Re: Why are there bubbles formed at bottom of a kettle of hot boilng water ?

I think I understand your question. Just to confirm: the bubbles you see forming are in the tap water that is being used to cool the hot pot/water.

The bubbles you see occur where they do because the surface of the metal pot has a number of nucleation points that allow the bubbles to form. The nucleation points could be scratches, contaminants or anything that makes the surface non-uniform.

These voids in the surface can hold trapped air, which can then form into a bubble or they can be a collection point for gasses that are dissolved in the water to deposit. Both of these cases could lead to the bubbles you see on the surface of the pot.

The actual science involved in the process is fairly complicated but involves balancing the thermal and mechanical forces present in the system. This web site has a much more in-depth discussion of the phenomena. Read the linked page and each of the links listed at the bottom of the page.