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Transformation of a supersaturated solution into solid crystals

It just takes one small nudge to spark the transformation of a seemingly stable liquid into a solid. The liquid – a supersaturated solution of sodium acetate – consists of water with more dissolved sodium acetate salt than can be stably sustained. Drop in a salt crystal and provide the kick needed to start the transformation into the more thermodynamically favourable solid state.

But if the solid state is more thermodynamically stable, why does it exist as a liquid at all? It’s to do with scale: the solution is only better off as a solid at the macroscopic scale – the scale of the whole solution. On a microscopic scale, the individual crystals are not thermodynamically favourable, so until you start it off with a salt crystal (which is, relatively, a huge disturbance), the transformation won’t begin.

Watch the full ‘Tales from the Prep Room’ film for more explanation, extra footage, and the curious problem we encountered when filming.