The specific latent heat of vaporisation (I) of a substance is the amount of heat energy required to change 1 kg of the substance from a liquid to a gas without a temperature change.
NOTE
When doing heat energy calculations, always take care with the units. You will find that individual parts of units will cancel and leave you with the required unit for your answer. For example, Q = mc∆θ in units is (J) = (kg)(J kg−1 K−1)(K). This leaves you with joules = joules.
The specific latent heat of fusion (I) of a substance is the amount of heat energy required to change 1 kg of the substance from a solid to a liquid without a temperature change.
The symbol for specific latent heat of fusion or vaporisation is l and it is measured in J kg–1.
Note that the same amount of energy is emitted in the change from liquid to solid and the change is still referred to as fusion change. Similarly, the same amount of energy is emitted in the change from gas to liquid and the change
is still referred to as vapour change. The formula for calculations involving specific latent heat is:
Specific latent heat Q = ml
Q = heat energy (J), m = mass (kg), l = latent energy (vapour or fusion) (J kg–1) NOTE
Unless particular values are given in a question, use the values in Table 11.2 or consult the Formulae and Tables booklet.
Fusion of ice Vaporisation of water TABLE 11.2 Specific latent heat values QUESTIONS AND ANSWERS
1. An energy bar that is rated as containing 2.5 kJ of energy is completely burned underneath a test-tube of 100 g water. Calculate the resulting rise in temperature of the water, assuming the test-tube is of negligible heat capacity and there is no heat lost to the surroundings. (SHC of water = 4190 J kg−1 K−1)
