In this experiment we heated sodium acetate trihydrate (CH3COONa x 3H2O) in the flask. Salt melting point is above room temperature, at 54°C/130°F. When the entire quantity of the salt we used melted, it gave a supersaturated solution of sodium acetate in its own crystal water that was left to cool down spontaneously. A supersaturated solution contains more of the dissolved material than could be dissolved by the solvent under normal circumstances.
Such liquid phase at room temperature, that is well below its melting point without forming crystals, is referred to as supercooled solution. Similarly we may find liquid cloud droplets in the atmosphere where the temperature is below freezing point.
When melted solution of sodium acetate trihydrate is cooled bellow its melting point, a supersaturated solution is formed and is very unstable. 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 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 acetate ions form a crystal lattice. Even scratched flask would easily initiate crystallization, providing appropriate space cavities for nucleation centers.
A supersaturated solution of sodium acetate crystallizes immediately upon contact with the seed crystal which acts as a nucleation center. Upon pouring the solution onto crystal, its color changes from clear to white while stalagmite like white solid is formed.
The process of crystallization is an exothermic process in which heat is given off to the surrounding. Enough heat is emitted that this reaction found its commercial application in hot pack. A chemical hot pack is an example of a commercially available supersaturated sodium acetate solution. It gives instant heat by clicking on a metal disk inside a sealed plastic bag holding the supersaturated sodium acetate solution. The crystallization is rapid once a seed crystal is available. Clicking the disk, forces a few molecules to flip to the solid state, while the rest start building up the crystal structure. In order for the crystallization to occur, very small particles of crystalline sodium acetate must form, and then aggregate into larger crystals. While crystal is growing which we can see through the liquid, heat is being emitted. The solidifying liquid temperature jumps up to 54°C/130°F. This can be noted as chain reaction since once it starts it causes the entire solution to solidify.
After it gradually cools down supersaturated solution returns to its original, more stable solid state. After boiling the hot pack in water we can again melt the crystals and use it again.
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