VOL.4. No. fi DEVICEFOR
DPMONETRATIKG BOYLE'SA N D CHARLES'LAW;
781
A DEVICE FOR DEMONSTRATING BOYLE'S AND CHARLES' LAWS F. SPENCER MORTIMEn, ILLrNOrs WESLEYAN UNIVERSITY.BLOOMINOTON, ILLINOIS
A limited inquiry has shown that the order of presentation of the principles involved in Boyle's and Charles' laws to students of chemistry and physics is very often from generalization to fact rather than from fact to generalization. This is especially true of the law of Charles dealing with the effect of temperature upon the pressure (or the volume) of an enclosed gas. It was thought that if a rapid method for the determination of the necessary data were described, more use might be made of the experimental approach to this important subject. The effect of pressure, a t constant temperature, upon the volume of an enclosed gas is easily demonstrated by a variety of simple experiments. The principles involved in Charles' law, however, are not so easily shown, neither are they so quickly grasped by the average student. For a number of years we have made use of a single piece of apparatus for the demonstration of both of these laws. The device we have used is shown diagrammatically in Fig. 1. Although there is nothing new in principle and, even though the diagram is practically self-explanatory, still a few words o$ explanation may serve to indicate how easily the device may be constructed from materials found in all laboratories. The apparatus consists of a eudiometer tube, A, containing a volume of air enclosed by a column of mercury contained in the rubber tube, B, and the mercury well, C. The tube, A , is graduated in cc. divisions and is conveniently made by fusing off a 25 cc. buret or Mohr's pipet at the zero mark. The tube, A , is enclosed in a larger tube, D, fitted with a two-holed stopper a t the top and a three-holed stopper a t the bottom. The middle hole of the lower stopper holds the eudiometer tube in the center of the jacketing tube, D. One of the other holes contains a tube, E, which admits water or the vapors of boiling liquids and the third hole is fitted with a tube and stopcock, F (or rubber tube with pinchcock), through which one may drain off the liquid condensed in D. An accurate thermometer, G, is held in place by the upper stopper which also carries an exit tube, H, attached to a condenser not shown in the diagram. I is a meter stick for measuring the difference in height of the balanced mercury columns. The complete apparatus is attached to a single ringstand on which it may be kept intact and ready for use from year to year as needed. We have sometimes attached a short worm condenser and a boiling flask for generating the vapors to the same stand. This is especially convenient when the apparatus is to be carried from place to place. It might he noted that the volume of air in the tube, A , should be so
adjusted that a t atmospheric pressure and room temperature i t will fill about threequarters of the calibrated part of the tube. It is also very necessary that the mercury be extremely dry.
Operation
G
-
-
-
-
3
A
;
1
FIG.1
~o~le's:lawstates that for a given mass of gas a t constant temperature, T, the volume, V,varies inversely as the pressure, P, or, P V = a constant. According to Charles' law the pressure a t constant volume or the volume a t constant pressure of a given mass of gas is directly proportional to the absolute temperature. That is,
I 1
PV/T=K.
We have used the apparatus both for class-room demonstration of the above laws and also for a laboratory exercise. Because of the limitation of time in the classroom we have generally demons t r a t e d Boyle's law for a single temperature, conveniently that of the tap water. The pressure on the enclosed gas is varied by raising or lowering the mercury well. The difference in height of the mercury in the well and in the tube added algebraically to the atmospheric pressure (the total mercury column being corrected for temperature) gives the absolute pressure acting on the gas. The corresponding volumes are read directly on the graduated eudiometer. These data are tabulated on the blackboard and the constancy of the P V product shown. ~ i verification r of Charles' law the apparatus is heated to another temperature, generally that of boil-
VOL. 4, NO. 6 DEVICE FOR DEHONSTRATING BOYLE'S AND CHARLES' LAWS
783
ing water, and the well is raised until the volume of the gas is the same as the volume occupied a t zero degrees and the pressure is then read. Likewise, to determine the increase in volume a t constant pressure the balanced columns of mercury are kept a t the same level, the absolute pressure thus being that of the atmosphere, and the volume is then read. Since the data a t zero degrees are determined with somewhat greater difficulty we have found i t best to obtain these values previous to class time. All of the data are then recorded on the blackboard and the constancy of the PIT and V / T quotients shown. As a laboratory exercise, especially for more advanced students, we have found that better results are obtained if the volumes for several pressures are measured a t as many temperatures as time permits. The temperatures we have used are those obtained by a mixture of ice and water, tap-water, room-temperature, and the vapors of boiling acetone, methanol, ethyl alcohol, and water. The pressures are then plotted against the volumes for each temperature, a series of equilateral hyperbolas being obtained representing the isothermals for the gas. From these curves the pressures a t constant volumes for each of the temperatures or the volumes a t constant pressure may be read a t as many points as desired. Likewise the mean values of the PV products, a t constant temperature, may be divided by the corresponding absolute temperatures as a verification of Charles' law. If desired the mean values of the PV products may be used to calculate the temperature of the absolute zero. To do this the variation of the PV product for one degree rise in temperature above its value a t zero degree is divided into the value of the product a t 0' C., or, P'V' - P'V" t'
- to
= -K.
and
P"V" -
K
-
T,the absolute zero.
The constant, K, is the specific gas constant for this mass of gas. The following data indicate the degree of precision which may be expected with no greater refinement of technic than that indicated above. In this table the values of the PV products have been determined with a slide-rule. Corrected
Temp.
O.OO
15.0
V0l.
Pressure.
PV
cc.
cm. H g
Crmst.
55.5 63.6 73.1 86.3 Mean
1008 1008 1007 1006 1007
57.1 61.1
1065 1062
18.16 15.82 13.75 11.66 18.65 17.40
PV T
3.688
Temp.
Vol..
Corrected mesnure,
cc.
cm. H g
14.35 12.86 11.75 10.76
74.1 82.6 90.3 98.5 Mean 67.45 82.5 92.3 100.8 Mean 73.4 84.9 95.4 109.4 123.2 Mean 66.0 75.3 86.85 98.6 107.25 115.8 Mean 67.99 90.00 105.34 122.25 138.5 Mean
PV canst.
1063 1062 1061 10M) 1062 1083 1084 1083 1082 1083 1223 1225 1223 1222 1222 1223 1311 1311 1309 1308 1304 1299 1307 1389 1391 1393 13% 1386 1389 Mean
Absolute Zero = 1007/3.7004 = 271.8".
It will be observed that these laws may be verified with an average deviation of about two parts per thousand for Boyle's and five parts per thousand for Charles' law. There are, of course, several sources of error for some of which correction might be made. No attempt has been made to allow for the unequal expansions of glass and mercury, nor the, increased vapor pressure of mercury a t the elevated temperatures. Moreover, the temperature correction of the mercury column, especially for the higher temperatures, is rather unsatisfactory due t o the fact that one end of the column is much warmer than the other. However, we believe the most important source of error is that already mentioned, namely, the mercury filling the tube must be very free from volatile liquids. The variation of the vapor pressures of such liquids with temperature vitiates the results.
