(N/A) On heating,when ice is converted into water and we continue heating,the temperature begins to rise. The temperature continues to rise until it reaches nearly $100^{\circ} C$,at which point it becomes steady.
The heat supplied is now utilized to change water from the liquid state to the vapour or gaseous state.
The change of state from liquid to vapour (or gas) is called vaporization.
It is observed that the temperature remains constant until the entire amount of the liquid is converted into vapour.
Both the liquid and vapour states of the substance coexist in thermal equilibrium during the change of state from liquid to vapour.
The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point.
The process of boiling of water:
$1$. Take a round-bottom flask,more than half-filled with water.
$2$. Place it over a burner and fix a thermometer and a steam outlet through the cork of the flask.
$3$. As water gets heated in the flask,note first that the air,which was dissolved in the water,will come out as small bubbles.