170 NON-METALLIC ELEMENTS IN BINARY COMPOUNDS
Place about 0.5 gram of powdered sodium bromide in a test
tube with a little concentrated phosphoric acid solution, heat,
and test the evolved gas for free halogen as well as for hy-
drogen bromide. Repeat, using sodium iodide in place of
the sodium bromide.
In both experiments, fogging of the breath, reddening of
litmus, and smoke with ammonia are noted. With the so-
dium bromide, the gas is entirely uncolored and no test with
iodide starch paper is given. With sodium iodide the gas
is at first entirely colorless but when the tube is heated
strongly the film of moisture condensing on the walls of the
tube becomes slightly brown. No test is given with starch
paper.Phosphoric acid is a much weaker oxidizing agent than sul-
phuric acid, and it fails to oxidize either hydrogen bromide or
hydrogen iodide. The brown color, it is true, indicates a trace
of free iodine, but this is accounted for by a direct decomposition
of hydrogen iodide by heat.
CHARACTERISTIC REACTIONS OF THE HALIDE IONS
The halides of all the metals except silver, lead, mercurous
mercury, and cuprous copper are soluble in water, but with the ions
of these metals, the halide ions give characteristic precipitates.
The precipitates are valuable as tests for identifying either the hal-
ogens or the metals in qualitative analysis.
- (a) Add a few drops of silver nitrate solution to a
few drops each of hydrochloric, hydrobromic, and hydriodic
acid solutions, diluted with 5 cc. of water, in separate test
tubes. Let each precipitate settle, pour off most of the
liquid, and find whether portions of the precipitate dissolve
in a large amount of boiling water. Test also the solubility of
each in ammonia.
(b) Add lead nitrate solution also to each of the three
acids and test the solubility of the precipitates in hot water.
Tabulate the results obtained in (a) and (6).
The exact figures for the solubility at different temperatures
will be found in the solubility table in the Appendix. The
solubility of the silver halides in ammonia solution depends on the