# Which salts precipitate from a potassium/sodium chloride/sulfate solution?

This is not a homework question. I am not in any course of study. The following question arises from my personal curiosity. If 1 mole of sodium sulfate and 2 moles of potassium chloride are dissolved in water, and the solution is gradually evaporated, which salts precipitate, and in which order?

There are four possible single salts that could be precipitated, namely $$\ce {Na2SO4, K2SO4, NaCl, KCl}$$, as well as mixed salts with the general formula $$\ce{((Na,K)Cl)_x.((Na,K)2SO4)_{1-x}}$$, but I don't know how to work out which ones, or what the sequence would be, since I am a complete ignoramus regarding the chemical thermodynamics of these salts, their solutions, and their precipitation from solution. I do understand the basic concept of Gibbs free energy, which I guess would be involved here.

• There is no simple answer to this problem. The result of the evaporation of concentrated solutions of two cations and two anions cannot be predicted by calculation, and are not dependent on the solubilities of the pure salts. It has to be established by experience. Sometimes double salts are obtained, sometimes not. it is entirely empirical. You have to establish the so-called phase diagrams. It is a rather special science. May 28 '20 at 20:15
• @Maurice :Thank you for your helpful answer, which at least establishes that this could not plausibly be a “homework” question. May 28 '20 at 20:46
• Actually @Maurice is again overestimating difficulty of problem, by talking about general situation. In this particular case you'd, most probably, first crystallise Na2SO4 and then KCl. While interactions between solute can complicate things, in this case I think the difference in solubilities to be enough. May 28 '20 at 21:43
• May 28 '20 at 21:48
• @ Mithoron. I agree that your opinion is the most probable. But it is only probable. To be sure, the phase diagram K-Na-SO4-Cl should be studied. Don't forget that Sodium Sulfate may produce $\ce{Na_2SO_4, Na_2SO_4·7H_2O, Na_2SO_4·10H_2O, 3K2SO4·Na_2SO_4}$ depending upon the temperature and the concentrations. May 29 '20 at 8:18