sium
chloride crystallizes out on cooling, while the very soluble magnesium
chloride remains in solution. The salt is very similar to sodium
chloride both in physical and chemical properties. It is used in the
preparation of nearly all other potassium salts, and, together with
potassium sulphate, is used as a fertilizer.

~Potassium bromide~ (KBr). When bromine is added to a hot concentrated
solution of potassium hydroxide there is formed a mixture of potassium
bromide and potassium bromate in accordance with the reactions already
discussed. There is no special use for the bromate, so the solution is
evaporated to dryness, and the residue, consisting of a mixture of the
bromate and bromide, is strongly heated. This changes the bromate to
bromide, as follows:

KBrO_{3} = KBr +3O.

The bromide is then crystallized from water, forming large colorless
crystals. It is used in medicine and in photography.

~Potassium iodide~ (KI). Potassium iodide may be made by exactly the same
method as has just been described for the bromide, substituting iodine
for bromine. It is more frequently made as follows. Iron filings are
treated with iodine, forming the compound Fe_{3}I_{8}; on boiling this
substance with potassium carbonate the reaction represented in the
following equation occurs:

Fe_{3}I_{8} + 4K_{2}CO_{3} = Fe_{3}O_{4} + 8KI + 4CO_{2}.

Potassium iodide finds its chief use in medicine.

~Potassium chlorate~ (KClO_{3}). This salt, as has just been explained,
can be made by the action of chlorine on strong potassium hydroxide
solutions. The chief use of potassium chlorate is as an oxidizing agent
in the manufacture of matches, fireworks, and explosives; it is also
used in the preparation of oxygen and in medicine.

~Commercial preparation.~ By referring to the reaction between
chlorine and hot concentrated solutions of potassium hydroxide,
it will be seen that only one molecule of potassium chlorate is
formed from six molecules of potassium hydroxide. Partly
because of this poor yield and partly because the potassium
hydroxide is rather expensive, this process is not an
economical one for the preparation of potassium chlorate. The
commercial method is the following. Chlorine is passed into hot
solutions of calcium hydroxide, a compound which is very cheap.
The resulting calcium chloride and chlorate are both very
soluble. To the solution of these salts pot