A Pseudo-Charles' Law Experiment to Teach Vapor Pressure Concepts I t has been found practical to introduce t,he concept of vapor pressure in freshman chemistry laboratories by means of a psendo-Charles' Law experiment. I n preliminary discussion i t is brought out that a test tube or other container inverted and completely immersed in a. liquid would be a conveuient device with which to roughly measure the expansion of air with temperature. The class is mked to suggest a liquid for use and a reprodrtcible temperature range to operate through. It is not difhcult to assme t h a t the liquid suggest,ed will he water, and the reproducible range suggested will be from the freezing point to t,he boiling point of water. The class is inst~uctedto design and carry out the experiment which they have suggested, predict,ing t,he results on the basis of Charles' Law. The inverted and immersed containers fill completely, or almost completely, upon being cooled but slightly below the boiling point,, instead of becoming approximately one third full upon coaling hack to ice bath temperature, sa had been predicted from Charles' Law. Most of the st,udents realize that something is wrong with either their experimental technique or their thearct,ical considerations. When t,he students find that their compa?~.iotsare all obtainilrg substantially the same results, they usually realize that the error is probably in bheir theoretical coi~siderntions. Subsequent discussion, with approprisle leading questions, leads to the concept of vapor pressure. Discussion also often leads to suggeslions for rnadificitf.io,r of the experiment and for other experiments which would be related. Two closely related experiments h w e been suggest,ed by several classes. These are the modifiiation of the experiment so as to carry out a true Charles' Law experiment, by snbstitut~inga liquid of negligible vapor pressure for t,he water but operating through the same range, cooking oil has been suggested as being suitable; and the modification of the experiment so as to measure the vapor pressure of water or other volatile liquids over a range of temperat,ures by eqriilibrating the system a t temperatures other than the boiling point. Interested students can be allowed t o develop such ideas for independent credit or the entire class may wish to carry ont the proposed experimenk This deceptively simple, and deliberately deceptive, experiment has been used by the writer for the past eight years in general chemist,ry classes with excellent results. The expe~imentstrongly impresses the basic ideas of vapor pressure and illustrates t,he scientific meihod in its testing and reassessment. phases quite uicely. DEANL. SINCLAIR
PIMA COLLEGE TUCSON, ARIZONA 85700
81 4 1 Journal of Chemical Educaiion
