Specific Latent Heat Latent heat of vaporisation: Energy is needed to turn a liquid into a vapour. (overcoming inter-atomic attractive forces increases the potential energy of the molecules) The energy required to change 1kg of liquid into vapour at the same temperature is called the specific latent heat of vaporisation ( L vap ).  U = m L vap eg L vap water = 2.3 x 10 6 J kg -1 Application: To evaporate sweat requires energy (its latent ht. of vap.) which it takes from the body and cools the body down. 100 o C

Specific Latent Heat Latent heat of Fusion: Energy is needed to turn a solid into a liquid. (overcoming inter-atomic attractive forces increases the potential energy of the molecules) 0oC 0oC 0oC 0oC The energy required to change 1kg of solid into liquid at the same temperature is called the specific latent heat of fusion ( L vap ).  U = m L fus eg L fus water = 3.34 x 10 5 J kg -1 Application: It is important to thoroughly defrost food before cooking. ( a lot more energy is needed to defrost than cook)

Specific Latent Heat Estimate the total energy which must be provided by a weapon to vaporise an iceberg of volume 1 cubic kilometre. Density of water = 1000 kg m o C 0 o C 100 o C  U 4 = mL vap  U 2 = mL fus  U 1 = m c  T  U 3 = m c  T