CHAPTER VIII NITROGEN AND THE ATMOSPHERE 53. The preparation and properties of nitrogen. (On account of its great affinity for oxygen, phosphorus must be kept and handled under water. Never bring the dry substance in contact with the skin, as it may ignite and cause a serious burn.) a. Cover the bottom of a pneumatic trough with water to a depth of 2 or 3 cm. Float on the water a porcelain crucible containing a small piece of phosphorus. Ignite the phosphorus by touching it with the hot end of a wire or file, and quickly invert over the crucible a large beaker or widemouthed bottle, being careful to keep the rim of the beaker below the surface of the water. The white fumes formed have the composition P2O. Leave the beaker in position until the fumes have entirely disappeared. Note that the water has risen in the beaker. Explain. Adjust the beaker or the water in the trough so that the level of the liquid inside and outside of the beaker is the same; then cover the beaker with a glass plate and turn it into an upright position. Test the gas with a burning splint. 5 b. In a 250-cc. flask place a mixture of 3 g. of ammonium chloride and 6 g. of sodium nitrite, and add 20 cc. of water. Provide the flask with a cork (one-hole) and delivery tube, so that the gas evolved may be collected over water as in the case of oxygen and hydrogen. Have at hand a vessel of cold water so that the flask may be cooled by lowering it into the water in case the action becomes too violent. Clamp the flask on a ring stand and apply a very gentle heat, moving the burner about with the hand. As soon as the action begins, withdraw the burner. After the air has been expelled from the apparatus fill two or three bottles (250-cc.) with the gas. If the action becomes too violent, immerse the flask in cold water. The reaction which takes place is expressed in the following equation : NH,C1+ NaNO, NH,NO, + NaCl = 2 The ammonium nitrite then decomposes into water and nitrogen (R). (The symbol (R) indicates that the equation for the reaction is to be written.) Note the physical properties of the gas. Test with a burning splint. 54. Determination of the relative volumes of nitrogen and of oxygen in the air.1 This determination may be made by bringing in contact with a definite volume of air a liquid which not only absorbs the oxygen but in doing so flows into the tube which contains the air and fills a space equal to that previously occupied by the oxygen. The volume of this liquid can be easily measured, and in this way the volume of the absorbed oxygen may be ascertained. The solution used to absorb the oxygen soon loses its strength on exposure to the air; hence the experiment must be performed rapidly. Before preparing the solution the student should practice the manipulations involved in the experiment. FIG. 32 Prepare an apparatus according to Fig. 32. In this apparatus Crepresents a test tube about 15 cm. in length (use the hardglass tube employed in the preparation of oxygen). The tube is fitted with a two-hole rubber stopper. Through one of the holes is fitted a glass tube, which, together with the accompanying rubber tube and funnel, is the same as was used to introduce the acid into the bottle A (Fig. 27). The remaining hole in the stopper is closed with a glass rod. Notice that 1 From Cooley's "Laboratory Studies." both the glass rod and the tube extend just through the small end of the stopper. Now close the rubber tube tightly with a screw clamp B. Disconnect the test tube and remove the glass rod from the stopper, preparatory to performing the experiment. Prepare an alkaline solution of pyrogallic acid, as follows: Dissolve 5 g. of potassium hydroxide in 5 cc. of water and cool the solution to room temperature. Add this to a solution of 4 g. of pyrogallic acid in 10 cc. of water, and at once pour the resulting liquid into the funnel A. Quickly open the screw clamp until both the rubber tube and the glass tube are filled with the liquid, then close tightly. Connect the test tube, holding it by the rim to avoid heating the contained air, and insert the glass rod in the cork. The air inclosed in the tube is now at the same temperature and pressure as the surrounding air. Now open the screw clamp. The liquid flows in, absorbing the oxygen. When the liquid ceases to enter, grasp the tube by the rim and invert it as shown in the dotted lines of the figure, adjusting it so that the level of the liquid is the same in both the tube and the funnel (why?). Then clamp the rubber tube and return the test tube to its original position. Mark the volume of the air originally inclosed in the tube by placing a narrow strip of gummed paper about the tube at the lower end of the stopper; also mark by a strip of paper placed at the level of the liquid in the tube, the volume of the oxygen absorbed. Disconnect the tube and rinse it. Measure the volume of the tube to each strip of paper by pouring in water from a graduated cylinder. From these data calculate the volume of oxygen and of nitrogen in 100 volumes of air. NOTE. This experiment disregards the presence in air of all constituents other than oxygen and nitrogen. The volume of such constituents, however, in the volume of air taken, is so small that it may be neglected. 55. Other constituents of the air. a. Expose a piece of calcium chloride to the air for two or more hours. Explain. b. Expose a few cubic centimeters of limewater to the air for a half hour. Explain the results. |