Question

what is meant by saying that the latent heat of vaporisation of water is 22•5×10^5 j/kg

Answer 1

Heat supplied to water for vaporisation takes place in types, one is sensible heat and other is latent heat. Sensible heat is responsible for rising the temperature till 100 deg C without changing the phase, while latent heat is responsible for changing the phase from water to water vapours. 22.5*10^5J/kg latent heat means that for vaporising 1kg of water which is already at it's boiling point you need 22.5*10^5 J of energy as heat.