What does a phase diagram illustrate?

A phase diagram is a graphical representation that illustrates the relationships between temperature, pressure, and the different states of matter (solid, liquid, gas) of a substance. It provides valuable insights into how the state of a substance changes under varying temperature and pressure conditions. For example, the phase diagram shows the boundaries, called phase lines, where phase transitions occur—for instance, the melting point, boiling point, or sublimation point.

This understanding is crucial because it helps predict the state a substance will be in under specific temperature and pressure conditions, aiding in various fields such as chemistry, material science, and even environmental science. The other choices do not accurately reflect the purpose of a phase diagram: it does not illustrate color changes, volume changes upon mixing, or energy level changes directly, as those topics involve different aspects of physical chemistry.