Synthetic Inorganic Chemistry

262 OXY-ACIDS AND SALTS OF THE NON-METALS

This experiment should be compared with Experiments 6 (6),
8 (b), and 9, in which sulphurous acid reduces iodic acid, bromine,
and permanganate, it itself being oxidized to sulphuric acid. The
latter is not an oxidizing agent in dilute solution.

13. Nitric Acid as an Oxidizing Agent, (a) To 3 grams of
    copper turnings in a test tube add 5 cc. water and 5 cc. of 6 N
    HNO 3. Fit the stopper with a delivery tube leading to a
    trough of water. Heat the test tube a very little to start the
    reaction; let the gas go to waste until it appears colorless in the
    test tube, then collect a test tube full of it at the water trough.
    Note that the gas is colorless and that it is not soluble in the
    water. Remove the test tube from the trough, turn it mouth
    up, and hold a sheet of white paper behind it; a red gas is seen
    where the gas in the tube meets the air. Note the suffocating
    odor of this gas.

Non-oxidizing acids have no effect on copper but they dissolve
copper oxide. Dilute nitric acid first oxidizes copper, it itself being
reduced to nitric oxide, 3Cu + 2HNO 3 -»• 3CuO + H 2 O + 2NO;
before any further oxidation of the copper occurs, the nitric acid
acts as an acid with the copper oxide giving the soluble copper
nitrate, and it is for this reason that we see no direct evidence of
the intermediate reaction. Nitric oxide has the most remarkable
property of combining spontaneously with oxygen at ordinary
temperature, forming the deep reddish brown gas nitrogen dioxide,
NO + §O 2 -»• NO 2.

(6) To 2 grams of granulated zinc in a test tube add 20 cc. of

water and 2 cc. of 6N HN0 3. Warm the mixture but not

to boiling; then let it stand 5 minutes. Test the solution by

pouring it from the undissolved zinc, adding sodium hydroxide

in excess, and warming, when the odor of ammonia becomes

apparent.

Zinc is a much more powerful reducing agent than copper:
furthermore, zinc displaces from an acid, hydrogen, which, at the
moment of its displacement, is in the atomic or " nascent " condi-
tion, and especially active. Under these conditions the nitrogen
is reduced to its lowest valence, which is shown in ammonia. The
ammonia does not escape from the solution because it combines
with the excess of nitric acid.