Why does it take so long to melt a solid?

In industrial applications, a solid such as wax often has to be melted so that it can be processed further. When the solid is heated for this purpose, the melting point is reached at some point. At this point, the aggregate state changes and the solid finally begins to melt. This is also referred to as a phase transition.

It can be observed that as long as residues of the solid are still present, the temperature does not rise any further despite the continued supply of thermal energy. This phenomenon is also known from everyday life when using ice cubes or pocket warmers.

The physical explanation for this is the thermal energy required for the phase transition from solid to liquid, which is referred to as latent (“hidden”) heat, enthalpy of fusion or heat of fusion.

Latent heat is a specific quantity related to the mass of the respective substance and has the unit J/kg. Example water: spec. heat of fusion 334 kJ/kg; spec. heat capacity 4.19 kJ/kg*K; melting temperature 0 °C.

Latent heat is a multiple of the heat required to increase the temperature further. The same applies to evaporation, i.e. the phase change from liquid to gaseous.

For example, melting 3 kg of ice with 1000 kJ requires exactly the same amount of energy as heating the same amount of water to 80°C! 1000 kJ corresponds to an output of 1 kW for a duration of 1000 s, i.e. about 17 minutes.

In practice, the efficiency of the heating system is also a factor, ranging between 50% (heating jacket) and 90% (immersion heater). The actual heating time increases accordingly.