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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 [what is] 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.