Which process generally increases the entropy of a system?

• A. Dissolving a solid in a solvent
• B. Cooling a solution
• C. Freezing a liquid
• D. Compressing a gas

Answer: (A)

Entropy measures disorder and the number of ways particles can be arranged. Dissolving a solid in a solvent disrupts the orderly crystal lattice of the solid, releasing those particles into more freedom as ions or molecules in solution. In this state, they can move and be oriented in many more ways, and mixing with the solvent further randomizes positions and surroundings. All of this increases the number of accessible microstates, so the entropy rises.

In contrast, cooling a solution slows molecular motion and reduces accessible microstates, lowering entropy. Freezing a liquid creates an orderly solid lattice, which is a much more ordered arrangement and thus has far lower entropy. Compressing a gas reduces the volume available to the molecules and typically lowers the number of accessible microstates, again decreasing entropy.

So, the process that generally increases entropy is dissolving a solid in a solvent.