Melting point of wax increase with increase in pressure but that of ice decrease. why?

Dear Student,

Melting is a process of state change under which solid changes its state to liquid. And the melting point is the particular temperature at which the solid and the liquid phase are in equilibrium. Now we know pressure decreases the volume of a substance. Thus when a pressure is applied on a substance that is undergoing phase change (solid to liquid) it disturbs the equilibrium of a system. The Le Chatelier's principle as we know the system in equilibrium will change in such a way to undo the effect of pressure that is the volume of the substance would decrease. Now since solid (like wax) which generally has a lower volume for a given mass of substance (since solid has higher density) therefore system tends to remain in the solid state to undo the effect of pressure. Hence melting point generally increases (for wax etc) but with water the thing is different, solid water (ice) has lower density than water at 4C, thus volume of ice is greater than water for a given mass of water.Thus, on application of pressure on water-ice system at equilibrium  favors a decrease in volume, that is the system tends to assume a low volume state and tends to be liquid (liquid water has low volume than ice) for a higher temperature under pressure, hence melting occurs at a lower temperature hence melting point is decreased.