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A phase diagram is a graphical representation depicting the pressure and temperature conditions at which various phases of a substance are stable. Areas represent single phases, while lines or phase boundaries specify the equilibrium between two phases. This visual tool helps predict which physical state a substance will occupy under specific conditions.

Water's solid-liquid equilibrium line has a negative slope because ice is less dense than liquid water. This unusual property means that increased pressure lowers the melting point of ice, causing solid ice to melt under pressure. Most substances show the opposite behavior, making water's phase diagram distinctive.

The triple point is where all three phase boundaries meet on a phase diagram, representing the unique conditions where solid, liquid, and gas phases coexist in equilibrium. For water, this occurs at 0.01°C and 0.006 atmospheres pressure. It is a fixed reference point used to define temperature scales and understand phase transitions.

The critical point is where liquid and gas phases become indistinguishable, occurring at specific critical temperature and critical pressure values. For water, this is 374°C and 218 atmospheres. Beyond this point, the substance enters a supercritical fluid phase exhibiting both liquid and gas properties simultaneously.

The solid-liquid boundary represents the melting point, where melting point decreases with increasing pressure for water. The liquid-gas boundary represents the boiling point, where boiling point increases with increasing pressure. The solid-gas boundary represents the sublimation point, showing conditions for direct solid-to-gas transitions.

The vapor-pressure curve represents the equilibrium between a condensed phase and its vapor. For water, the line between ice and water vapor shows the solid's vapor-pressure curve, while the line between liquid water and water vapor shows the liquid's vapor-pressure curve. These curves indicate conditions where the two phases coexist in equilibrium.

In water's phase diagram, increasing pressure raises the boiling point of the liquid phase along the liquid-gas boundary. Conversely, increasing pressure lowers the melting point of ice along the solid-liquid boundary due to water's unusual density relationship. These pressure-dependent changes are critical for understanding reactions at equilibrium and the equilibrium constant.