Laboratory Experiment on Deliquescence and Efflorescence O m 0 F. STEINBACH, City College, College of the City of New York, New York City A SIMPLE experiment on deliquescence and &Iescence that can be performed by the individual student should be of distinct value in the teaching of the chemical principles involved. The following experiment illustrates the phenomena of deliquescence and d o r e s cence and emphasizes the distinction between them. It also furnishes a good application of the principles of the kinetic molecular theory and of the hygrostat. Place a small amount of anhydrous copper sulfate in the bottom of a clean dry test tube. Holding the tube horizontally, place four or five clear crystals of hydrated sodium carbonate in the mouth of the test tube. Using a stirring rod, push the crystals into the test tube until they are withim an inch or so of the copper sulfate. Stopper the tube and set aside in a horizontal position. In a short time (one-half hour) the anhydrous copper sulfate turns blue and the surface of the sodium carbonate becomes covered with dehydrated salt. Thus i t has been demonstrated that the crystals of hydrated sodium carbonate must have lost water vapor ( d o r esced) and likewise the anhydrous salt gained water to form crystals (solid) of hydrated copper sulfate. To illustrate the phenomena of deliquescence and efflorescenceand the distinction between them is relatively simple. Place four or five crystals of hydrated sodium carbonate in a clean dry test tube. Holding the tube horizontally, place one lump of anhydrous calcium chloride in the mouth of the test tube and use a stirring rod to push it to within an inch or so of the bot-
tom. Stopper and set aside in a horizontal position. In about two hours the calcium chloride deliquesces and forms a saturated solution of calcium chloride, and the hydrated sodium carbonate efflorescesand becomes coated with dehydrated salt. Thus, both deliquescence and efflorescenceand the distinction between them have been shown. Variations of the experiment can easily be made. Thus, various hydrates may be used in this test tube hygrostat, the distance between the two different salts can be varied, and the temperature can be changed simply by using an oven set at, say, 40°C. to accelerate the change. Obviously, the time it takes for the desired result to appear depends upon the temperature, as this determines the vapor tension of the hydrate and also the velocity of the water molecules in the vapor state. The effect of the distance between the two salts (temperature constant) can be easily shown and also explained in terms of the kinetic molecular theory and illustrates the effect of having "molecules" of air interposed between the two salts in terms of molecular collision. To illustrate mean free path, the student could be asked to predict the result if the test tube containing the hydrated sodium carbonate and the anhydrous copper sulfate were evacuated. Finally, the fact appears that the vapor tension of a hydrate, in equilibrium with its saturated solution, must be the same as that of the saturated solution.
