Final answer:
The solid phase has the smallest temperature range, particularly during the freezing process, as it occurs at a specific temperature and involves fewer microstates compared to other phases. This transition reflects a decrease in entropy and corresponds to the freezing point, a narrowly defined temperature for the phase change.
Step-by-step explanation:
The phase that exists across the smallest temperature range is the solid phase, particularly when considering the process of freezing. During the phase transition from liquid to solid, known as freezing, there is a release of thermal energy which corresponds to a decrease in entropy (ΔS < 0). This change occurs at a specific temperature, the freezing point, which is a relatively narrow range when compared to other phase transitions like vaporization, which happen over a broader range of temperatures because evaporation can occur at temperatures below the boiling point. Moreover, for a given pressure, the sublimation point is specific, occurring at a single temperature when a solid transitions directly to a gas, again representing a narrow temperature range for that specific transition.
To summarize the microscopic scale activities during phase changes, as temperature rises, the number of microstates that atoms or molecules can occupy increases, leading to a greater dispersal of thermal energy. This correlates with an increase in entropy. Solid molecules occupy the fewest microstates, with tightly bound atoms or molecules, followed by liquid and then gas, which has the highest number of accessible microstates and thus the highest entropy. When phase changes occur, such as during freezing or condensation, the entropy will decrease, indicating a transition to a state with fewer microstates and a smaller temperature range for that phase.
Considering phase diagrams, discrete regions represent the stability of different phases under varying temperature and pressure. The solid phase is typically stable under a combination of high pressure and low temperature, indicating a limited temperature range where the solid exists before it undergoes melting.