Final answer:
The entropy increases when a solid melts into a liquid due to the higher number of microstates available in the liquid's less-ordered structure, which allows more freedom of motion for the molecules. The entropy change during melting is greater than zero (ΔSfus > 0).
Step-by-step explanation:
When a solid melts to become a liquid, the entropy of a system increases because the microstates of the system increase. A crystalline solid has molecules, ions, or atoms arranged in an orderly fixed lattice, whereas in a liquid, these particles can move and tumble freely within the volume.
This freedom of motion in a liquid state corresponds to a greater number of ways in which the system can be arranged (microstates), which in turn means higher entropy. Thus, the change in entropy during melting (ΔSfus) is greater than zero.
Conversely, when a liquid freezes to become a solid, the entropy decreases because the particles are moving into a more ordered structure, reducing the number of microstates. The change in entropy (ΔS) will, therefore, be less than zero.