Ehrenfest classification has been put forward by Paul Ehrenfest. According to this classification phase transitions were labeled by the lowest derivative of the free energy that is discontinuous at the transition. In this classification, solid/liquid/gas transitions are termed as first order phase transition.
A highlighting feature of first-order phase transition is the discontinuous changes in variables. Whereas in the second order phase transition the variables are continuous. We will not be going into details of this here but would clearly mention that Ehrenfest classification is not perfect and has some loopholes. Discussing them in our current article would not be feasible. These loopholes have led to the development of the modern classification.
In the modern classification, the first order transitions are governed by the principle of latent heat. During such transition, the system either releases or absorbs a certain amount of energy per given volume. Here the temperature of the system remains constant as the heat is added. That means the excess heat is given off while the process is in place. In modern classification, the second order is also called continuous phase transition. It is a very complex phenomenon to study. But it is more concrete than the earlier classification.
Now you know the basics of the phase transition, its time to understand the answer to the question that this article intends to resolve. See, any substance which has combustion temperature which is lower than its melting point will burn instead of melt.
Before they have a chance of changing their phase they react with oxygen in the atmosphere and combust (burn). On the other hand, substances which have a combustion temperature higher than their melting point will begin the phase transition process and melt (change their phase). It’s as simple as that.
Hope this suffices the answer that you guys were looking for.