The heating curve is a graph of temperature versus the amount of heat added. The heating curve shows the changes in temperature that result as the substance absorbs an increasing amount of heat. We can also observe that the temperature remains constant while the change of state is in progress. The temperature plateaus because all the heat energy is used to break the intermolecular forces.
Figure 1. Heating curve of water at a constant pressure of 1 atm. 1: Ice is heating towards the melting point; 2: The ice is melting. H2O(s) --> H2O(l) ; 5: The steam is heating to certain temperature.