In the heating curves of a body it seemed that the increase in its temperature was proportional to the energy absorbed.  However, there are cases in which this rule does not apply.  Especially when the body is changing its state of aggregation. We all know that matter basically presents itself to us in three possible states: solid, liquid and gaseous.  Each of these states is due to the existence of different binding energies among the molecules of a body.  When the body changes state we are altering this form of internal energy instead of increasing the temperature. 
  

How is this phenomenon translated onto the heating curve of a body? To study this we will observe the case of a block of ice of 100 g mass which we progressively heat until we convert it into steam.

We should tell you that in this visual, for the sake of simplicity, we are ignoring the quantity of water which evaporates while the temperature increases from 0ºC to 100ºC.  We should also take into account that the fusion and boiling point temperatures observed are only true at the pressure of one atmosphere and with distilled water. The change of state temperature depends on the pressure conditions in the environment where the substance is and the latter's degree of purity.