THE HALOGENS 163In (a) the yellow soluble substance is lead tetrachloride PbCU
and is formed by the reaction
PbO 2 + 4HC1 -» PbCl 4 + 2H 2 O
Lead tetrachloride is very unstable and decomposes rapidly into
the lower chloride PbCl2 and free chlorine. This chloride of lead
is the same one that is obtained when lead monoxide is treated
with hydrochloric acid:
PbO + 2HC1 -» PbCl 2 + H 2 OSince the 2 oxygens of lead dioxide are replaced by 4 chlorines,
and the lead is exchangeable for 4 hydrogens of the acid, the
valence of lead is established as 4 and therefore each of the divalent
oxygens is held as a separate unit by the lead.
In (6) the barium peroxide acts as we should expect with hydro-
chloric acid, yielding hydrogen peroxide. Since hydrogen peroxide
is unstable, breaking down into water and oxygen, we should expect
that it would react, at least to some extent, with the excess of
hydrochloric acid and set free chlorine.
H2O2 + 2HC1 -> 2H 2 O + Cl 2
This experiment therefore has shown that lead dioxide is an
ordinary oxide. The prefix di indicates the quantity of oxygen
and that the valence of the metal is sufficient to hold all the
oxygen in the ordinary manner.
THE HALOGENS
Recall that chlorine is a greenish yellow gas, bromine is a dark
red liquid which readily vaporizes to a red gas, and iodine is a
nearly black solid which is changed by heat to a beautiful violet-
colored gas.
- Test for the Presence of Iodine. Volatility of Iodine.
Place some small crystals of iodine in the bottom of a 2-liter
bottle and suspend a piece of filter paper moistened with
starch paste in the upper part of the bottle. Cover the
whole with a watch glass, allow it to stand 15 minutes or more,
and observe that the paper slowly turns blue.
This experiment not only shows the volatility of iodine — it must
pass through the space between the solid crystals and the paper as