Question

How much energy is transferred when 1 gm of boiling water at $100^\circ$ C condenses to water at $100^\circ$ C?

Answer 1

Hint: Whenever there is change of state the temperature remains constant but still either energy is absorbed or evolved. The energy involved is called latent heat. Here when boiling vapour is condensed to water at $100^\circ $C the energy is released called latent heat of vaporisation and for water the latent heat of vaporisation is 540 cal/gm or 2257J/gm. Heat/energy required= mass*latent heat

Complete step by step answer:
Given mass of water, m=1 gm.
Latent heat of vaporization of water, LV=540cal/gm.
We know that latent heat is the amount of energy required to change the state of a substance of unit mass without a change in its temperature, L=Q/m.
Where, Q=Energy required
               L=Latent heat.
Here the latent heat is in the form of latent heat of vaporisation since condensation takes place (L=LV)
Therefore,
Q=m×LV
$\implies$ Q =1×540
$\implies$ Q =540cal/gm
$\therefore$ Q =2257J/gm

Note:
All the temperature involved in the calculation should be in the same unit to avoid error in the final answer.
Latent heat is defined as the heat or energy that is absorbed or released during change of state of a substance. It could either be from gas to liquid or liquid to solid and vice versa.
Latent heat of vaporisation is the amount of energy that must be added to a liquid substance, to transform it into vapour at its boiling point.
Latent heat of fusion is the amount of energy that must be added to a solid substance, to transform it into liquid at its melting point.