remove the cork from the tube (?). What is the source of the oxygen? Place the tube and contents aside for use as described in d. c. Properties of oxygen. 1. Note the physical properties (color, odor, taste, solubility in water) of the gas. (The slight cloud that is often present when oxygen is prepared from potassium chlorate is due to an impurity and will disappear if the gas is allowed to stand over water.) 2. Repeatedly thrust a glowing splint into a bottle of the gas. 3. Heat some sulfur in a deflagrating-spoon until it begins to burn. Note the color and size of the sulfur flame. Now lower the burning sulfur into a bottle of oxygen and note the change. 4. Tip the piece of picture-frame wire with sulfur by wrapping a very small bit of cotton about the end of the wire and dipping this into melted sulfur (for this purpose melt a little sulfur in a deflagrating-spoon). Ignite the sulfur by holding it in a Bunsen flame for an instant, and then thrust the wire into a bottle of oxygen. Describe the results obtained in 2, 3, and 4. What becomes of the oxygen? d. Separation of the compounds present in the residue left in the preparation of oxygen. Heat the tube containing the residue until no more oxygen is evolved. After the tube is cool, nearly fill it with water and shake the contents thoroughly. After a few minutes, filter off the solid matter (what is it?), repeating the filtration, if necessary, in order to obtain a perfectly clear filtrate. Evaporate about one third of the liquid, and set it aside until crystals are deposited. Convince yourself that the substance is different from the potassium chlorate with which you started. What is the compound (p. 26 of text)? EXERCISE 8 THE PREPARATION AND PROPERTIES OF HYDROGEN Apparatus. Test tube; apparatus, bottles, and trough as shown in Fig. 21 (the bottles are 250-cc., wide-mouthed); beaker; stirringrod; 60-cc. bottle; burner; evaporating-dish; * funnel. Materials. Bit of sodium (size of a small pea); wooden splints; 10 g. granulated zinc; copper sulfate solution (R. S.); sulfuric acid. filter paper; a. Preparation from water. Fill a test tube with water and invert it in a beaker of water. Wrap a piece of sodium in a bit of filter paper previously moistened with coal oil. Raise the inverted test tube until its mouth dips just below the surface of the water in the beaker, and quickly insert the sodium. Stand at arm's length, as a slight explosion sometimes occurs. Notice that the sodium decomposes the water, liberating a gas which is caught in the tube. Place your thumb tightly over the mouth of the tube to prevent the gas from escaping, and bring the tube to an upright position. Light a splint, remove the thumb from the tube, and quickly bring the flame to the mouth of the tube. Does the gas act like oxygen? What is the source of the gas? What other methods may be employed for obtaining it from the same source? b. Preparation from acids (usual laboratory method). Prepare a hydrogen generator according to Fig. 21. D represents a wide-mouthed bottle of about 250-cc. capacity. The gasdelivery tube B, C is the same as that used in the preparation of oxygen (Fig. 20). A rubber stopper for the bottle D is preferable, although a good cork stopper will do. The funnel tube A must extend nearly to the bottom of the bottle (?). Put 10 g. of granulated zinc (why granulated ?) into D. Pure sulfuric acid will not react with pure zinc; hence it is advisable to add to the zinc 8 or 10 drops, of a solution of copper sulfate, which will start the reaction. Now pour just enough water through the funnel tube to cover the zinc. Prove that the apparatus is air-tight by blowing into the delivery tube C until the water is forced nearly to the top of the funnel tube; then quickly close the rubber tube either by tightly pinching it or by placing the tongue firmly against its end. If the apparatus is air-tight, the water in the funnel tube will not fall. Prepare some dilute sulfuric acid by pouring slowly, a few drops at a time, 15 cc. of concentrated acid into a beaker containing 50 cc. of water. Stir the water with a glass rod while the acid is being added. Notice that the acid is poured into the water never the reverse (?). Cool the mixture and pour a few drops of it through the funnel tube. Hydrogen is at once evolved. Enough of the acid must be added from time to time to cause a gentle and continuous evolution of the gas. An excess of the acid should be avoided, however, or the action will become too violent and a large quantity of zinc will have to be added at the close of the exercise. It is evident that the gas which passes over first is a mixture of hydrogen and air. The student must remember that such a confined mixture of hydrogen and air explodes with great violence if ignited. Hence see that the end of the delivery tube is not brought near any flame. Determine when the hydrogen is free from air by repeatedly collecting a test tube full of gas and igniting it, holding the tube mouth downward (?). If pure, the gas burns quietly; otherwise there is a slight explosion. After all the air has been expelled from the generator, collect four bottles (250-cc., widemouthed) of the gas. What is the source of the hydrogen? What is the use of the zinc ? Remove the cork from the generator, add a few more pieces of zinc, and set aside. Sufficient zinc should be used so that at least a small portion of it remains undissolved. c. Properties of hydrogen. 1. Thrust a lighted splint into a bottle of the gas held mouth downward. Slowly withdraw the splint and again thrust it into the gas. Describe the results. What do they prove? 2. Fill a small (60-cc.) wide-mouthed bottle or test tube one-third full of water and invert it in a pneumatic trough. Displace the remaining water with hydrogen from one of the bottles. What does the bottle now contain? Withdraw it from the water and, holding it at arm's length, quickly bring it, mouth downward, over a flame. What do the results prove? 3. Uncover a bottle (mouth upward) of the gas. After a minute, test for the presence of hydrogen with a lighted splint. Repeat, holding the bottle mouth downward. Describe the results. Is the gas heavier or lighter than air? *4. Without removing the fragments of undissolved zinc, pour the liquid set aside in Experiment b into an evaporatingdish and boil gently on a ring-stand support. As soon as white crusts begin to form on the side of the dish, just above the liquid, filter the hot liquid into a beaker and set it aside to cool. How does the product which separates from the filtrate compare in properties with the original zinc? What is the product (p. 41 of text)? How does it differ in composition from sulfuric acid? EXERCISE 9 THE COMBUSTION OF HYDROGEN; THE OXY- Apparatus. Hydrogen generator A (Fig. 22) attached by rubber tubing C to a drying-tube B. This tube is filled with granulated calcium chloride, held in place by loose plugs of cotton placed at each end of the tube. D is a glass tube drawn to a jet. Materials. Granulated calcium chloride; cotton; 8 g. zinc; copper sulfate solution (R. S.); dilute sulfuric acid; picture-frame wire 10 cm. long; bit of charcoal. a. Charge the generator A (Fig. 22) with 6 or 8 g. of zinc, add the solution of copper sulfate, cover with water, and add dilute sulfuric acid as in b, Exercise 8. Slip a piece of rubber tubing over the tube D, collect samples of the gas in test tubes over water, and test with a flame to see whether the hydrogen is free from air. After all the air has |