litmus paper. What is the solution called? How does it compare with the so-called "sulfur water" of many springs?

Drop a silver coin into the solution and account for the results. Why do certain foods, as eggs, blacken silver spoons?

c. Pass a few bubbles of hydrogen sulfide through 3 cc. of nitric acid. Sulfur separates as a white solid (?). Account for the fact that sulfur waters deposit sulfur on exposure to air. *d. Filter off the liquid left in the generator and evaporate (hood) to dryness in an evaporating-dish. Note the results.

EXERCISE 39

THE PREPARATION AND PROPERTIES OF THE SALTS OF HYDROSULFURIC ACID (SULFIDES)

Apparatus. Hydrogen generator and connections, as shown in Fig. 42; 6 test tubes; funnel; watch glass.

Materials. Ferrous sulfide and dilute hydrochloric acid, as used in Exercise 38; separate solutions of silver nitrate, copper sulfate, cadmium chloride, lead nitrate, and sodium chloride, made by dis solving about 0.3 g. of the solid in 5 cc. water (solutions on reagent shelf may be used); 5 filter papers; sulfuric acid; lead acetate (R. S.).

a. Charge the hydrogen sulfide generator as in Exercise 38 and pass a few bubbles of the gas (Fig. 42) through each of the following solutions: (1) silver nitrate, (2) copper sulfate, (3) cadmium chloride, (4) sodium. chloride, (5) lead nitrate. The exit tube C through which the gas bubbles into

the solutions must be thor

A

B

FIG. 42

oughly cleaned each time (?). Note the color of the precipitates obtained. Write the equations for the reactions

involved. Do any of the solutions fail to give a precipitate when the gas is passed through them? How do you account for this?

b. Intimately mix 5 g. of sulfur with 3 g. of powdered lime. Transfer to a beaker and add 150 cc. of water. Stir the mixture and heat just to boiling for ten minutes. Now fill a test tube with the resulting mixture, cork the tube loosely, and set it aside until the end of the laboratory period; then examine. Describe the results. For what is the solution used? What is its composition (p. 232 of text)?

c. Test for hydrogen sulfide. Dip a strip of filter paper into a solution of lead acetate. Remove the cork from the hydrogen sulfide generator and insert the paper for a moment. Note the results (R). This serves as a convenient test for the gas. What property would also serve to detect it if present in any marked quantity?

d. Test for sulfides. Filter off the precipitated sulfides obtained in a, and wash them with water until the odor of hydrogen sulfide is no longer noticeable. Transfer each in succession to a watch glass and add to the solid 1 or 2 drops of sulfuric acid. Carefully note the odor. Most of the sulfides when treated in this way evolve hydrogen sulfide, which can be detected by the odor and by its action on paper moistened with lead acetate.

All sulfides when heated in air evolve sulfur dioxide (formed by the combustion of the sulfur present), which has the characteristic odor of burning sulfur. Heat a little iron sulfide in the flame of the burner and note the odor.

EXERCISE 40

SULFUR DIOXIDE AND SULFUROUS ACID

Apparatus. 250-cc. flask fitted with funnel tube and glass exit tube, as shown in Fig. 43; ring stand and burner; 3 bottles (250-cc.); 2 test tubes; evaporating-dish.

Materials. 10 g. copper; 25 cc. sulfuric acid; blue litmus paper; sodium hydroxide solution; strips of colored calico or a red flower.

a. Preparation of sulfur dioxide and sulfurous acid. (Hood.) Place about 10 g. of copper turnings or small pieces of sheet copper in a generator arranged as in Fig. 43. Add 25 cc. of concentrated sul

furic acid and apply a gentle
heat. As soon as the action
begins (R), lower the flame,
regulating it so as to obtain
a uniform evolution of the
gas. Collect two bottles of
the gas by displacement of
air; then cause it to bubble
through 50 cc. of water as
long as any is dissolved.
b. Properties. Note the
odor of the gas.
Is the gas

combustible?

