166 NON-METALLIC ELEMENTS IN BINARY COMPOUNDSAlthough this experiment is almost a repetition of Experiment 9,
it does illustrate another method of recognizing small amounts of
free bromine and iodine. It emphasizes again that, corresponding
to their position in the electromotive series, chlorine is more ac-
tive than bromine and bromine in turn more active than iodine.
Iodine can be liberated from its salts by the weakest oxidizing
agents. It should be noted that in this connection the halogens
themselves are oxidizing agents; for, in the reaction
2KI + Br 2 -> 2KBr + I 2
the bromine has been reduced from a valence of 0 to — 1, while
the iodine has been oxidized from valence — 1 to valence 0.
Fluorine is the strongest oxidizing agent of all. It liberates
the other halogens from their compounds. It will even liberate
oxygen from water. It is impossible to liberate fluorine from
hydrofluoric acid or fluorides by any of the chemical oxidizing
agents because there is no other electro-negative element which
exceeds it in activity. It is impossible to liberate it from an
aqueous solution by electrolysis because the less active oxygen is
set free instead. Fluorine can be prepared by electrolysis of
a solution of potassium fluoride in anhydrous hydrogen fluoride.
This solution is a good electrolyte, and since it contains no other
negative ion than fluoride, it is fluorine that has to be discharged
at the anode.
FORMATION AND PROPERTIES OF THE HYDROGEN HALIDES- Hydrogen Chloride. Add 2 cc. of concentrated sul-
phuric acid (36 N) to about 0.5 gram of sodium chloride
in a test tube and warm it a very little if necessary. The salt
effervesces in the concentrated acid. Test for hydrogen
chloride gas by blowing gently across the mouth of the tube,
by holding moistened litmus in the gas, and by bringing a
strip of filter paper moistened with ammonium hydroxide
near the mouth of the tube. The gas issuing from the tube
creates a dense fog with the breath. The gas turns moistened
litmus paper red, and it produces a dense white smoke in the
vicinity of the paper moistened with ammonium hydroxide.
The main reaction consists of the displacement of a volatile
acid from its neutral salt by means of a non-volatile acid.
NaCl + H2SO4 -» NaHSO 4 + HC11