Question

why is the there no further raise in temperature, when we heat a liquid at its boiling point

Answer 1

This because at this time the heat is used up by water to convert into Gaseous state steam. This heat can not be measured in the thermometer and it is called latent heat.

Answer 2

There is no further raise in temperature, when we heat a liquid at its boiling point because the extra heat is being utilized to break the bonds (intermolecular attractions) to undergo transition. This heat is being absorbed by the liquid in form of latent heat of vaporization.