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steam heats the dish and causes the water in the dish to evaporate. After the solution has evaporated to dryness, cover the dish with the watch-glass and heat it directly with the burner, regulating the flame so that the tip barely touches the dish. Continue the heating until all the moisture has been expelled and the under part of the watch-glass is free from moisture. (Why use the watch-glass cover?)

Now withdraw the burner, and after the dish is cool (room temperature) weigh once more. From your results calculate the approximate number of grams of salt that will dissolve in 1 liter of water at the temperature of the original solution.

b. If time permits, the solubility of other substances such as calcium sulfate and potassium dichromate may be determined in like manner; or different students may select different substances and compare their results.

EXERCISE 24.

*A METHOD FOR DETERMINING WHETHER OR NOT A GIVEN LIQUID IS A CONDUCTOR OF ELECTRICITY

Apparatus. Apparatus as shown in Fig. 34 (p. 153 of text); current from electric-lighting system.

Materials. 5 g. common salt; 5 g. sugar; 5 cc. sodium hydroxide solution added to 20 cc. water; 3 cc. sulfuric acid added to 20 cc. water; tap or well water.

a. Obtain from your instructor the apparatus shown in Fig. 34. Polish the copper-wire electrodes with emery paper until they are bright and free from oxide. At the beginning of each experiment see that the electrodes are bright and dry, and that the cell is also perfectly clean. Unscrew a lamp C from a convenient socket above your desk, screw it loosely into the socket on your apparatus, and attach the apparatus to the empty socket on the lighting system by means of the extension cord and plug B. Every time a change is to

be made in the cell, loosen the lamp in the socket, and do not screw it down to make contact until all of the connections of the cell have been arranged.

b. Partly fill the cell A with dry powdered salt, dip the electrodes into the powder, arrange the connections at the

[graphic][subsumed][subsumed][subsumed][merged small]

binding-posts, and screw down the lamp C. Does the salt conduct the electric current ?

c. Repeat b, using distilled water (?).

d. Repeat b, using a solution of the salt in distilled water (?). e. Test the conductivity of the following substances and interpret the results: (1) dry powdered sugar; (2) a solution of sugar in water; (3) tap or well water; (4) distilled water containing a few drops of sulfuric acid; (5) a solution of sodium hydroxide.

EXERCISE 25

THE PREPARATION AND PROPERTIES OF CHLORINE

Apparatus. Apparatus as shown in Fig. 35 (A is a 250-cc. flask and B and C 250-cc. bottles; B contains some sulfuric acid); 4 additional 250-cc. bottles (dry); glass plates; 200-cc. beaker with glass plate cover.

Materials. 25 g. manganese dioxide; hydrochloric acid; bit of powdered antimony; strip of copper foil; strips of colored calico; piece of printed paper (printer's ink); paper written over with ordinary ink; 1 g. potassium permanganate.

PRECAUTION. All of the following experiments must be performed in the hood, and great care must be taken not to inhale the chlorine.

a. (Two students may work together.) Arrange an apparatus according to Fig. 35. Put into the flask from 20 to 25 g. of manganese

dioxide. Insert the cork and pour 150 cc. of hydrochloric acid through the funnel tube. Shake the flask SO as to mix the contents thoroughly. Warm gently, applying just enough heat to cause a gentle evolution of the gas, but not sufficient to boil the liquid. The chlorine generated

[blocks in formation]

bubbles through the sulfuric acid in B (which removes the moisture) and is collected in C. Fill four bottles with the gas (note the color), cover them with glass plates, and set them aside.

b. Grind a fragment of antimony to a fine powder and sprinkle a pinch of the powder into one of the bottles of the gas. SbCl, is formed (R).

c. Support by forceps a small piece of copper foil, heat it to redness, and immediately thrust it into a bottle of the gas. Describe the result. What is formed?

d. Suspend strips of colored calico in a bottle of the dry gas; also two strips of paper, the one with writing in ink on it, the other with printing (printer's ink) on it.

e. Repeat d, using similar strips, but moistened with water. Describe the results in d and e. What part does the water play in the bleaching?

f. Place a few crystals of potassium permanganate on the bottom of a clean beaker. Pour over these 3 cc. of hydrochloric acid and cover the beaker with a glass plate. Examine after a few minutes and test (?) the gas in the beaker for the presence of chlorine (?).

EXERCISE 26

THE PREPARATION AND PROPERTIES OF HYDROGEN CHLORIDE AND OF HYDROCHLORIC ACID

Apparatus. Flask and bottle connected as shown in Fig. 36 (this is same as shown in Fig. 35, except that the glass tube extending into the bottle B does not touch the liquid (water) in B); two 250cc. bottles (dry); burner; large beaker.

Materials. Dilute sulfuric acid prepared by slowly pouring (3 or 4 drops at a time with constant stirring) 30 cc. of the concentrated acid into 10 cc. water; 50 g. sodium chloride; splint; blue litmus paper.

a. Usual laboratory method of preparation. Put about 50 g. of common salt into the generator flask (Fig. 36), insert the cork, pour the cold dilute sulfuric acid through the funnel tube, mix the contents by a gentle motion of the flask, and after two or three minutes warm gently with a

small flame. Notice the currents in the water in B. What causes them? As soon as the gas is evolved regularly, disconnect the generator flask at D long enough to collect two bottles of the gas by displacement of air (as in Fig. 30). Cover these tightly with glass plates and set them aside; then connect the generator with B again and continue to apply a gentle heat as long

as any gas is evolved (R).

b. What is the color of the gas (examine that in the generator)?

c. Test the gas in one of the bottles with a lighted splint. Is it combustible? Is it a supporter of combustion?

B

F

d. Fill a large beaker with water. Now uncover the remaining bottle, invert it, and at once bring its mouth under the surface of the water in the beaker. Describe the results. What does the experiment prove? Why not extend the tube in bottle B (Fig. 36) to the bottom of the bottle?

FIG. 36

e. Put a drop of the aqueous solution of the acid from bottle B on a bit of blue litmus paper. Note the result. Pour 2 drops of the solution into 3 or 4 cc. of water and taste a drop. Perform a test-tube experiment to prove the presence of chlorine in the acid (R); also one to prove the presence of hydrogen (R). How does the solution compare with the hydrochloric acid on your desk?

f. Distinguish clearly between hydrogen chloride and hydrochloric acid.

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