336 GROUP VII: THE HALOGENS
exposed to ultraviolet light or by the action of ozone on chlorine
dioxide:
6C1O 2 + 2O 3 -> 3C1 2 O 6It is a liquid at room temperature, melting point 276.5 K. The
molecular weight, determined in carbon tetrachloride, indicates the
dimefic formula, but magnetic measurements show the presence of
small quantities of the paramagnetic monomer C1O 3 in the pure
liquid. It is rather an unstable compound and decomposes slowly
even at its melting point, and more rapidly on heating, forming
finally oxygen and chlorine. It is a powerful oxidising agent and
reacts violently even with water with which it forms a mixture of
chloric(V) and chloric(VII) acids.
Dichlorine htptoxide, C1 2 O 7 , is the most stable of the chlorine
oxides. It is a yellow oil at room temperature, b.p. 353 K, which will
explode on heating or when subjected to shock. It is the anhydride
of chloric(VIl) acid (perchloric acid) from which it is prepared by
dehydration using phosphorus(V) oxide, the acid being slowly
reformed when water is added.
Bromine oxides
These are all unstable substances and little is known about them.
Dibromine monoxide, Br 2 O, is prepared, similar to the corres-
ponding dichlorine compound, by the action of a solution of
bromine in carbon tetrachloride on yellow mercury(II) oxide:
2HgO + 2Br 2 -» Hg 2 OBr 2 + Br 2 OIt is a dark brown liquid, m.p. 256 K, which decomposes rapidly at
room temperature.
Tribromine octoxide, Br 3 O 8 , is a white solid obtained when ozone
and bromine react together at 273 K at low pressure. It is unstable
above 200 K in the absence of ozone. It is known to exist in two
forms, both soluble in water.
Bromine dioxide^ BrO 2 , is prepared by passing an electric discharge
through a mixture of oxygen and bromine at low temperature and
pressure. It is a yellow solid, stable only below 230 K, decomposing
above this temperature to give oxygen and bromine.