following facts. It will be recalled (Chapter XVI of the text) that both acids and salts are ionized in solution. In the case of sulfuric acid and sulfates, ions are formed as follows:

Likewise, barium chloride solution contains the ions Ba++ and Cl-, Cl. Now when a solution of barium chloride is mixed with any solution containing the SO--ion, the two ions Ba++ and SO-- unite to form the insoluble BaSO, which precipitates; hence the reaction proceeds to completion (p. 225 of text). The barium chloride test is therefore really a test for the presence of the SO-ion. Since only sulfuric acid and its salts give this ion, however, it is customary to say that it is a test for sulfuric acid and the sulfates.

EXERCISE 43

HYDRATES; EFFLORESCENCE

Apparatus. Burner; test tubes; porcelain crucible; ring stand; pipe-stem triangle; balance; evaporating-dish.

Materials. 2 g. zinc sulfate crystals; 6 g. copper sulfate crystals; clear crystal of sodium sulfate.

a. Hydrates. Heat some small crystals of zinc sulfate in a dry test tube. What evidence have you of the presence of water in the crystals? Examine the residue. How does it differ from the original crystals in form and composition?

Select some small crystals of copper sulfate. Do they appear to be dry? Fill a test tube one-fourth full of these crystals, and heat until no further apparent change takes place. Compare the residue in form, color, and composition with the original crystals. Dissolve the residue in as little hot water as possible, pour the solution (note its color) into an evaporating-dish, and set aside until crystals are deposited.

Do these appear to be identical with the original crystals of copper sulfate in shape and color?

The water evolved when hydrates are heated is sometimes called "water of crystallization." Is the term appropriate? Distinguish between the terms "hydrate," "anhydrous," "anhydride."

b. Efflorescence. Expose a clear crystal of sodium sulfate to the air for one or two hours. Note the change in its appearance. To what is the change due? What are such bodies called?

c. Quantitative determination of the amount of water expelled on heating the hydrate of copper sulfate. Accurately weigh (or counterpoise) a porcelain crucible and cover. Then add 2 or 3 g. of crystals (no larger than a pea) of the hydrate of copper sulfate and again accurately weigh. Place the covered crucible on a pipe-stem triangle and heat it with a gentle flame until the crystals lose their color. This will require from twenty to thirty minutes. The tip of the flame should not quite touch the crucible. The product is anhydrous copper sulfate. When the crucible is cool, reweigh. From your results calculate the percentage of water of crystallization present in the crystals. Compare your results with those obtained by other members of the class.

EXERCISE 44

THE PREPARATION AND PROPERTIES OF HYDROGEN

FLUORIDE

Apparatus. Piece of window glass; small lead dish (laboratory outfit).

Materials. 2 or 3 small pieces of paraffin (size of a pea); 3 g. fluorite; sulfuric acid.

PRECAUTION. Hydrogen fluoride is very corrosive and must not be inhaled; neither must its solution be brought in contact with the skin.

Place some pieces of paraffin on a glass plate and gently warm over a small flame. When the paraffin is melted, tilt

the plate about so as to form a uniform layer of the wax. When the wax is cold, scratch your name through the wax with a pin. Place 3 g. of fluorite in a lead dish and add sufficient sulfuric acid to make a paste of it. Cover the dish tightly with the waxed side of the glass plate and set it in the hood for an hour; then scrape off the paraffin and examine the glass. Write the equations for all the reactions involved.

EXERCISE 45

THE TEST FOR HYDROCHLORIC ACID AND ITS SALTS (CHLORIDES)

Apparatus. 6 test tubes.

Materials. 2 cc. silver nitrate solution (R. S.); hydrochloric acid; nitric acid; ammonium hydroxide; crystals of different chlorides, such as those of sodium, calcium, and iron.

