LECTURE DEMONSTRATION OF T H E LAW OF COMBINING VOLUMES T. H. JAMES University of Colorado, Boulder, Colorado
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AY-LUSSAC'S law of comb'ming volumes is customarily demonstrated on the lecture table by two experiments: (1) the union of hydrogen and oxygen to form water vapor, and (2) the decomposition of hydrogen chloride by sodium or sodium amalgam. The latter experiment is, from the point of view of greatest accuracy, best performed by introducing a ribbon of metallic sodium into a tube of hydrogen chloride, the lower end of which is sealed by mercury. This experiment, however, requires several hours for its completion, and hence is not well suited to demonstration on the lecture table. The awalgam method, although much more rapid, presents difficulties of technic which render the results uncertain. Much of the inaccuracy involved in the use of sodium amalgam can be eliminated if hydrogen bromide is used in place of the chloride. The former gas presents the following distinct advantages. (1) It is decomposed completely and almost instantaneously by the amalgam. (2) Its greater density allows the coutamer to be filled more completely and with a minimum amount of air retained. (3) It fumes readily in even relatively dry air and thus acts as its own indicator of the point a t which the container is well filled. These advantages lead to considerable accuracy in the demonstration and more than offset the only important disadvantage; i. e., that the detailed study of hydrogen bromide customarily follows that of the law of combining volumes. The construction of the simple apparatus used in this demonstration is clearly indicated by the figure. The
gas jar should be of about 500-cc. capacity, and the glass tubes 8-10 mm. in diameter. A small, rimless test-tube will serve for tube A. Hydrogen bromide is passed into the jar (with A removed) until the gas issuing from B fumes strongly in the air. (The hydrogen bromide is best obtained by the action of bromine on red phosphorus and water.) The rubber tube, B, is now closed at its middle by a a screw clamp, and tube C by a clamp or glass plug. The tube A, which has been charged with 10-15% in excess of the theoretical amount of 20% sodium amalgam, is now put in place above the screw clamp. It is essential at this point that all joints be absolutely tight. The amalgam is readily introduced into the jar by removing the screw clamp from B. The reaction is accompanied by a beautiful pyrotechnic display, and considerable heat is evolved. The hydrogen is cooled to room temperature by immersing the apparatus in water. The jar is then opened under water by removing tube B, the water inside is quickly leveled with that outside, and the jar is removed from the water bath. If the latter operations are carried out with leasonable speed, the amount of hydrogen formed by the reaction of the excess amalgamwith the waterwill not appreciably affect the results.
