Physically distinct state of a substance.

• A. Equilibrium
• B. Phase Diagram
• C. Phase
• D. Phase Boundaries

Answer: (C)

A phase is a physically distinct state of matter with uniform properties throughout, separated by interfaces from other such states. Ice, liquid water, and water vapor are three different phases of the same substance, each having its own density, structure, and behavior. The phrase “physically distinct state of a substance” lines up exactly with what a phase is. Equilibrium describes a balance of processes, a phase diagram is a chart showing which phase exists under certain temperature and pressure, and phase boundaries are the interfaces between two phases. So the best answer is the one that denotes a single, uniform form of matter with its own characteristic properties.