rface
instantly becomes dim through oxidation by the air. It decomposes water
very vigorously, the heat of reaction being sufficient to ignite the
hydrogen evolved. It is somewhat lighter than sodium and is preserved
under gasoline.

[Illustration: Fig. 78]

~Potassium hydroxide~ (_caustic potash_) (KOH). Potassium hydroxide is
prepared by methods exactly similar to those used in the preparation of
sodium hydroxide, which compound it closely resembles in both physical
and chemical properties. It is not used to any very great extent, being
replaced by the cheaper sodium hydroxide.

~Action of the halogen elements on potassium hydroxide.~ When any one of
the three halogen elements--chlorine, bromine, and iodine--is added to a
solution of potassium hydroxide a reaction takes place, the nature of
which depends upon the conditions of the experiment. Thus, when chlorine
is passed into a cold dilute solution of potassium hydroxide the
reaction expressed by the following equation takes place:

(1) 2KOH + 2Cl = KCl + KClO + H_{2}O.

If the solution of hydroxide is concentrated and hot, on the other hand,
the potassium hypochlorite formed according to equation (1) breaks down
as fast as formed:

(2) 3KClO = KClO_{3} + 2KCl.

Equation (1), after being multiplied by 3, may be combined with equation
(2), giving the following:

(3) 6KOH + 6Cl = 5KCl + KClO_{3} + 3H_{2}O.

This represents in a single equation the action of chlorine on hot,
concentrated solutions of potassium hydroxide. By means of these
reactions one can prepare potassium chloride, potassium hypochlorite,
and potassium chlorate. By substituting bromine or iodine for chlorine
the corresponding compounds of these elements are obtained. Some of
these compounds can be obtained in cheaper ways.

If the halogen element is added to a solution of sodium hydroxide or
calcium hydroxide, the reaction which takes place is exactly similar to
that which takes place with potassium hydroxide. It is possible,
therefore, to prepare in this way the sodium and calcium compounds
corresponding to the potassium compounds given above.

~Potassium chloride~ (KCl). This salt occurs in nature in sea water, in
the mineral sylvine, and, combined with magnesium chloride, as
carnallite (KCl.MgCl_{2}.6H_{2}O). It is prepared from carnallite by
saturating boiling water with the mineral and allowing the solution to
cool. The mineral decomposes while in solution, and the potas