When a solid is converted into liquid, entropy:
A. Becomes zero
B. Remains the same
C. Decreases
D. Increases

Hint: The key to answering this question is the definition of entropy. Entropy is generally understood to be linked to the amount of disorder and chaos within a system. To answer this question, we must think about the amount of disorder in a system when it is in a solid state and when it is in a liquid state. Once this is determined, we will be able to figure out how entropy changes when a solid turns into a liquid.

Complete Step by Step Solution:
Entropy can be described as a measure of the degree to which energy (such as thermal energy) becomes spread out or dispersed within a system. Here, it is useful to know what microstates are. Consider a system of molecules in random motion. The positions of the particles and the energy of individual particles are constantly changing because of the non-stop collisions that they undergo, with each collision changing the energy of the individual particle.

The locations, momenta, and energies of each molecule within the system are going to be fixed at a particular instant in time. The set of values of locations, momenta, and energies of each molecule within the system, at a particular instant in time is called a microstate of that system. The system has a fixed value of energy in each of its various microstates since a particular microstate corresponds to a constant set of values of the locations, momenta, and energies of each molecule within the system.

Each microstate of a system represents a possible way (configuration) in which the particles and energies of a system can be arranged in it.
Consider two systems, one of which has 2 possible microstates (system A) and the other has 20 possible microstates (system B). Therefore, system B can have its molecules and their energies configured in 20 different ways whereas the molecules (and their energies) of system A can exist in only 2 different ways. This also means that if system B wants to change its configuration from its current one, it has 19 different configurations to choose from. Whereas, if system A wants to change its configuration from the one it's currently in, it has only 1 other configuration available to it. Thus, it can be said that system B has more disorder/chaos in it than system A.

The energy of a system with a greater number of possible microstates can be more dispersed/spread out than a system with a lower number of possible microstates. Since entropy is the degree to which energy can be spread out within a system, it can be said that systems with a greater number of possible microstates have a higher entropy. Since a system having a higher number of possible microstates is said to be more disordered and such systems are said to have higher entropy, it is easy to see how the concept of entropy got linked with the idea of disorder and chaos.

In a solid, atoms and molecules are very restricted in their movements compared to liquids. Thus, it can be said that solids have a lower number of possible microstates than liquids. Consequently, liquids are higher in entropy than solids. Therefore, when a solid is converted into a liquid, entropy increases.
Thus, option D is correct.

Note: The conversion of a solid into liquid is called fusion. Since fusion takes place at a constant temperature (\[{T_{fus}}\]), the increase in entropy (\[\Delta {S_{fus}}\]) can be calculated from the formula:
\[\Delta {S_{fus}} = \dfrac{{\Delta {H_{fus}}}}{{{T_{fus}}}}\] where \[\Delta {H_{fus}}\]is called the enthalpy of fusion.