Spontaneous Crystallisation:

This experiment was carried out with three substances - sodium acetate, sodium sulphate and sodium thiosulphate.

Distilled water (approx. 25 ml) is stirred into the salt (e.g. 250 g sodium sulphate): Thereby, the solution cools down!

Afterwards, the solution is heated to 50 - 60°C, filtered and stored in the fridge in a beaker covered with aluminium foil. If you manage to produce a solution that contains no impurities, cooling down will supersaturate the solution and no crystals will form!

As soon as a 'seed crystal' - that is a small crystal - is added, the whole solution turns solid all of a sudden. This is called spontaneous crystallisation. Thereby, heat is released, and the beaker becomes hot!

In our experiments, this had only worked with sodium acetate, and no seed crystal was necessary. After covering the beaker, the spontaneous crystallisation occurred immediately, probably due to a contamination that fell into the beaker.

In the other two solutions, crystals had already formed on the bottom while in the fridge. However, it was still possible to induce crystallisation of the whole solution in one of the solutions.

The production of the sodium acetate solution

Warning! The beaker becomes considerably hot!

The whole solution has become solid!