potassium chromate as an indicator.

Chlorine and oxygen do not combine directly, but compounds can be
obtained indirectly. Three oxides are known: chlorine monoxide, Cl2O,
chlorine peroxide, ClO2, and chlorine heptoxide, Cl2O7.

Chlorine monoxide results on passing chlorine over dry precipitated
mercuric oxide. It is a pale yellow gas which can be condensed, on
cooling, to a dark-coloured liquid boiling at 5 deg. C. (under a
pressure of 737.9 mm.). It is extremely unstable, decomposing with
extreme violence on the slightest shock or disturbance, or on exposure
to sunlight. It is readily soluble in water, with which it combines to
form hypochlorous acid. Sulphur, phosphorus, carbon compounds, and
the alkali metals react violently with the gas, taking fire with
explosive decomposition. A.J. Balard determined the volume composition
of the gas by decomposition over mercury on gentle warming, followed
by the absorption of the chlorine produced with potassium hydroxide,
and then measured the residual oxygen.

Chlorine peroxide was first obtained by Sir H. Davy in 1815 by the
action of concentrated sulphuric acid on potassium chlorate. As this
oxide is a dangerous explosive, great care must be taken in its
preparation; the chlorate is finely powdered and added in the cold, in
small quantities at a time, to the acid contained in a retort. After
solution the retort is gently heated by warm water when the gas is
liberated:--3KClO3 + 2H2SO4 = KClO4 + 2KHSO4 + H2O + ClO2. A mixture
of chlorine peroxide and chlorine is obtained by the action of
hydrochloric acid on potassium chlorate, and similarly, on warming a
mixture of potassium chlorate and oxalic acid to 70 deg. C. on the water
bath, a mixture of chlorine peroxide and carbon dioxide is obtained.
Chlorine peroxide must be collected by displacement, as it is soluble
in water and readily attacks mercury. It is a heavy gas of a deep
yellow colour and possesses an unpleasant smell. It can be liquefied,
the liquid boiling at 9.9 deg. C., and on further cooling it solidifies
at -79 deg. C. It is very explosive, being resolved into its constituents
by influence of light, on warming, or on application of shock. It is a
very powerful oxidant; a mixture of potassium chlorate and sugar in
about equal proportions spontaneously inflames when touched with a rod
moistened with concentrated sulphuric acid, the chlorine per