Which scenario increases the entropy of a system?

• A. Increasing the pressure of a gas at constant temperature.
• B. Removing a molecule from a mixture.
• C. Crystallizing a liquid.
• D. Mixing two gases at the same temperature and pressure.

Answer: (D)

Entropy measures how many ways the energy and particles can be arranged. When two different gases are allowed to mix at the same temperature and pressure, the molecules can spread throughout the entire volume in many more ways than when they’re separate. This increase in the number of possible configurations—the system becoming more disordered—increases the entropy. For ideal gases, this mixing results in a positive entropy change, because there are more microstates available after mixing.

The other scenarios reduce entropy: compressing a gas at constant temperature squeezes fewer microstates into a smaller volume; crystallizing a liquid imposes order and greatly lowers disorder; removing a molecule reduces the number of ways the remaining molecules can be arranged. So mixing two gases at the same temperature and pressure best increases entropy.