An improved heat of vaporization demonstration - Journal of Chemical

An improved heat of vaporization demonstration. Vinton R. Rawson. J. Chem. Educ. , 1940, 17 (2), p 94. DOI: 10.1021/ed017p94. Publication Date: Februa...
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THE classical experiment to demonstrate heat of vaporization is to freeze a wet w a t d glass or small beaker to a block of wood by the rapid evaporation of ether or carbon disulfide placed in the glass vessel. This demonstration has several disadvantages. (1) The layer of ice formed between the glass and wood is usually so thin that it quickly melts. In a large class, only a few of the pupils actually see that ice has formed. (2) The fumes of ether or carbon disulfide, blown into the air of the room during the evaporation process, make many pupils ill. In fact, after the experiment has been repeated several times for the benefit of many sections, the odor of the air in the room is almost unbearable. (3) Most high schools do not have a hood placed a t the front of the lecture room so that the fumes may be conducted up a flue. However, if such a fortunate condition exists, the pupils get a better view of the


1 Present address: White Plains High School. White Plains, New York.

instructor's back during the actud freezing operation than of the demonstration itself. To overcome these difficulties, the author has devised

the following demonstration. The accompanying diagram shows the set-up of the apparatus. Test-tube A measures ten inches by one inch and contains 25 ml. of ether. Suspended in the ether by a stout cotton thread is a four inch by one-half inch test-tube (C) which is filled to a depth of one inch with distilled water. The thread is useful in later withdrawing i t from the ether. Test-tube B measures twelve inches by one and onehalf inches and is half filled with alcohol. The short bent glass tube leading from tube B is connected to an aspirator. To operate this apparatus, the aspirator is slowly tumed on. Air enters the ten-inch test-tube and buhbles through the ether, vaporizing it. The ether vapor is camed over into the alcohol, and is absorbed. After the air bubbles have agitated the alcohol, the aspirator

may be fully turned on. This causes rapid evaporation of the ether, and within a short time, the water in the small tube is frozen. The pupils actually see the water freeze,and later may examine the tube of ice, which does not melt for several minutes. Even a t the end of several demonstrations, the odor of ether in the air of the room is scarcely noticeable. The waste water from the aspirator may be conducted directly to the sink outlet by means of a large diameter metal pipe. This further cuts down the amount of ether vapor entering the air of the room and prevents splashing of the water. There is apparently little loss of efficiency in the aspirator. If the glass tube between the two large testtubes is cut in two and connected by a short length of rubber tubing, the removal of the stopper from testtube A is facilitated.