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Why does water’s melting curve slope downward as pressure increases?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

The melting curve of water slopes downward as pressure increases because of the unique relationship between its solid and liquid phases. According to the Clapeyron equation, the slope of a phase-transition line is determined by the ratio of the latent heat of the transition to the product of the temperature and the change in volume ( $Δ V$ ).1

For most substances, the solid phase is denser than the liquid phase, meaning the substance expands upon melting ( $Δ V > 0$ ), resulting in a positive slope. Water is an exception: because of its open, hexagonal crystalline structure, ice is less dense than liquid water. Consequently, when ice melts, it undergoes a negative volume change (it contracts).231

Because $Δ V$ is negative for the transition from ice to water, the slope of the melting curve ( $d p / d T$ ) is negative. This means that increasing the pressure lowers the temperature at which ice melts, a phenomenon that persists until the pressure reaches approximately 2.1 kilobars, at which point ice transitions into a denser crystalline form.1