Question

Why doesnt a pure liquid in a distilling flask vaporize all at once when the boiling point is reached?

Answer 1

Boiling point is the temperature at which vapor pressure is equal to the atmospheric pressure. The vapor pressure of the liquid is the pressure created by its vapor. Liquid is vaporized only if the intermolecular force is lower than then then energy of molecule to escape in vapor phase from its liquid phase.

We know on the surface of the liquid the intermolecular force is lower as compare to the molecules in the bulk of the liquid. So the molecules at the surface escape easily than the molecules in the liquid. At boiling point the molecules at the surface escapes and go into the gas or vapor phase and so liquid is converted into vapor slowly since after removing one layer of the molecules at the surface second layer start to evaporate.

This is the reason why liquid does not vaporize all of once.