In most pure substances, melting occurs at a specific temperature called the what?
Pure substances have a certain temperature at which they become liquid (they melt). This point is called the melting point.
At this temperature, the energy of the molecules have reached a certain temperature at which the bonds between the molecules are weakened. This results in the observation that the substance becomes liquid! The molecules are more moving instead of sitting at a certain position.
When cooling down again, the energy of the molecules decrease again and the motion of the molecules will decrease resulting in a solid compound (like ice!)