Sodium thiosulfate pentahydrate, Na2S2O3 x 5H2O, is a colorless crystalline compound. It melts at 48°C (118°F). When solid sodium thiosulfate pentahydrate is heated over a weak flame, the entire content melts after some time. Heating sodium thiosulfate pentahydrate causes it to dissolve in its own crystal water. It gives a saturated solution of sodium thiosulfate in water.
If we allow it to cool undisturbed bellow its melting point, and if the crystal sodium thiosulfate pentahydrate we used was pure, an unstable supersaturated solution is formed. A supersaturated solution contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. This is a supercooled solution – a liquid phase that remains liquid far below its freezing point. Salt melting point is above room temperature, after cooling the solution below salt’s melting point, such liquid is referred to as supercooled.
A system in a metastable state easily falls into lower energy state with induced action, but only after overcoming an activation energy barrier (as shown on the diagram bellow). When it is disturbed, stirred with a glass rod or when a few crystals of the same salt, so called crystallization initiators are added, it crystallizes rapidly. Seed crystal, but also any other impurity particle gives a structure around which the sodium and thiosulfate ions form a crystal lattice. Even scratched flask would easily initiate crystallization, providing appropriate space cavities for nucleation centers. It is necessary that the glassware is completely clean and sound.
A supersaturated solution of sodium thiosulfate pentahydrate crystallizes immediately upon contact with the seed crystal which acts as a nucleation centre. Upon adding one crystal of sodium thiosulfate to the centre of the Petri dish to the cooled supersaturated solution we can observe the slow crystallization of the entire content. Once it starts it causes the entire solution to solidify.
The process of crystallization is an exothermic process in which heat is given off to the surrounding.