Principle

The course of sodium thiosulfate’s temperature during melting
and subsequent freezing is measured. Moreover, it can be clearly
seen that heat is necessary for the melting process: the
temperature does not rise until all the salt has melted. During
freezing this energy is released. This can be seen especially well
when the molten salt first undercools and
then immediately freezes when a salt crystal is dropped into
it. The temperature rises immediately to the melting point.