EXPERIMENTS 259the anhydride of chloric acid, however, does not appear; it breaks
down into the oxide C1O 2 , which is the dark yellow gas, and free
oxygen. Chlorine dioxide is not the anhydride of any known acid,
but it dissolves in water and forms equivalent amounts of chlorous
and chloric acids, in which the valence of chlorine is +3 and
+5 respectively,
2C1O 2 + H 2 O -»HCIO2 + HCIO3Chlorine dioxide is extremely explosive, and it is very dangerous
to make larger amounts of it than that directed in this experiment.
The instability of the oxy-compounds is further shown in (a) in
which chlorine is reduced from a valence of +5 to — 1.- Reduction of Iodic Acid, (a) Dissolve a little potassium
iodate in water and add some starch paste. No color is ob-
served. Add a little potassium iodide to part of the mixture,
and still no color is observed. Now add an acid, for example,
HNO3, and observe that instantly the solution turns deep
blue.
Iodate and iodide ions alone have no action on each other, but
with hydrogen ions present a mutual oxidation and reduction of the
iodine takes place.
6H+ + 51" + IO 3 " -»• 3H 2 O + 3I 2No oxidation or reduction of the hydrogen occurs, but the hydro-
gen ion is used up, which explains why the presence of acid is
necessary to make the reaction take place.
(6) Dip filter paper in the rest of the potassium iodate-
starch mixture and suspend it in a bottle containing a little
sulphurous acid. A deep blue color immediately appears in
the paper. If the paper remains in the bottle the blue is very
quickly bleached, leaving the paper white again.The sulphur dioxide escaping from the solution dissolves in the
water on the paper and the sulphurous acid reduces the iodate to
free iodine which gives the blue color.H 2 SO 3 + 2KIO 3 -» K 2 SO 3 + 2HIO 3
2HIO 3 + 5H 2 SO 3 -»• 5H 2 SO 4 + H 2 O + I 2