What happens when pcl5 is heated?Explain its reaction and the product??

Actually PCl5 is thermally  less stable than PCl3. 

So, 

When it is strongly heated, Two P-Cl bonds which are of higher bond length (that is, at axial positions) break down and two Cl atoms are released which combine to form Cl2. 

So, when PCl5 is heated reaction is follows:

PCl5 gives PCl3 and Cl2! 

I Hope it helps you!