EXPERIMENTS 261
- Oxidizing Action of Sulphur Dioxide and Sulphurous
Acid, (a) Fill a 250-cc. wide-mouth bottle with sulphur
dioxide and lower into the gas a burning strip of magnesium
ribbon held by pincers. The magnesium continues to burn
brilliantly, forming the same white smoke as if it burned in
air. There is also noticed a yellowish deposit on the glass
(sulphur).
(6) Pass hydrogen sulphide into a solution of sulphurous
acid. A milky precipitate is formed.
Although sulphur has considerable affinity for oxygen, its
strength in this respect is far less than that of the active metal
magnesium.
In (6) we have another case similar to that in Experiment 6 (a)
in which an element existing in different states of oxidation mu-
tually oxidizes and reduces itself.
H 2 SO 3 + 2H 2 S -»3H 2 O + 3S j
4 + 2(-2) = 0
- Dehydrating Action of Sulphuric Acid. Add 10 cc. of
36 N H2SO4 to 5 grams of sugar in a porcelain dish. If
necessary warm the mixture a little to start a reaction. Once
started the reaction proceeds vigorously giving off a good deal
of heat, and the sugar swells up and grows black. Finally,
there is left a bulky brittle charcoal-like mass.
The formula of sugar, C12H22O11, shows that it contains hydrogen
and oxygen in the proportion to form water. The affinity of sul-
phuric acid for water is so great that it causes the sugar to decom-
pose so as to yield carbon and water.
- Oxidizing Action of Sulphuric Acid. In separate test
tubes heat sulphur, charcoal, and copper turnings with 36 N
H2SO4, and hold potassium iodate-starch paper in the mouth
of each tube. In each case the test paper is turned blue.
Concentrated sulphuric acid is an oxidizing agent (cf. Experi-
ments 13 and 14, pages 167-168), it being reduced usually to sul-
phurous acid (SO 2 ).
2H2SO4 + S -^ 3SO 2 + 2H 2 O
2H2SO4 + C -• 2SO 2 + CO 2 + 2H 2 O
{ H2SO4 + Cu -* CuO + SO 2 + H 2 O
I CuO + H 2 SO 4 -» CuSO 4 + H 2 O
