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~Hydrochloric acid~ (_muriatic acid_) (HCl). The preparation of
hydrochloric acid may be discussed under two general heads:

1. _Laboratory preparation._ The product formed by the burning of
hydrogen in chlorine is the gas hydrochloric acid. This substance is
much more easily obtained, however, by treating common salt (sodium
chloride) with sulphuric acid. The following equation shows the
reaction:

2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.

The dry salt is placed in a flask furnished with a funnel tube and an
exit tube, the sulphuric acid is added, and the flask gently warmed. The
hydrochloric acid gas is rapidly given off and can be collected by
displacement of air. The same apparatus can be used as was employed in
the preparation of chlorine (Fig. 53).

When a _solution_ of salt is treated with sulphuric acid there
is no very marked action. The hydrochloric acid formed is very
soluble in water, and so does not escape from the solution;
hence a state of equilibrium is soon reached between the four
substances represented in the equation. When _concentrated_
sulphuric acid, in which hydrochloric acid is not soluble, is
poured upon dry salt the reaction is complete.

2. _Commercial preparation._ Commercially, hydrochloric acid is prepared
in connection with the manufacture of sodium sulphate, the reaction
being the same as that just given. The reaction is carried out in a
furnace, and the hydrochloric acid as it escapes in the form of gas is
passed into water in which it dissolves, the solution forming the
hydrochloric acid of commerce. When the materials are pure a colorless
solution is obtained. The most concentrated solution has a density of
1.2 and contains 40% HCl. The commercial acid, often called _muriatic
acid_, is usually colored yellow by impurities.

~Composition of hydrochloric acid.~ When a solution of hydrochloric acid
is electrolyzed in an apparatus similar to the one in which water was
electrolyzed (Fig. 18), chlorine collects at the anode and hydrogen at
the cathode. At first the chlorine dissolves in the water, but soon the
water in the one tube becomes saturated with it, and if the stopcocks
are left open until this is the case, and are then closed, it will be
seen that the two gases are set free in equal volumes.

When measured volumes of the two gases are caused to unite it is found
that one volume of hydrogen combines with one of chlorine. Other