From a macro point of view, any substance has a melting point, that is, any substance heated to a certain temperature will melt and vaporize.
From a molecular point of view, heating can make the molecular movement intensified, so that the movement range becomes larger, making the gap between molecules and molecules larger, making the solid slowly become liquid properties.
Here to say, the classification and nature of solid liquid gas is mainly the molecular spacing changes?
For example, the paper is heated and burned because there is oxygen. When you isolate the oxygen, you find that the paper is carbonized, that is, a lot of things turn into gas. Carbon also has its own melting point, but the temperature is higher Bale.