The phase diagram for a substance shows a solid-liquid coexistence line with a negative slope. This implies which of the following?
Correct: A
A negative slope for the solid-liquid coexistence line in a phase diagram indicates that the melting point of the substance decreases as the external pressure increases. This is a unique characteristic of substances that expand upon freezing (i.e., the solid phase is less dense than the liquid phase, like water or bismuth). When pressure is applied, it generally favors the denser phase. If the liquid is denser than the solid, increasing pressure shifts the equilibrium towards the liquid state, thereby lowering the temperature at which the solid can melt (or liquid can freeze). So, 'the substance expands on freezing' is the correct implication.