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A small domestic experiment

In order to understand the number of factors that make a water undersaturated or oversaturated, there is an experiment that we can carry out in the kitchen while we wait for the water to boil before cooking pasta. If we take a glass of water and add salt to it, after a relatively short amount of time, the salt crystals disappear, dissolved in the water, which becomes a water solution (if we stir the solution, the operation takes place faster, because by agitating water we bring aggressive water in contact with the salt crystals, at the same time dispersing the water  saturated with the salt that was near by). If we continue to add salt, initially this will continue to dissolve, but once a certain quantity has dissolved, part of the salt will remain in the form of crystals, on the bottom of the container: we have reached the salt solubility limit at room temperature and our solution is saturated. If we heat the water, the solubility of the salt changes, it increases, and the salt on the bottom of the cooking pot disappears, dissolved in the water that has become undersaturated again. However, if we let the water boil, and we forget the cooking pot on the fire, because of evaporation the solution will become concentrated (with evaporation water is removed, but not the Na and Cl ions) and in a short amount of time the solution will become  oversaturated and begin to deposit a whitish rim of salt on the sides of the cooking pot (together with other salts that were dissolved in the water that we were unaware of, such as calcium carbonate): if we let the process continue, at the end we will find our cooking pot covered with large salt crystals.

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
Eni S.p.A. - P.IVA 00905811006