Changes of states can be explained by using the idea that particles are constantly in motion .
This is called the kinetic particle theory
we can explain a heating curve using ideas about the energy and motion of the particles.
Heating Curve
 
At point A Physical status is Solid
From A to B increasing heat energy increases vibration of the solid particles so the temperature of the solid increases
 
From B to C increasing the heat weakens the forces of attraction between particles so they begin to slide over each other . there is no temperature change because all energy supplied is used to overcome the forces between the particles instead of raising the temperature . the substance melts
At C , the substance is all at liquid state.
 
 

 
 
From C to D : increasing the energy increases the motion of the particles in the liquid so the temperature of the liquid increases.
From D to E : forces of attraction between liquid particles weakens to particles start to move away from each other . there is no temperature change as the energy supplied is used to break the forces between the particles , the substance boils .
From E to F : increasing the energy increases the speed of the gas particles . so the temperature increases and the gas particles are getting far away from each other.
 
 
Cooling Curve
Almost opposite is happening