I. COLLECTION OF GASES Fill a wide-mouthed bottle (250 cc.) with water. Cover its mouth with a glass plate, being careful to exclude all air bubbles. Hold the plate firmly in place, invert the bottle, and dip its mouth into the water in a pneumatic trougn. Remove the glass plate. Why does the water remain in the bottle? Now fill the bottle with ex aled air by placing one end of a piece of glass or rubber tubing under the mouth of the bottle and blowing gently through the other end. Before the bottle, so filled, is removed from the trough, cover its mouth tightly with a glass plate. The bottle so covered may then be placed on the desk in either an upright or an inverted position. (When should it be placed in an inverted position?) Fill a bottle with exhaled air and then transfer the air to another bottle. Draw a diagram to show the method of doing this. FIG. 13 II. PREPARATION OF OXYGEN FROM MERCURIC OXIDE In the bottom of a clean, dry test tube place about g. of mercuric oxide. This is done by placing the oxide near the end of a narrow strip of folded paper and introducing it carefully into the tube, as shown in Fig. 13. On inclining the tube and gently tapping the paper the oxide will be deposited in the bottom of the tube. The paper is now withdrawn, leaving the sides of the tube perfectly clean. Now hold the tube between the thumb and fingers (Fig. 6), and apply a gentle heat to the oxide. The tube must be rotated constantly to distribute the heat, otherwise it may be melted. During the heating insert a glowing splint from time to time into the mouth of the tube. Note the result. Continue to heat as long as any gas is evolved. What remains in the tube? How has the heat affected the mercuric oxide? I. PREPARATION OF OXYGEN FROM POTASSIUM CHLORATE Usual laboratory method. Prepare an apparatus according to Fig. 14. B is a hard-glass test tube, D a glass tube bent at right angles, and C a piece of rubber tubing. Mix intimately on paper 6 g. of potassium chlorate and 3 g. of manganese dioxide. The presence of impurities in the materials may lead to a serious explosion when heat is applied; hence test a small portion of the mixture, say g., by heating it in a test tube. In the absence of impurities the oxygen is evolved quietly, unaccompanied by any very marked sparking in the materials. FIG. 14 If pure, transfer the remainder of the mixture to a hard-glass tube, and insert the cork (Fig. 14); then, holding the burner in the hand, heat the mixture gently with a small flame, applying the heat at first to the upper part of the mixture. The flame must not strike the upper part of the test tube, as the cork may be ignited. At first the heat expands the air and a few bubbles escape; then the oxygen is evolved. Regulate the heat so as to secure a uniform and not too rapid evolution the gas. Collect three or four bottles (250 cc., wide mouthed of the gas. Before the heat is withdrawn remove the cork from the tube. Why? What is the source of the oxygen? What is the function of the manganese dioxide? II. PROPERTIES OF OXYGEN 1. Note the physical properties 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 sulphur in a deflagrating spoon until ignited. Note the color and size of the sulphur flame. Now lower the burning sulphur into a bottle of oxygen and note the change. 4. Tip a piece of picture-frame wire (12 cm.-15 cm. long) with sulphur by wrapping a very small bit of cotton about the end of the wire and dipping this into melted sulphur. Ignite the sulphur 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. Are the changes physical or chemical? What becomes of the oxygen? |