Charles's and Boyle's Laws A Class room Demonst ration ARTHUR W . DEVOR' Adelphi College, Garden City, New York
T
HE author finds it somewhat confusing to many beginning . . chemistry students to perform and understand gas law experiments as directed in most laboratory manuals. In the demonstration described here the student can see the change of pressure, temperature, and volume. There is no correction for vapor pressure which is confusing to some beginning students. The setup and use of this type of apparatus (see Figure 1) is described by the author in a previous arti~le.~ However, two details were omitted, one of
The volume of the system was determined by filling it with mineral oil and then draining the oil into a graduated cylinder. It was then flushed out for several minutes with dry gas (air, 0 2 , or N2 are suitable) to eliminate all traces of moisture which might cause errors. The screw clamp A was closed; mineral oil was introduced into the funnel C; the stopcock was opened until the air in the tube was displaced and then screw clamp A was opened long enough to allow the oil to fill the capillary tube. With screw clamp A closed and screw clamp D open, the suction flask was lowered into a beaker of water which was heated to boiling until the temperature inside the flask was above 90°C. The pressure on barometer B was recorded, the heat removed, and as soon as the temperature of the bath lowered to the temperature inside the flask the screw clamp D was closed and the temperaturerecorded. The hot water was siphoned out of the beaker and all connections were greased to prevent air leaks. Cold water and a little ice were added to the beaker for rapid cooling. At this point the students can see the Hg drop due to the decrease in pressure. When the temperature inside the flask was approximately 10 degrees above room temperature the bath was warmed very slowly until the temperature of the bath and the temperature inside the flask were the same (very near room temperature). The temperature and pressure were then recorded. TABLE Obsoued Pressure
(Mm. HZ) pirat trial
i'f
20
Volume (MI.)
cdc. V0l. (Boyle's
LOW) 452 ml. 457 rnl.
Mineral oil was added to or removed from the dropping funnel by means of a pipet, to bring the oil level to the which is the correction due to the surface tension ef- mark on the funnel. Then 200 ml. more of oil were fect in the barometer, and the other is that, when added from a graduated cylinder and screw clamp A was filling the barometer, the air bubbles were best elimi- opened (temperature constant) until the pressure inside nated by introducing a small amount of Hg a t a time the flask was the same as it was when recorded a t the and carefully bumping the closed end down against a beginning of the experiment. The screw clamp was rubber pad. In Figure 1 it should be noticed that two closed immediately and the temperature and pressure capillary tubes, one of which is connected to a dropping again recorded. The excess oil was removed by means funnel, by means of an oil-resisting rubber tube, have of an oil-rinsed pipet, until the level of the oil was a t the mark on the funnel, and transferred back to the been added. 200-ml. graduated cylinder. From the decrease in 1 Present address: University of Southern California, Los volume of the oil in the cylinder and from the total Angeles, California. volume of the system (inside the flask), the volume of J. CHBU.EDUC.. 22,144 (1945).
268
the gas, a t the lower temperature and a t the higher pressure, was calculated and recorded. The decrease in the volume due to decrease in temperature was checked by draining the flask into a clean graduated cylinder. The temperature, pressure, and volume were recorded, and the volume was calculated from the gas laws and compared with the observed volume, as shown in the table. With these data the student can see that the temperature is constant in one case, the pressure in another. The effect of vapor pressure due to water vapor can be explained as described in the previous article.%
For more advanced students an excellent method of proving these laws is described in an article by Steinbach and C ~ n e r y . Other ~ methods of demonstrating Boyle's law are described by Dutton4 and by Hays and Gustavs~n.~ There was a slight decrease in volume as the mercury in the barometer B dropped. This was determined by finding the volume per decimeter of tube and from the drop in the Hg column.
a
mid., 21,216-19 (1944).
'Ibid., IS, 15-16 (1941). Ibid., 16, 115 (1939).
