Howard D. Murdock and Robert M. Hawthorne Jr. Purdue University North Central Campus Westville, Indiana 46391
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1 Charles' Law Experiment a Balloon or a Plastic Bag
General chemistry experiments which demonstrate Charles' Law (direct variation of V with absolute T, in a gas) clearly, quickly, and reasonably quantitatively are hard to find. We have devised an experiment which is exceedingly quick and simple, and satisfactorily quantitative as well. using either an ordinan, balloon or a nlastic sandwich bag as a (nearly) constant-pressure gas-sample holder. Use of a balloon gives an exneriment of extreme quickness and simplicity,-with fair quantitative results; use of a plastic bag which, unlike the balloon, has no residual pressure above atmospheric, gives good precision and accuracy, but involves a somewhat more complicated procedure. Both methods show the variation of V with T very graphically, and can be used as lecture demonstrations as well. In our procedure measurement of gas volume is done by water displacement in a graduated beaker, the water serving also as a constant-temperature bath a t various temperatures. If a balloon is used, it is blown u p to a size which fits comfortably into a 1000-ml graduated beaker (volume of balloon about 400 ml), and kneaded when inflated to break down wall tension for lowest nossible nressure. If a plastic sandwich bag is used, it is partially inflated and closed by twisting the nlastic film and knottine it (the twist ties subplied with the bags are not satisfactory, as they allow leakage). Water vapor from the breath can be avoided by "snapping" the bag open as one does with a paper hag. When tied the bag should be limp, to allow for expansion without pressure increase; its volume should be 300-400 ml, as accuracy falls off with smaller volumes. The balloon can be held in the water bath with the fingertips, with fair accuracy in the volume measurements. The bag requires a holder, which is made from a 10-12-in. length of coat hanger wire, bent into a hook a t one end, with a right-angle bend about 2 in. from the hook. The knot is slipped into the hook, the excess plastic film is trimmed off, and the bag is held knot side down in the water bath. Once the choice of sample holder is made, the remainder of the procedure is the same for each. The balloon or
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bag is placed in the 1000-ml graduated beaker, and tap water is poured in to give a total volume of 800-900 ml. Volume of water with and without the bag or balloon can be estimated to 5 ml, and the volume of the gas sample calculated by difference. Temperature of the water is measured after 15-30-see immersion of the balloon or bag, to ensure temperature equilibrium with the gas sample. This procedure is repeated with hot tap water (50-60'C) and with ice water; in each run temperature equilibrium (i.e., no further gas volume change) is established surprisingly quickly, within 30 sec in all cases. Three runs at different temperatures can easily be done within 15-20 min, even by quite unskilled students; this leaves ample time for a Boyle's Law experiment in the same laboratory period. Typical student results from our laboratories give a V/T which varies by 1 4 % from 4" to 50°C, using a balloon, and by f0.870 from 3" to 55"C, using a plastic bag. The T-V data obtained a t different temperatures can also be used to calculate a value for absolute zero on the Centigrade scale, but both precision and accuracy are poorer here. The T-V pairs from the three different runs are plugged into the equation V/(n + t ) = k, where n = number of degrees absolute zero lies below O"C, and k = a calculable constant. The resulting equations, in which n and k are unknown, are taken in all possible pair combinations (for each experiment) and solved simultaneously. Each pair gives a different value for n, and the variations are large. The balloon gives , t h e poorer results: -167' 23% Values from the plastic bag experiment are better, but still only fair: -250" 119°C. A value which is more effective for instructional purposes, though not legitimately derived, can he obtained by dividing the average V/T factor into each measured gas volume, to find how many degrees below each corresponding temperature absolute zero lies. When each of these values is subtracted from the respective Centigrade temperature, the results obtained invariably average -273°C. With the above data, the variations are 112°C for the balloon, 12°C for the plastic bag.
