What is the effect of pressure upon the melting and boiling points of a substance?
When we apply pressure on a substance, its melting point increases or decreases depending upon the change of volume which occurs during the phase change of that substance. To be specific, if the volume of the substance's liquid phase is greater than the volume of the solid phase, its melting point will increase upon the increase of volume. Again, if the volume of the substance's liquid phase is less than the volume of the solid phase, its melting point will decrease upon the increase of volume. 
The boiling point of liquids always increases when pressure is applied on that liquid. This is because the molecules of the liquid will require comparatively more energy to turn into gaseous state when pressure is applied on that substance.