RELATIVE ACTIVITY 171formation of the complex ion, Ag-2NH 3 + (see Chapter III, page
118). Silver bromide and silver iodide are decreasingly less sol-
uble in pure water than the chloride, although all three seem by the
direct experiment to be completely insoluble. Thus, In a solu-
tion containing ammonia, the Ag-2NHS+ ion does not dissociate
enough for the simple silver ions and chlorine ions to reach the
solubility product of silver chloride. The solubility product of
silver bromide is sooner reached, and silver bromide is only spar-
ingly soluble in ammonia solution. The solubility product of silver
iodide is still smaller, and this accounts for the apparent complete
insolubility of silver iodide in ammonia solution.
RELATIVE ACTIVITY OF THE HALOGENS, OXYGEN, AND SULPHUR
The activity of an element is judged by its power to combine
with other elements, or by its ability to pass from the elementary
condition into solution in the form of ions, whereby it forces other
less active elements out of the ionic into the elementary condition.
There can be no doubt as to the relative activity of the halogens
as regards their tendency to form simple negative ions. The
position of oxygen in this respect is harder to define, for no simple
ions of oxygen exist in solution. Under certain conditions chlo-
rine displaces oxygen from water, as in Experiment 10, page 69;
yet some substances which give up oxygen readily (oxidizing
agents, see Experiment 10 of this chapter) set chlorine free from
hydrochloric acid.
- Observe the color of the stock solution of hydriodic
acid, or of a dilute solution of KI acidified with a little
H 2 SO 4. If this should chance to be fresh and perfectly color-
less, pour a few cubic centimeters into the bottom of a beaker
and let it stand some time exposed to the air. Apply tests
to determine whether the color is caused by free iodine.
Are hydrobromic and hydrochloric acids similarly affected
by exposure to the air? Is a solution of potassium iodide so
affected?
Hydriodic acid solution can be prepared perfectly colorless and
kept colorless so long as it is kept out of contact with the air.
In contact with air it becomes brown in a very few minutes and
the brown color gradually increases in depth. This color is due