Latent heat of fusion
Just as latent heat is taken in when water changes to vapour at the same temperature, so the same thing occurs when ice melts to form water. But in this case the latent heat is not so great. It requires only 336 000 J to convert 1 kg of ice at 0 °C to water at the same temperature. Likewise, when water at 0 °C freezes into ice, the same quantity of heat is given out for every 1 kg of ice formed. This is called the specific latent heat of ice.
As already mentioned, the phenomenon of latent heat is not confined to water alone. Other substances also absorb latent heat when they melt; conversely, they give out latent heat on solidifying. This heat is called latent heat of fusion. The specific latent heat of fusion of a substance is the quantity of heat required to convert unit mass of the substance from the solid to the liquid state without change of temperature. (Symbol = t.) The same units, J/kg, or alternatively kJ/kg or Ml/kg, are used for fusion as for
vaporization. The measurement of specific latent heat is best done by electrical methods which are fully dealt with in chapter 42.