FORMATION AND PROPERTIES OF HYDROGEN HALIDES 167Although hydrogen chloride is excessively soluble in water it is
not soluble in sulphuric acid.
The fogging of the breath is possible because of the extreme
solubility of hydrogen chloride in water. The fog then consists of
countless minute globules of hydrochloric acid solution.
The hydrogen chloride of course dissolves in the water in the
moist litmus paper and makes hydrochloric acid.
The smoke with ammonia is due to the precipitation of solid
ammonium chloride where the gases HC1 and NH 3 meet. The
ammonium hydroxide dissociates non-electrolytically
NH4OH ^ NH 3 + H 2 O
and the ammonia combines with the hydrochloric acid
NH 3 + HC1 -» NH4CI
These properties of fogging the breath, reddening litmus, and
making a smoke with ammonia are characteristic of volatile acids
and particularly of the hydrogen halides.- Hydrogen Bromide. Add 1 cc. of 36 N sulphuric acid
to about 0.5 gram of powdered potassium or sodium bromide
in a test tube. The salt effervesces in the concentrated acid.
Apply the same tests as in Experiment 12 then look for
other new substances formed, applying the following tests,
and towards the last heating the tube a little. Observe the
color of the gas and also the effect of lowering iodide-starch
paper for a moment only into the tube. Continued exposure
of iodide-starch paper to strong acid fumes will develop a
blue color in any case. Observe the odor, but with great
caution. Test the gases with a strip of filter paper mois-
tened with lead acetate solution. The gas evolved fogs
the breath even more strongly than hydrogen chloride.
It reddens litmus, and it gives a dense smoke with ammonia.
The gas is quite strongly tinged with red; it turns iodide-
starch paper blue immediately; it does not darken lead
acetate paper. Sometimes one is able to distinguish the
odor of sulphur dioxide.
The fogging of the breath, litmus test, and smoking with am-
monia indicate that hydrogen bromide is freely evolved. The
reddish tinge to the gas and the coloring of iodide-starch paper
indicate the presence of a rather small amount of free bromine.