Why does the solid-liquid phase boundary line have a negative slope in the pressure-temperature phase diagram of water?
Answer and explanation
Correct answer: D. Liquid water is denser than solid ice due to open hydrogen-bonded ice crystal lattice structure
According to the Clapeyron equation dP/dT = ΔH_fusion / (T * ΔV_fusion), the slope of the melting curve depends on the sign of ΔV_fusion = V_liquid - V_solid. In water, the hexagonal hydrogen-bonded structure of ice leaves open cage-like voids; upon melting, these collapse, making liquid water denser than solid ice (ΔV < 0). Thus, dP/dT is negative, meaning increasing pressure lowers the melting point of ice.
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