ice melts, adding more heat will cause the temperature of the water to rise. A similar situation occurs during liquid gas change of state at the boiling point. Adding more heat to boiling water causes vaporisation, without increase in temperature. The heat required during a change of state depends upon the heat of transformation and the mass of the substance undergoing a change of state.
Thus, if mass m of a substance undergoes a change from one state to the other, then the quantity of heat required is given by Q = m L or L = Q/m ( . ) where L is known as latent heat and is a characteristic of the substance. Its SI unit is J kg – . The value of L also depends on the pressure.
Its value is usually quoted at standard atmospheric pressure. The latent heat for a solid- liquid state change is called the latent heat of fusion ( L f ), and that for a liquid-gas state change is called the latent heat of vaporisation ( L v ). These are often referred to as the heat of fusion and the heat of vaporisation. A plot of temperature versus heat for a quantity of water is shown in Fig.
. . The latent heats of some substances, their freezing and boiling points, are given in Table . .
Fig. . Temperature versus heat for water at atm pressure (not to scale). Note that when heat is added (or removed) during a change of state, the temperature remains constant.
Note in Fig. . that the slopes of the phase lines are not all the same, which indicate that specific heats of the various states are not equal. For water, the latent heat of fusion and vaporisation are L f = .
× J kg – and L v = . × J kg – , respectively. That is, . × J of heat is needed to melt kg ice at ° C, and .
× J of heat is needed to convert