164 NON-METALLIC ELEMENTS IN BINARY COMPOUNDSa vapor — but it also illustrates one of the striking properties of
free iodine, namely its power of turning starch deep blue. We shall
not concern ourselves as to what the blue substance is, but we shall
employ starch in testing for iodine, and look for the blue color to
indicate its presence.
- Iodide-Starch Paper. Add 1 cc. of potassium iodide
solution to 10 cc. of a starch solution. Wet a number of
strips of filter paper with the solution, allow them to dry, and
save for use in certain of the following experiments. The
test papers thus prepared are colorless.
Free iodine colors starch blue, but iodine in combination has
altogether different properties. - Test for the Presence of Chlorine or Bromine. Place
5 cc. each of chlorine water and bromine water in separate wide-
mouthed bottles. Lower for a moment strips of moistened
iodide-starch paper in the mouth of each bottle. The papers
are immediately turned deep blue.
Chlorine and bromine are higher in the electromotive series
than iodine and thus are able to drive it out of the ionic form - Chlorine from Hydrochloric Acid. Place about 0.5
gram each of manganese dioxide, lead dioxide, sodium dichro-
mate, and potassium permanganate in separate test tubes.
Add about 2 cc. of 6 N hydrochloric acid to each and test for
chlorine by holding iodide-starch paper in the mouths of the
tubes. Also after warming a very little observe the odor and
color of the gas. Rinse out the tubes immediately at the sink
under the hood. Compare the action of the oxides used above
with that of copper oxide CuO and lead oxide PbO.
The action of the lower oxides is one of simple metathesis,
chlorine and oxygen simply exchange places, no chlorine being
set free.
CuO + 2HC1 -> CuCl 2 + H 2 O
MnO 2 + 4HC1 -> MnCl 2 + Cl 2 + 2H 2 O
In the case of MnO 2 the reaction is one of oxidation and reduction
(see page 122).
One atom of manganese changes its valence from +4 to +2, and