Invert one of the bottles

FIG. 43

of the gas so that its mouth is under water, and examine after several minutes (?). Account for the results.

Test the saturated aqueous solution of the gas with blue litmus paper. Is the gas combined with the water or simply dissolved in it?

Set aside 10 cc. of the solution for a future experiment (Exercise 41) and divide the remainder into two equal parts.

Immerse in the one part some small strips of colored calico or some petals of a red flower and note any results.

c. Salts of sulfurous acid — the sulfites. To the remainder of the liquid add a solution of sodium hydroxide, drop by drop, until neutral (R), and evaporate just to dryness. What is the residue? Moisten it with 2 or 3 drops of sulfuric acid and note the odor of the evolved gas (?). All sulfites evolve sulfur dioxide when treated with sulfuric acid. This reaction serves as a good test for sulfites.

EXERCISE 41

A STUDY OF SULFURIC ACID

Apparatus. Burner; 5 test tubes.

Materials. Sulfuric acid; 2 pieces of granulated zinc; splint; 0.5 g. sugar; barium chloride solution (R.S.); hydrochloric acid; nitric acid; sulfurous acid prepared in Exercise 40; bit of charcoal (size of a bean).

a. Heat a bit of charcoal with 1 or 2 cc. of sulfuric acid. What gas is evolved (odor)? Account for the formation of this gas, recalling that carbon has a strong affinity for oxygen.

b. Into one test tube pour 3 cc. of water and add 5 drops of sulfuric acid; into another test tube pour 3 cc. of concentrated sulfuric acid. Drop a small piece of zinc into each tube. If no reaction takes place, heat the acid gently. Test with a lighted splint the gas evolved by the action of the dilute acid on zinc. Note the odor of the gas evolved by the action of the concentrated acid on the metal. Account for the difference in the action of the dilute and the concentrated acid.

c. Put a drop of concentrated sulfuric acid on a splint. Pour a few drops on 0.5 g. of sugar in a test tube. Examine after a few minutes. Account for the results.

d. Recall the action of sulfuric acid on sodium nitrate (Exercise 33). What property of sulfuric acid enables it to be used in the preparation of nitric acid?

e. Add 3 drops of sulfuric acid to 5 cc. of water in a test tube. To this add a few drops of a solution of barium chloride. Note that a precipitate forms (R). Now add 3 or 4 drops of hydrochloric acid. Does the precipitate dissolve? The formation with barium chloride of a precipitate which is insoluble in hydrochloric acid constitutes a good test for sulfuric acid. f. Divide the solution of sulfur dioxide obtained in Exercise 40 into two portions. To the one, apply the test for sulfuric acid (?). To the other, add 1 cc. of concentrated nitric acid and heat gently nearly to boiling; then cool and apply the test for sulfuric acid (?).

EXERCISE 42

SALTS OF SULFURIC ACID (SULFATES)

Apparatus. 6 test tubes.

Materials. Crystals or small amounts (0.1 g.) of the sulfates available in the laboratory; 2 cc. barium chloride solution (R.S.); hydrochloric acid.

a. Examine the physical properties of (1) sodium sulfate, (2) calcium sulfate, (3) barium sulfate, (4) copper sulfate, (5) magnesium sulfate, (6) ferrous sulfate, (7) such other sulfates as are available.

Test the solubility of each of the above sulfates in water. What sulfates are insoluble (p. 395 of text)?

b. Prepare a dilute solution of different sulfates by dissolving a crystal of each in 2 or 3 cc. of water. Add to each 1 drop of barium chloride solution (?). Add 1 or 2 drops of hydrochloric acid to the mixture in each tube. Does the precipitate dissolve? All soluble sulfates give in solution a white precipitate (BaSO) with barium chloride solution, which precipitate is insoluble in hydrochloric acid. This reaction serves as a good test for sulfates.

It will be noted that both sulfuric acid and its salts give with barium chloride the same product; namely, a white precipitate of barium sulfate. This is evident from the