Distinguishing between potassium chloride and sodium chloride 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.
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:

$\ce{2 KCl + MgSO4 &lt;=&gt; MgCl2 + K2SO4}$

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.