Laboratory exercises arranged to accompany "First course in chemistry"

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 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

H

A

FIG. 19

as a splint, a piece of picture-frame 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.

b. Examine the structure of the oxyhydrogen blowpipe. Draw a diagram representing a cross section of it.

EXERCISE 10

MEASUREMENT OF GAS VOLUMES (OPTIONAL)

Apparatus. Graduated tube and cylinder, as shown in Fig. 20; thermometer.

a. (It is suggested that the instructor arrange one or more pieces of apparatus as shown in Fig. 20. 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. 20. Adjust the tube until the level of the liquid inside and outside of the tube is the same; then take the reading of the

be a partial separation of the two liquids? In this way a mixture of liquids boiling at different temperatures may generally be separated more or less perfectly. The process is termed fractional distillation.

EXERCISE 12

THE QUALITATIVE SYNTHESIS OF WATER; 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, Cis a straight glass tube, and D is a hard-glass test tube); burner; apparatus used in preparing oxygen (Fig. 13).

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

a. Recall a, Exercise 9. The experiment proves the presence of what elements in water?

b. 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.

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 visible evidence of change in the copper oxide? Explain.

c. Disconnect the bottle A at E from the rest of the apparatus and force a little air through the tube B to remove the hydrogen present. Now connect the apparatus used in preparation of oxygen (Fig. 13) 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 (?). d. Explain the terms reduction and oxidation and give an example of each process from the above experiment.

EXERCISE 13

THE PROPERTIES OF HYDROGEN PEROXIDE

Apparatus. 3 test tubes; funnel.

Materials. 10 cc. hydrogen peroxide solution; 1g. manganese diox ide; wooden splint; black hair; filter paper.

a. What is the strength of the hydrogen peroxide solution sold by druggists (p. 52 of text)? Pour 3 cc. of the solution into a large test tube and add 1 or 2 g. of finely powdered manganese dioxide. Test the gas evolved with a glowing splint (?).

Filter the mixture remaining in the tube. The solid is the unchanged manganese dioxide. In what other experiment has manganese dioxide brought about a change without apparently undergoing any change itself?

b. Immerse some black hairs for a few seconds in a solution of the peroxide; remove the hairs, place them on a piece of paper, and examine them at the close of the laboratory period.

c. If time permits, the student is advised to prepare the apparatus shown in Fig. 26, Exercise 15, and set it aside for use when that exercise is reached.

EXERCISE 14

BOILING POINTS AND FREEZING POINTS

Apparatus. Flask (250-cc.) fitted with a two-hole cork, through which pass the thermometer B and the glass tube C, as shown in Fig. 24; test tube; large beaker; small beaker; ring stand; burner.

Materials. 300 g. ice; 100 g. common salt.

a. Boiling points. Fill the flask A (Fig. 24) about one-third full of water. Lower the thermometer until the bulb is immersed in the liquid. Heat the water to boiling and note the temperature after it has become con

stant. Apply a greater heat and note the effect upon the temperature.

Raise the thermometer until the bulb no longer touches the boiling water but is simply exposed to the steam. Again note the temperature. Note your general conclusions.

b. Freezing points. Place some pieces of ice in a small beaker. Introduce 15 or 20 cc. of water and stir the mixture gently with the bulb of the thermometer. Continue until the temperature is constant (?).

B

FIG. 24

Half fill a test tube with pure water and dip the tube into a freezing-mixture made by mixing 3 parts of powdered ice with 1 part of common salt. Stir the water in the test tube from time to time with a thermometer and note the temperature at which it begins to freeze. Compare the freezing point of water with the melting point of ice.