Question

if it is said that a liquid converts to a gas on heating , why does an egg' s yolk turn to a solid ?

Answer 1

The liquid turns to a gas on heating only when a boiling point of liquid is reached.
And egg is made up of proteins ,so when an egg is boiled at high temperature,entropy of it's molecule increases . This increase in entropy causes denaturation of protein present in egga and results in coagulation of proteins. Thus it gets solidified.
Details of the entropy concept and its effect on egg will be taught to you in your higher classes.