54 WATER AND SOLUTIONhigher temperature is cooled to 10° and stirred until an equilibrium
is attained, all but 21 grams of the salt crystallizes out, and the
solution has exactly the same concentration as that obtained in the
other way.
A very important fact concerning solubilities is that the solu-
bility of a given salt is practically unaffected by the presence of
another salt in the solution, provided only that the other salt does
not possess one of the same ions as the first salt.
For example, suppose that sodium nitrate and potassium chloride
in equivalent amounts are added to 100 grams of water at 10°, so
that the total weight of K+ and NO 3 ~ radicals will be 42 grams;
the potassium nitrate in excess of its solubility will then crystallize
out, and 21 grams of the crystals will thus be obtained. The
presence of the radicals of sodium chloride in the solution is with-
out effect on the potassium nitrate.
In the following procedure 2 formula weights (F.W.) of NaN0 3
(170 grams) and 2 F.W. of KCl (149 grams) are together treated
with 210 grams of water at the boiling temperature. The 170
grams of NaN0 3 will dissolve in the water since this amount will
not nearly saturate 210 grams of water, but from the table it is
seen that it will take only 118 grams of KCl to saturate this amount
of water. If this were the only consideration we should expect
118 grams to dissolve and 31 grams of KCl to remain undissolved.
But we must consider the presence of the Na+ ions also in conjunc-
tion with the Cl ~~ ions, and since only 84 grams of NaCl are soluble
in 210 cc. of water and in all 2 F.W. or 117 grams of NaCl are
available we may conclude that 33 grams will crystallize out.
This removes CP from the solution and upsets the equilibrium
which would otherwise exist between the solid and dissolved KCl.
Thus the entire KCl will dissolve and furnish the entire 2 F.W. of
Cl" ions for the 2 F.W. of NaCl. The K+ and NO 3 ~ ions thus
made available constitute 2 F.W. or 202 grams of KN0 3 , which
according to the table would be completely soluble in 210 cc. of
water at 100°. Hot filtration at this point will retain a part of
the NaCl as solid in the filter but allow the whole of the 2 F.W. of
KN0 3 to pass into the filtrate. Cooling of the filtrate will allow
a new state of equilibrium to be established corresponding to the
solubilities at the lower temperature.
In preparing a preliminary report for this preparation, copy
thoughtfully into the laboratory note book the flow sheet of the