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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 further processed. If the solid is heated for this purpose, the melting point will be reached at some point. At this point the state of aggregation changes and the solid finally begins to melt. One also speaks of a phase change.

It can be observed that as long as there are still residues of the solid, the temperature does not rise any further, despite the continued supply of heat 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 solid-to-liquid phase transition, which is called latent (“hidden”) heat, enthalpy of fusion or heat of fusion.

Latent heat is a specific quantity related to the mass of each 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

The latent heat is a multiple of the heat required to further increase the temperature. 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 by 80°C! 1000 kJ corresponds to an output of 1 kW for a duration of 1000 s, i.e. around 17 minutes.

In practice, the efficiency of the heating system also comes into play, which is between 50% (heat jacket) and 90% (immersion heater). The actual heating time increases accordingly.


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