Gas Law Demonstration Apparatus F. C . H I C K E Y , O.P. Providence College, Providence. Rhode Island
T .
HE fact that practically all textbooks in general chemtstry begin their discussion of the gas laws w ~ t ha drawing of a cylinder fitted with a frictionless piston to which weights may be applied to vary the pressure, indicates that such an apparatus is, in the minds of the authors, the simplest type for demonstration, yet the standard laboratory apparatus makes use of hydrostatic pressure applied through a column of mercury of variable height. While this type of apparatus gives very satisfactory results from a quantitative viewpoint, i t is not well adapted to large group demonstration. The application of pressure by a column of mercury draws the attention of the student away from the gas and focuses i t on the mercury. While the student is attempting to understand the mechanism which applies the pressure he is likely to miss the point of the demonstration. A simple apparatus built around a 100-ml. hypodermic syringe approaches the clarity of the textbook illustration. The fact that the cylinder is transparent helps to direct the student's attention to the gas while the application of pressure through simple weights resting on the piston leaves his mind free to consider just what is happening to the gas. Apparatus: The syringe had a bore of 1.453 inches, was made of resistant glass, and was provided with a metal stopcock firmly attached to the delivery end. I t was mounted in a stand which allowed clear vision of the cylinder and made the application of weights to the piston convenient. A piece of 3-inch hard copper tubing about 30 inches long was cut out as indicated in the figure, the cut-out being large enough to allow the largest weight to be slipped into the tube above the syringe plunger. At the top of the tube a pulley was mounted, its shaft passing through holes drilled in the wall of the large The syringe was inserted through the hole in the tube where it was brazed in position. The pulley was so mounted that the cable running over it on one side plywood ring and allowed to rest on the lips a t the followed the axis of the tube and, on the other, cleared upper end of the barrel. A water jacket was made the back of the tube. A slit was cut in the hack of the from 2'/*-inch pyrex tubing shaped as shown in the tube to accommodate that side of the pulley. Three diagram and provided with side arms as shown. A legs made of 6/8-inchround steel were bent as indicated rubber stopper was fitted around the upper end of the in the figure. They were of such length that the barrel, below the supporting ring. The water jacket bottom of the copper tube was elevated 13 inches. was fitted over the rubber stopper and supported by a This permitted the water jacket to be attached and re- condenser clamp attached to the rear leg of the tripod. moved without lifting the apparatus. The legs were The piston of the syringe was lubricated with mineral ground flat where they made contact with the copper oil. Glycerin was found to cause trouble through the absorption of water. tube and were brazed in position. The cable for lowering the weights onto the piston A ring was cut from 1/2-inch plywood, its outer diameter equal to the inner dianeter of the copper tube was made from 3/16-inchflexible wire rope. One end and its inner diameter large enough to accommodate was spliced to a steel hook while the other was fitted the syringe. The ring was held in the bottom of the with a brass handle. The weights were made by filling a length of 2-inch tube by three wood screws, 491
copper tubing with molten lead and then cutting off the calculated lengths. The lead in the upper portion was then remelted to pennit the insertion of eyebolts (with nuts attached) and final adjustrent of the weight to proper value. The sizes corresponding to the various weights are given in Table I. DEMONSTRATION
Boyle's Law: Following an explanation of the difference between force and pressure, the increase in pressure in pounds per square inch caused by each weight was calculated a t the blackboard by dividing the known weight by the area of the piston. To these pressures were added 15 pounds per square inch (atmospheric pressure plus the constant pressure resulting from the weight of the plunger). The volume within the cylinder was adjusted to 100 ml. The weights were then carefully lowered to the piston top by means of the cable and pulley. The volume corresponding to each weight was added to the table a t the blackboard. The product of the pressure and the volume was computed. Actual results along with the approximate length of the weights are given in Table 1. TABLE I Lcnglh
(In.)
No.
Weigh1
(Lbr.)
pw
-w A
PV Voi.
P =Po+P.
(MI.)
(Pounds
X MI.)
piston, the volume was adjusted to approximately 70 ml. (room temperature). The water jacket was then placed over the syringe. The rubber tubing from the upper side arm was led to the drain while that from the lower side arm was connected to a Niter reservoir of ice water. A pinch clamp on the latter was used to control the flow of ice water. The reservoir of ice water was elevated above the level of the water jacket. By opening the pinch clamp the water jacket was filled with ice water and sufficient water was allowed to pass through the apparatus to assure the attainment of a temperature close to O°C. The volume of gas within the syringe was then noted. By removing the rubber tubing from the ice water reservoir and leading i t to the drain, while the tube from the upper side arm was connected to a steam generator, the water jacket was filled with steam and the temperature of the gas raised to approximately 100°C. It is well toremove the weight during temperature changes to minimize leakage. While temperature equilibrium was 'being established the volume a t 100°C. was calculated at the blackboard. The actual volume was then observed. If necessary, satisfactory results can be obtained by using hot and cold water from service taps. In this case the temperature can be read by allowing the water, after it has passed through the water jacket, to drain into a beaker placed in the sink. The thermometer can be placed in thebeaker t o determine the hot and cold temperatures. Actual results of the two procedures are given in Table 2.
Po = pre~surein lbr./rq. in. w i t h no weights on piston P, = presure in lbr./rq. in. due to applied aeixht. P total pre9rure on gar.
TABLE 2
-
Charles' TAW:
With Number :3 weight on the
c T ('C.)I. lee W n f e r 4 f e a m 2. not
water-Cold
Water
VI
I
2
0 16
100 41
62
60
Va
Vv
ICn1.l
(Obr.,
85 65 .
83 64