NOTE. Experiments on chlorine and hydrochloric acid were included under Exercises 25 and 26. The student should carefully review the results obtained, since they have an important bearing upon the experiments now to be performed on the remaining members of the Chlorine Family.

a. Add 4 or 5 drops of hydrochloric acid to 4 cc. of water, mix well, and add 2 or 3 drops of silver nitrate solution (?). Divide the resulting mixture into two equal parts: to the one add ammonium hydroxide until the liquid is alkaline (?); to the other add 2 or 3 drops of nitric acid (?).

b. Examine the physical properties of such chlorides as are available. Test their solubility in water. What ones are insoluble (p. 394 of text)? Dissolve a small crystal of· different chlorides each in 5 cc. of water and add silver nitrate solution as in a (?). The formation of a white precipitate (Ag Cl) with silver nitrate, which precipitate is soluble in ammonium hydroxide and insoluble in nitric acid, serves as a good test for hydrochloric acid and its salts.

How do you account for the fact that both hydrochloric acid and its salts react toward silver nitrate in the same way?

EXERCISE 46

THE PREPARATION AND PROPERTIES OF BROMINE AND OF HYDROGEN BROMIDE

Apparatus. Retort, test tube and beaker, as shown in Fig. 40; burner; funnel; 2 test tubes.

Materials. 3 g. potassium bromide or sodium bromide; 4 g. manganese dioxide; 10 cc. sulfuric acid dissolved in 40 cc. water; strips of colored calico; 1 cc. silver nitrate solution (R.S.).

PRECAUTION. The vapor of bromine must not be inhaled. Perform the experiments in a hood.

a. Put into the retort a mixture of 2 g. of potassium bromide or of sodium bromide and 4 g. of manganese dioxide, and add to this through a funnel a cold dilute solution of sulfuric acid. Shake the retort so as to mix the contents thoroughly. The test-tube receiver should contain sufficient water to allow the end of the retort to dip just below its surface.

Now heat the retort gently. The bromine is liberated and distills over (R). Continue the heating until all the bromine has distilled over. Remove the stopper from the retort before the heat is withdrawn.

b. Note the properties of the bromine collected in the bottom of the receiver. Has any dissolved in the water? What property is implied in the name of the element?

Test the bleaching property of the aqueous solution. How does it compare with chlorine as a bleaching agent?

c. Add 3 or 4 drops of sulfuric acid to about 1 g. of potassium bromide in a test tube. Some hydrogen bromide is evolved, which attracts moisture as it escapes from the tube, forming a light cloud (test the vapor with a moist piece of blue litmus paper). At the same time there appears in the tube a reddish vapor. Explain (p. 270 of text). Distinguish between hydrogen bromide and hydrobromic acid.

d. Hydrobromic acid is unstable and is but little used. Its salts (bromides) are stable. Study the properties of potassium bromide as well as of any other available bromides.

Dissolve a crystal of potassium bromide in 5 cc. of water and apply the silver nitrate test as outlined for testing for chlorides in Exercise 45 (?). Would this test alone serve to distinguish between chlorides and bromides?

EXERCISE 47

THE PREPARATION AND PROPERTIES OF IODINE AND OF HYDROGEN IODIDE

Apparatus. Retort and connections, as shown in Fig. 40; burner. Materials. 4 g. potassium iodide (or sodium iodide); 4 g. manganese dioxide; sulfuric acid; chlorine water (R.S.); silver nitrate solution (R. S.); starch solution (R.S.); alcohol (R.S.).

a. Introduce into the retort a mixture of 2 g. of potassium iodide and 4 g. of manganese dioxide. Pour over this mixture 5 cc. of sulfuric acid. Insert the stopper and apply a gentle heat (R). Note the vapor of the iodine in the bulb of the retort; also note the grayish-black crystals, which are soon deposited in the neck of the retort. What property does the name of the element suggest?

b. Half fill two test tubes with starch solution. To the first add a few drops of a solution of iodine prepared by shaking a small crystal (obtained in a) in water (?). To the second add a few drops of an aqueous solution of potassium iodide (?). Now add to the second tube 2 or 3 drops of chlorine water and mix the contents (?). Determine whether the chlorine water alone changes the color of the starch. What is the function of the chlorine water added to the second tube? What other substance studied might be substituted for the chlorine in this experiment? (See Exercise 15.) Dissolve a crystal of iodine in alcohol. What is the solution called?