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A phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure. It shows the boundaries between solid, liquid, and gas phases and the conditions at which these phases coexist in equilibrium. An area in a phase diagram represents a single phase, whereas lines or phase boundaries represent the equilibrium between two phases.
In the phase diagram of water, the boundary line between the solid and liquid states illustrates the melting point of ice. It can be observed that the melting point of ice decreases slowly with increasing pressure. The phase boundary between the liquid and gas phases represents the boiling point of water. As we move along this line, the boiling point of water increases with increasing pressure.
In addition, the phase boundary between solid and gas phases represents the sublimation point of water. It shows the conditions under which water can transition directly from a solid to a gas, bypassing the liquid phase.
A triple point is a point on the diagram where all three phase boundaries meet. For water, the triple point occurs at a specific pressure of 0.006 atm and a temperature of 0.01°C. Under this unique set of conditions, water can exist simultaneously as a solid, liquid, and gas. In the phase diagram, the critical point is the point beyond which a substance's liquid and gas phases become indistinguishable. For water, the critical point is at a temperature of 374 °C and a pressure of 218 atm. These particular pressures and temperatures are known as the critical temperature and critical pressure of water. Beyond this point, water enters a supercritical fluid phase exhibiting both liquid and gas properties.
