experiments show that the volume of hydrochloric acid formed is just
equal to the sum of the volumes of hydrogen and chlorine. Therefore one
volume of hydrogen combines with one volume of chlorine to form two
volumes of hydrochloric acid gas. Since chlorine is 35.18 times as heavy
as hydrogen, it follows that one part of hydrogen by weight combines
with 35.18 parts of chlorine to form 36.18 parts of hydrochloric acid.

~Physical properties.~ Hydrochloric acid is a colorless gas which has an
irritating effect when inhaled, and possesses a sour, biting taste, but
no marked odor. It is heavier than air (density = 1.26) and is very
soluble in water. Under standard conditions 1 volume of water dissolves
about 500 volumes of the gas. On warming such a solution the gas
escapes, until at the boiling point the solution contains about 20% by
weight of HCl. Further boiling will not drive out any more acid, but the
solution will distill with unchanged concentration. A more dilute
solution than this will lose water on boiling until it has reached the
same concentration, 20%, and will then distill unchanged. Under high
pressure the gas can be liquefied, 28 atmospheres being required at 0 deg..
Under these conditions it forms a colorless liquid which is not very
active chemically. It boils at -80 deg. and solidifies at -113 deg.. The
solution of the gas in water is used almost entirely in the place of the
gas itself, since it is not only far more convenient but also more
active.

~Chemical properties.~ The most important chemical properties of
hydrochloric acid are the following:

1. _Action as an acid._ In aqueous solution hydrochloric acid has very
strong acid properties; indeed, it is one of the strongest acids. It
acts upon oxides and hydroxides, converting them into salts:

NaOH + HCl = NaCl + H_{2}O,
CuO + 2HCl = CuCl_{2} + H_{2}O.

It acts upon many metals, forming chlorides and liberating hydrogen:

Zn + 2HCl = ZnCl_{2} + 2H,
Al + 3HCl = AlCl_{3} + 3H.

Unlike nitric and sulphuric acids it has no oxidizing action, so that
when it acts on metals hydrogen is always given off.

2. _Relation to combustion._ Hydrochloric acid gas is not readily
decomposed, and is therefore neither combustible nor a supporter of
combustion.

3. _Action on oxidizing agents._ Although hydrochloric acid is
incombustible, it can be oxidized under some circumstances, in which
case the hydrogen combines with oxygen, while the c