Why does pressure usually have only a small effect on a substance’s melting temperature?

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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 temperature of a substance is generally little affected by changes in pressure because the melting curve—which represents the equilibrium between the solid and liquid phases—is significantly steeper than the vapour-pressure curve.12

This relationship is described by the Clapeyron equation, which relates the slope of a phase-equilibrium line ( $d p / d T$ ) to the latent heat of the transition ( $Δ H$ ) and the change in volume ( $Δ V$ ) associated with that phase change. Because the change in volume during melting is typically very small, the resulting slope of the melting line is quite large, meaning that a substantial change in pressure results in only a minimal shift in the melting temperature.231