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been expelled, wrap a towel carefully about the generator and cautiously ignite the hydrogen. The flame is nearly invisible and is very hot. Test

the heat of the flame by holding in it different objects, such as a splint, a piece of pictureframe wire, a bit of charcoal.

Finally, hold over the flame a cold, dry beaker or bottle and note the liquid deposited on the sides of the

vessel. Explain.

FIG. 22

B

D

b. Examine the structure of the oxyhydrogen blowpipe. Draw a diagram representing a cross section of it. Compare it with the blast-lamp used in the laboratory. Why not have a short inner tube?

EXERCISE 10

OXIDATION AND REDUCTION

Apparatus. Hydrogen generator and tubes as shown in Fig 23 (A is the hydrogen generator, B is a drying-tube filled with calcium chloride, C' is a straight glass tube, and D is a hard-glass test tube); burner; apparatus used in preparing oxygen (Fig 20).

Materials. 2 g. copper oxide; calcium chloride sufficient to fill the drying-tube B; 8 g. zinc; dilute sulfuric acid for preparing hydrogen (see Exercise 8); 4 g. potassium chlorate; 2 g. manganese dioxide.

a. Remove the tube D (Fig. 23), introduce into it 1 or 2 g. of copper oxide, and return it to the position shown in the figure.

Now generate hydrogen in A as in Exercise 8. After all the air has been expelled from the apparatus and the generator wrapped in a towel (see Exercise 9), cautiously heat

the copper oxide to redness, being careful to keep the flame away from the mouth of the tube D (?). Note the condensation of moisture in the cold portions of the tube. Is there any invisible evidence of change in the copper oxide? Explain. What is the object of the calcium chloride in tube B?

b. Disconnect the bottle A at E from the rest of the apparatus and force a little air through the tube B to remove

A

B

D

E

FIG. 23

the hydrogen present. Now connect the apparatus used in preparation of oxygen (Fig. 20) at E to the tubes B, C, and D. Generate oxygen and conduct a slow current of the gas through B, C, and D, at the same time heating the residue in D (?).

c. Explain the terms reduction and oxidation and give an example of each process from the above experiment.

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EXERCISE 11

MEASUREMENT OF GAS VOLUMES

Apparatus. Graduated tube and cylinder as shown in Fig. 24.

a. (It is suggested that the instructor arrange one or more pieces of apparatus as shown in Fig. 24. The students will then take the readings and solve

the problems.)

Partially fill a graduated tube with water and invert it in a cylinder (or other vessel) of water as shown in Fig. 24. Adjust the tube until the level of the liquid inside and outside of the tube is the same; then take the reading of the volume of the air in the tube. Note the temperature of the air (place a thermometer by the side of the tube) and likewise the pressure of the atmosphere as indicated by the barometer. Insert these values in their appropriate places in the following table:

FIG. 24

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From the above values calculate what volume the air inclosed in the tube would have under standard conditions.

EXERCISE 12

THE DETERMINATION OF THE WEIGHT OF ONE LITER OF OXYGEN; ALSO, OF THE PERCENTAGE OF OXYGEN IN POTASSIUM CHLORATE (QUANTITATIVE)

Apparatus. Apparatus shown in Fig. 25; burner; 500-cc. beaker. Materials. 1 g. potassium chlorate.

Prepare the form of apparatus shown in Fig. 25. A represents the hard-glass test tube used in the preparation of oxygen, B is a common (narrow-mouthed) bottle having a capacity of about 1 liter; rubber stoppers should be used in both A and B. The rubber tube C is provided with a screw clamp D, for closing the tube, and has in its end a glass tube E. The end of this glass tube is drawn out to a jet, the internal diameter of the jet being about 2 mm. F is a 500-cc. beaker.

B

FIG. 25

--E

The bottle is nearly filled with water, as shown in the figure, and allowed to stand until it acquires the room temperature. The tube A is now removed and a gentle suction is applied to the glass jet E. The water siphons over, through the tube C, into the beaker and is allowed to run for a moment so as to fill completely both the rubber and the glass tubes (C and E). The rubber tube is then quickly closed with the screw clamp D.

Now thoroughly clean and dry the tube A and carefully weigh it; then introduce about 1 g. of potassium chlorate into the bottom of the tube by means of a folded paper (Fig. 18), and reweigh. Attach the tube, as shown in Fig. 25, care being taken to have the apparatus air-tight.

The pressure of the air within the bottle is now adjusted to that of the air outside as follows: Water is added to the beaker F, if necessary, until the end of the glass tube E is covered. The screw clamp is then opened and the beaker at once raised vertically until the water in the beaker is at the same level as the water in the bottle and is retained in this position until the screw clamp D is closed. The beaker is then emptied and returned to the position shown in Fig. 25.

Now open the clamp D and gently heat the potassium chlorate in A. The oxygen is evolved and forces the water from the bottle into the beaker. Gradually increase the heat, and continue the heating until all the oxygen has been expelled (shown by the fact that no more water passes over into F). Let the apparatus stand until it has acquired the room temperature, care being taken that the glass jet E is kept below the surface of the water in the beaker (?).

Now bring the level of the water in the beaker to that of the water left in the bottle, and while holding it in this position close the screw clamp (?). Carefully measure the water in the beaker; also take the reading of the thermometer and the barometer. Disconnect the tube A and carefully reweigh. Insert the values in the following table:

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Weight of oxygen evolved (loss in weight of contents
of tube A on heating)

Volume of water in beaker = volume of oxygen evolved
Temperature of water

Barometric reading

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Value of vapor pressure (see Appendix)

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From your results calculate the weight of 1 liter of oxygen under standard conditions; also calculate the percentage of oxygen in potassium chlorate. Compare your results with the actual values (pp. 25 and 29 of text).

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