conclusions do you draw from the experiment? How do you account for the fact that a burning candle gradually loses weight, while iron on burning increases in weight? EXERCISE 5 CHEMICAL ACTION; ELEMENTS, COMPOUNDS, MIXTURES Apparatus. Burner; evaporating dish; ring stand; magnet; magnifying glass; test tubes. Materials. Iron wire, 10 cm. in length; wooden splint; 2 or 3 g. of sugar; 1g. common salt; piece of granulated zinc; sulfuric acid; 2 g. sulfur; 2 g. powdered iron. a. Chemical action. Hold a piece of iron wire in the Bunsen flame for a few seconds. Is the iron changed? Examine it when it has cooled. Have the properties changed? Has a chemical action occurred? b. Repeat a, using a splint of wood in place of the iron wire. How does the change produced differ from that in a? Has a chemical action occurred ? c. Place enough sugar in a clean, dry test tube to cover the bottom to a depth of 1 cm. Heat it gently in the tip of the flame as long as any changes are produced. Note all the changes. Is the product sweet? Is it soluble in water? Do any properties remain unchanged? What grounds do you have for assuming that a chemical action has taken place? d. Place about 1 g. of common salt in a test tube and dissolve it in a little water. Pour the clear solution into an evaporating-dish and evaporate to dryness (Fig. 15). What is the solid left? How do its properties correspond to those of the original salt? e. Cover a small piece of zinc in a test tube with about 5 cc. of water and add carefully 3 or 4 drops of sulfuric acid. Notice that the zinc dissolves with the evolution of a gas. Hold a burning splint at the mouth of the test tube and note the result. After the action has entirely ceased, filter off any undissolved zinc and evaporate the solution to dryness (hood) as in d. How does the change differ from that in d? f. Elements; compounds; mixtures. What is an element? Are iron and sulfur included in the list of elements? Weigh out separately (on paper) 2 g. of sulfur and 2 g. of powdered iron and make a careful list of their properties. Try the effect of a magnet on each. Now mix the two, and grind them together intimately in a mortar. Examine the product with a magnifying glass. Can you distinguish the iron from the sulfur? Can you separate them with a magnet? Have they undergone any change in properties? What is such a material called? mutue Now place the product in a clean test tube and heat gently. As soon as the mass begins to glow, quickly withdraw the tube from the flame. Does the mass continue to glow? Now heat it strongly for one or two minutes; then cool the tube, remove the substance (it may be necessary to break the tube to do so), and examine the product with a magnifying glass. Can you now distinguish between the iron and the sulfur? Try the effect of the magnet. Of what is the substance composed? When elements combine chemically, do they retain their original properties? What is the product of such a combination called? EXERCISE 6 COLLECTION OF GASES; PREPARATION OF OXYGEN FROM (4) MERCURIC OXIDE, (B) SODIUM PEROXIDE Apparatus. 250-cc. wide-mouthed bottles; glass plate; pneumatic trough or granite-ware pan; hard-glass test tube; * apparatus Fig. 19. Materials. 0.5 g. mercuric oxide; wooden splint *; 5 g. sodium peroxide. a. 1. 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 FIG. 18 a clean, dry test tube b. Preparation of oxygen from mercuric oxide. In the bottom of place about g. of mercuric oxide. ing the oxide near the end of a paper and introducing it carefully into the tube, as shown in Fig. 18. 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. 9), 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? What kind of change has the mercuric oxide undergone (p. 14 of text)? mik *c. Preparation of oxygen from sodium peroxide. Sodium peroxide is a white solid containing 41 per cent of oxygen, and when treated with water, a part of this is set free. Arrange an apparatus according to Fig. 19. By means of a short piece of rubber tubing A connect the funnel B with a glass tube C, pinching the rubber tube shut with a screw clamp. Place about 5 g. of sodium peroxide in the bottom of D and partly fill the funnel with warm water. Very cautiously open the screw clamp so that the water will run down and fall, drop by drop, upon the peroxide. (The funnel must be kept partially filled with water (?)). A steady current of oxygen is given off. Collect a bottle of the gas and test it by thrusting a glowing splint into the bottle. What is the source of the oxygen? What substances remain in the generator bottle (p. 27 of text) ? EXERCISE 7 USUAL LABORATORY PREPARATION OF OXYGEN; PROPERTIES OF OXYGEN Apparatus. Test tubes; test tube prepared in Exercise 2 (Fig. 2), connected with rubber tube as shown in Fig. 20; trough and widemouthed bottles shown in Fig. 20; burner; deflagrating-spoon ; 4 pieces of window glass 10 cm. square; funnel; evaporating-dish. Materials. 10 g. potassium chlorate; 4 g. manganese dioxide; wooden splint; 1 g. of sulfur; picture-frame wire 20 cm. long; bit of cotton; filter paper. a. Preparation of oxygen from potassium chlorate (preliminary experiment). Select two test tubes of the same size, and clean and dry them thoroughly. Into the one introduce 2 g. of potassium chlorate; into the other introduce 2 g. of potassium chlorate mixed intimately with 1 g. of manganese dioxide. Now heat the contents of the two tubes, applying the flame so that both tubes are equally heated. Repeatedly thrust a glowing splint into each tube in order to detect any oxygen that may be evolved. Note the results. What effect has the manganese dioxide? Does the addition of the manganese dioxide enable one to obtain an increased amount of oxygen (p. 26 of text)? What term is applied to substances which act like the manganese dioxide ? b. Preparation of oxygen; usual laboratory method. 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 0.5 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. Place the apparatus prepared in Exercise 2 (Fig. 2) on a ring stand as shown in Fig. 20, and connect with it a piece of rubber tubing C; also fill 4 wide-mouthed bottles (250-cc.) with water and invert them in a pneumatic trough, as shown in the figure. Now transfer the mixture of potassium chlorate and manganese dioxide to the glass tube A, and insert the cork (Fig. 20); 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 of air escape; then the oxygen is evolved. Regulate the heat so as to secure a uniform and not too rapid evolution of the gas. Will the gas which passes over at first be the pure oxygen? Collect four 250-cc. widemouthed bottles of the gas. Before the heat is withdrawn, |