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This is a very basic question I found myself wondering. For simplicity's sake, we'll say it is a super saturated aqueous solution of an equivalent measure of sugar and salt. As the solution cools and crystals form, do you get sugar crystals, salt crystals, cocrystals or discrete crystals of both?

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When the solution contains several substances, it is very often possible to achieve their separate sequential crystallization due to the different nature of the solubility of the substances as a function of temperature. In a certain temperature range, the solution will always be saturated with respect to one substance and unsaturated with respect to the other. This leads to the isolation of only the first substance from the solution, while the other remains in the mother liquor.

The difference in the compositions of the liquid (vapor) and solid phases formed during the partial crystallization of the solution, melt, and gas phase always leads to the fractions enriched with one component, and sometimes even to practically pure components. Possible degree of separation during crystallization is determined by the type of the phase diagram (composition vs temperature) in the solid-liquid(gas) system.

Obviously, salt and sugar do not form solid solutions, therefore depending on the concentration and temperature one component will crystallize first, and, when water is fully evaporated, a mechanical mixture of crystals is left.

This is a basis of various crystallization methods of mixtures (binary or multicomponent) separation, such as:

  • Fractional crystallization;
  • Crystallization in the presence of auxiliary substances;
  • Fractional melting and sublimation;
  • Multiple recrystallization.
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