What changes occur during a phase transition?

During a phase transition, such as when a substance changes from solid to liquid (melting) or from liquid to gas (vaporization), both entropy and density undergo significant changes. Entropy, a measure of disorder or randomness in a system, generally increases as a substance transitions to a higher energy state. For example, when ice melts into water, the orderly arrangement of molecules in the solid becomes more disordered in the liquid phase, leading to increased entropy.

Density, on the other hand, commonly changes during phase transitions due to differences in how tightly packed the molecules are in each phase. For instance, when water freezes into ice, the density decreases because the crystalline structure of ice holds molecules further apart compared to the more compact arrangement in liquid water. Conversely, when ice melts, the density increases as the molecules transition closer together in the liquid phase.

While temperature does play a pivotal role in phase transitions, it doesn't always increase or only change in one direction. Additionally, density changes distinctly depend on the specific phase being compared. Thus, the chosen answer accurately reflects the fundamental characteristics of phase transitions—highlighting the role of entropy and the varying density of substances as they move between different states of matter.