What is the energy required for one molecule of water to evaporate, given the following data? Surface tension is exhibited by liquids due to force of attraction between molecules of the liquid. The surface tension decreases with an increase in temperature and vanishes at boiling point. Given that the latent heat of vaporisation for water (L v ) = 540 k cal kg –1 , the mechanical equivalent of heat (J) = 4.2 J cal –1