more hits, so more pressure."