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I have these very large blocks of salt intended for a water softener in my house, but I'm not sure if they're potassium chloride or sodium chloride, according to what I've read they're both widely used for water softeners. I've thought about distinguishing them by weight but I don't have any sodium chloride with the same granularity as the salt blocks.

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    $\begingroup$ If the blocks are pure salts, flame coloring may be possible. $\endgroup$ – TAR86 May 1 at 20:27
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    $\begingroup$ @TAR86 Flame coloring worked great, turned out it was sodium chloride in case anyone was wondering $\endgroup$ – Faissaloo May 1 at 22:45
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    $\begingroup$ Wonder how pure the KCl for such a use would be. Sodium gives a very strong flame test, potassium much weaker. So for even like 5% sodium (or less..) you'd see a yellow Na flame. It would help to have a piece of cobalt blue glass to look though. $\endgroup$ – MaxW May 2 at 0:49
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    $\begingroup$ @MaxW According to this site they're going to be upwards of 99% purity blog.watertech.com/… $\endgroup$ – Faissaloo May 2 at 0:54
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    $\begingroup$ Flame test is the best test for distinguishing alkali metal ion but it can also be distinguished chemically. In basic qualitative analysis, there is a method of distinguishing $\ce{Na+}$ and $\ce{K+}$. For $\ce{Na+}$, potassium pyroantimonate is used and for $\ce{K+}$ sodium cobaltinitrate/picric acid is used. See: chemistry.stackexchange.com/questions/51653/… $\endgroup$ – Nilay Ghosh May 2 at 5:14
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Per Wikipedia:

..., MgSO4·H2O, can be combined with a solution of potassium chloride to produce potassium sulfate.

So, cooling an aqueous solution KCl in the presence of Epsom's Salt:

$\ce{2 KCl + MgSO4 <=> MgCl2 + K2SO4}$

creates potassium sulfate, which falls out of the solution forming distinct orthorhombic crystals.

If Na2SO4.7H2O is created instead from aqueous NaCl in place of KCl, you will get something that resembles a gluey paste.

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  • $\begingroup$ Nice. Pure KCl might be hard to find at home, but a grocery store will have plenty of NaCl. Wonder what would happen with Morton Lite salt Na:K::1.4:1 $\endgroup$ – MaxW May 2 at 0:42
  • $\begingroup$ Cream of tartar is potassium bitartrate. It would be low in Na. Not sure if there would be so little as to not see any yellow in the flame. $\endgroup$ – MaxW May 2 at 0:52

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