What is the triple point on a phase diagram?

The triple point on a phase diagram is defined as the single point where a substance can exist in equilibrium as a solid, liquid, and gas. At this unique combination of temperature and pressure, all three phases coexist simultaneously. This means that at the triple point, the conditions allow the solid, liquid, and gas forms of the substance to be present together without favoring any one phase over the others.

Understanding the triple point is important because it highlights how phase changes depend on both temperature and pressure. Each substance has a specific triple point that can be identified on its phase diagram, which is a graphical representation showing the state of a substance at various temperatures and pressures.

In contrast, the other options do not accurately define the triple point. For instance, the point where a substance can exist in two phases refers to phase boundaries, not the triple point. The boiling point pertains to the transition from liquid to gas at a specific pressure, and while the temperature is defined as zero for absolute zero on a temperature scale, this doesn’t relate to the concept of a triple point and its significance in phase diagrams.