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I have noticed (during an experiment) that the boiling point of the solutions created by H2O+KBr; H2O+CaCl2 and H2O+AlCl3 see their boiling point increase.

During the experiment, I thought that, since every time the number of Ions dissociated in each solution increases, the boiling point of each solution should increase with the number of Ions formed: BP(H2O+AlCl3)>BP(H2O+CaCl2)>BP(H2O+KBr)

But apparently this is true only when the concentration of the salt in the water is small: in fact, with high concentrations, the solution formed by CaCl2 reaches much higher boiling points than the one formed by AlCl3.

Is there someone who can help me to understand?

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    $\begingroup$ At high concentrations you'll run into all sorts of problems: non-linear effects (which can't be predicted quite as easily), solubility limit, and hydrolysis of AlCl3. $\endgroup$ – Ivan Neretin Oct 14 '17 at 11:16
  • $\begingroup$ I believe that Raoults law (as all properties of solutions) is only correct at finite concentrations if you take don't take the concentration, but the activity of each ion. $\endgroup$ – Karl Oct 14 '17 at 11:56

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