DEMONSTRATION OF AN INVERTED SOLUBILITY CURVE KENNETH A. KOBE University of Washington, Seattle, Washington
I N A recent paper' Bateman and Feruelius give the details of a lecture demonstration of a negative temperature coefficient of solubility. They recommend cerous sulfate, Cez(SO&, the solubility of which d e creases from 19.09 g. per 100 g. water a t Oo to 0.78 g. per 100 g. water a t 100". This twenty-five fold decrease in solubility gives a considerable precipitation of crystals when the tube of solution is heated in boiling water. A consideration of the system manganous sulfatewater (see the figure) shows that i t has a number of advantages in demonstrating solubility relationships. Both positive (normal) and negative (inverted) solubility relationships and transition points may be demonstrated by means of this system. A solution is saturated with manganese sulfate a t 27'C. It is desirable to add a small amount of sulfuric acid, as 0.5 per cent., to insure that basic salts cannot be formed. The clear solution is decanted from the excess salt and sealed in a tube as shown by Bateman and Fernelius. The demonstration may be carried out in a number of ways. Probably the best is to show the class the clear solution a t 27'C. and then immerse the solution in an ice-salt mixture. In a short time large pink crystals of MnSOa7Hz0 will appear in the liquid to fill the bottom of the tube. The tube may then be heated up, the crystals will melt, and the solution become clear again. Continued heating in boiling water will cause the appearance of small white crystals of MnSOrH20 which will slowly settle out and fill the bottom of the tube. The crystals will slowly dissolve when the solution is again cooled, and a clear solution will be formed. To carry out the demonstration in a shorter time the ' BATEMAN,L. A. AND w. C. FERNELIUS, J. CREM.EDUC., 14, 315 (1937).
tube may be placed in an ice-salt mixture before the demonstration, which then starts by beating the tube to give first a clear solution and then the white precipitate. The precipitation from manganous sulfate solution may be compared with that from cerous sulfate solu-
-20
0
+20 40 60 80 Temperature degrees C.
100
tion. Using 50 g. of saturated solution the cerous sulfate will precipitate 7.65 g. Cez(S0d3when heated from 0' to 100°C. Using 50 g. manganous sulfate solution saturated a t 27', 10.5 g. MnSOaHzO precipitate out when the solution is heated to 100°C. When cooled from 27' to - lo0, 16.35 g. of MnS04.7H20 crystallize out. The advantage is thus seen to lie with the manganous sulfate, especially as equal volumes of the two solutions contain relatively more manganous sulfate because of its greater density.
