Question

Below the boiling point of a liquid evaporation takes place. Then when the liquid is being heated to vaporise it, before reaching its boiling point does the liquid not evaporate?

Answer 1

Evaporation is a different process to boiling.Both evaporation and boiling are processes by which a liquid turns into vapour but the mechanism through which the liquid gets converted into vapour varies.

Evaporation always takes place at any temperature below the boiling point of that liquid ,for example water will evaporate at a temperature below 100 degree celcius at a pressure of 1 atm . The liquid molecules on the top surface of the liquid gain heat energy from the surrounding and the gained energy is enough to break the cohesion force between it and the molecules below it and now it gets converted into vapour ,thus evaporation is a surface phenomena

Boiling takes place only when the temperature of the liquid reaches the boiling point temperature of the liquid at that pressure ,when heat is supplied to the liquid ,the liquid molecules that are closer to the heat source gain heat and their temperature rises,move up and leave the liquid surface as bubble ,the vaccant spot is filled by the adjacent molecule and the process continues , thus boiling is a bulk phenomena.

So when the liquid is being heated to the boiling point , before reaching the boiling point evaporation takes place. This is because evaporation takes place at all temperatures, but only a small quantity of water will be thus evaporated since it is a surface phenomenon. But on reaching the boiling point, the vaporisation process takes place rapidly.