PHILIP S. BAKER Bradley University, Peoria, Illinois
EXPERIMENTS to demonstrate the effect of a dissolved solute upon the vapor pressure of a solvent are common. Perhaps the most familiar involves the use of two beakers-one containing concentrated sulfuric acid and the other containing water-both under a bell jar. (Some demonstrators evacuate the system to speed up the effect.) The proposed piece of apparatus is designed to improve this type of demonstration. The apparatus, shown in the accompanying figure, consists of a 250-ml. Erlenmeyer flask A, rounded a t the bottom, and joined by 15-mm. tubing to a rounded 25ml. Erlenmeyer flask C, and a 10- to 15-ml. side-bulb B. The dimensions are not a t all critical; pieces of broken equipment and scrap tubing at hand may well determine the final form. The system may be filled as follows: an aqueous solution of ammonium chloride' which is not quite saturated (approximately 25 ml. of water added to 150 ml. of saturated solution) is introduced into the system through a small sidearm D. After dividing the solution so that some is in each bulb, all three portions are heated simultaneously to boiling and the hoiling is continued for a short time in order to sweep the air from the system. Aspiration through the sidearm may be used to aid in removing the air. Then, with the portions still barely boiling and with steam passing through the sidearm, the latter is sealed offwith a small blast flame. The heat is withdrawn immediately from under the bulbs. Upon cooling, we have a system under reduced pressure containing in the gaseous phase only water vapor. I t is possible now to manipulate all of the solution into the largest bulb A. Then, upon immersing this bulb in hot water while cooling C in tap water or ice water, water vapor leaves A and condenses in C. In a very short time any desired amount may be collected there. Now, as the system stands, there will be a gradual return of the pure water in C to the solution in A as a result of the difference in respective vapor pressures. Over a period of a few hours (overnight is convenient) the water level in C will he lowered considerably and eventually all of the water will end up in the solution in
comes supersaturated with respect to the normal (room) conditions:, unon coolie. the excess solute ~recinitates out. Now, as the water gradually returns to A, it redissolves the excess solute. (A little experimentation will indicate to the demonstrator just how much water to condense in C so that the excess solid will have redissolved in a convenient length of time.) One advantage of the design of the apparatus is that the resultant migration of water vapor upon standing is from the small to the large area. This anticipates arguments by students who might think that the migration should be in the opposite direction. Also, by sealing the system entirely, one avoids uncertainties concerning leaks in the hell jar, loss of moisture to saturate the air within the jar, etc. The apparatus is described as "versatile," because in addition to what already has been discussed, it has other uses: (1) I t may be used as a cryoph~rus.~(2) It may be used t o illustrate saturated and unsaturated solutions. (3) It may be used to show s c h l i ~ e n . ~ ~ A. (4) It may be used as a water-hammer. (5) It may be The purpose of using a solution which is not quite used to demonstrate the fact that durine evanoration ~-~ saturated is to show that the pure water, which evapo- it is the hot,^ molecules which are leaving the liquid. rates from C in an effort to re-establish equilibrium, ac(1) A, a ~ r y o p ~ o r u s ~t , is apparent that bulb B is tually returns the in pure water is not essential to the demonstration described above, and condensed in the cool side of the system during the set- that a simple inverted u-tuhe prepared in an analogous ting up of the demonstration, the mother solution bez B ~R. ~A,, ~J. CHEM. ~ , EDUC., 25, 259 (1948): 26. 481 1 Ammonium chloride is recommended because of its availabil(1949). ity, solubility, low moleculm weight, and failure to cake upon a EMICH, F.,Monatsch., 50,269 (1928); 53-54,312 (1929). ' SCHALLY, E., ibid., 58, 399 (1931). overheating. A
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manner should suffice. To illustrate the cryophoric principle, all of the solution is manipulated into A which is then immersed in hot water as already described. This time, however, the condensate is collected by cooling B instead of C. When a few milliliters of water have been condensed, A is removed from the hot water. If the empty bulh C is now immersed in a mixture of dry ice and acetone, the water in B will freeze within a few seconds. (2) To Illustrate Saturated and, Unsaturated Solutions. Of course, if the cooling in (1)is continued by leaving C in the dry ice-acetone mixture, the solution in A will also cool by evaporation and in a short time solid ammonium chloride will start to precipitate. This effect, along with the original demonstration, is a very helpful and convincing contribution to the consideration of saturated and unsaturated solutions. (8) T o Show Schlieren. Accompanying the cooling in (2) is the appearance of schlieren in the solution in A as the temperature drops. This effect is also observable while A is being heated and C cooled in the original
JOURNAL OF CHEMICAL EDUCATION
evaporation. The schlieren can be seen with the eye or may be projected on a screen with a bright light. (4) A s a Water-Hammer. To illustrate the waterhammer effect, the apparatus need only be shaken. As liquid strikes liquid or glass without being cushioned by air, there is a noise resembling that of marbles being shaken together. If the liquid is properly distributed in the bulbs and the apparatus is inverted suddenly, the resulting sound simulates the cracking of glass from thermal shock. (6) T o Demonstrate "Hot" Mobc~les. If the entire contents are put into A and heated to a temperature of 7040°C. (not critical), condensing water vapor in C by holding the bulh under cold water illustrates a very interesting point. If the bulb is held in the cold water for a minute or t w e u n t i l a small amount of water has condensed and the bulb is cool-and then removed. it will be found to warm up quite rapidly. This is a rather convincing piece of evidence that during evaporation it is the "hot" molecules which are escaping from the evaporating solution.
