rod into limewater and remove it gently so that a drop of the clear liquid clings to the end of the rod; then quickly remove the cork from the test tube and lower the end of the rod into the tube (?). Write all reactions involved.

b. Repeat a, substituting cream of tartar for the alum (?). c. What compounds remain in food as a result of the use of an alum baking-powder? of a cream of tartar baking-powder? For methods of analysis of baking-powders, see Exercise 98, Appendix A.

EXERCISE 80

A STUDY OF IRON, COBALT, AND NICKEL

Apparatus. 2 beakers; watch glass; funnel; flask (250-cc.); 8 test tubes; ring stand and burner.

Materials. Piece of watch spring, 10 to 15 cm. in length; 5 g. of small tacks or fine iron wire; 0.5 g. powdered iron; filter paper; hydrochloric acid; sulfuric acid; nitric acid; ammonium hydroxide; potassium ferrocyanide (R. S.); potassium ferricyanide (R. S.); potassium sulfocyanide (R.S.); 0.5 g. cobalt nitrate dissolved in 10 cc. of water: 0.5 g. nickel nitrate dissolved in 10 cc. of water.

a. Heat a piece of watch spring (from 10 to 15 cm. in length) to a white heat in a Bunsen flame. Let it cool slowly, and when cold bend it to determine if it is brittle (?). Again heat and at once plunge into a beaker of cold water. Bend the piece as before (?). Reheat the piece, allow it to cool slowly, and again examine it (?).

b. Place 5 g. of fine iron wire or small tacks in a beaker and pour over it 15 cc. of water. Now add 4 cc, of concentrated sulfuric acid and heat very gently (hood) until a vigorous evolution of gas takes place (R), then cover the beaker with a watch glass and set it aside in the hood until near the end of the laboratory period. Then add 10 cc. of water and heat slowly until the liquid boils, stirring the mixture constantly. Filter off any undissolved solids, collecting the filtrate in a beaker. Set the filtrate in your desk until the next laboratory period; then examine the crystals (?).

c. Place about 0.5 g. of iron powder in a small flask, pour over it 5 cc. of water, and then from 1 to 2 cc. of hydrochloric acid. Mix the contents of the flask, heat the flask slightly, and set aside in the hood for five minutes (?). Add 50 cc. of water, mix well, and filter. Divide the filtrate into two equal parts. Mark one of these A and set it aside. Add to the other portion about 1 cc. of hydrochloric acid and heat it nearly to boiling; then withdraw the flame and add nitric acid (about 2 cc. of the concentrated acid will be required), a drop at a time, with constant stirring, until the solution, which is at first dark brown, becomes light yellow in color (?). Cool the resulting solution. Call this solution B. How do solutions A and B differ in composition? Now compare the action of the following reagents upon solutions A and B (add 2 or 3 drops of the reagents to 5 cc. of the solutions in test tubes): ammonium hydroxide, potassium ferrocyanide, potassium ferricyanide, potassium sulfocyanide (KCNS). Tabulate your results as follows:

d. Test separate solutions of a salt of cobalt and nickel with a borax bead (b, Exercise 62); with a solution of sodium hydroxide; with ammonium sulfide. Note the results.

e. Write on a piece of paper, using the cobalt nitrate solution as ink. Now heat the paper gently over a flame (?) (invisible ink); moisten the paper with a damp cloth (?).

EXERCISE 81

A STUDY OF COPPER AND ITS COMPOUNDS

Apparatus. 3 test tubes; beaker; ring stand and burner.

Materials. Nail; copper sulfate solution (R. S.); sodium hydroxide; ammonium sulfide (R. S.); ammonium hydroxide; 10 cm. copper wire; hydrochloric acid.

a. Recall the action of nitric acid and of sulfuric acid on copper (Exercises 33, 40); also the action of chlorine and sulfur on copper (Exercises 25, 37). Place a nail in a solution of copper sulfate. Account for the result.

b. To a cold solution of copper sulfate add one half its volume of sodium hydroxide solution. Copper hydroxide (Cu(OH),) is precipitated. Now heat to boiling. The hydroxide is decomposed into water and cupric oxide (black).

c. Try the action of ammonium sulfide on copper sulfate (R). d. Add 1 drop of ammonium hydroxide to a dilute solution of copper sulfate; now continue to add the ammonium hydroxide, drop by drop, until the precipitate which is at first formed is dissolved. How does the color of this solution compare with that of the original solution? This reaction is characteristic of copper compounds.

e. Recall the formation of cuprous oxide (Exercise 53). f. Moisten the end of a copper wire with hydrochloric acid and hold it in the edge of a Bunsen flame (?).

EXERCISE 82

A STUDY OF MERCURY AND ITS COMPOUNDS

Apparatus. 100-cc. beaker; 2 test tubes.

Materials. Globule of mercury (size of a grain of wheat); nitric acid; copper penny; 0.5 g. mercuric oxide; 3 cc. solution of mercurous nitrate (R.S.); hydrochloric acid.

a. Note the physical properties of mercury. Place a globule of it in a small beaker and add (hood) just enough nitric acid to dissolve it. Dilute the product with 10 cc. of water and place a copper penny in the solution. After a few minutes remove the coin and polish it with a piece of cloth. Account for the result.

b. For what purpose have we used mercuric oxide? Place 0.2 g. of it in a test tube and dissolve it in as little nitric acid as possible (R). Then add water until the test tube is one-fourth full. Into a second test tube pour a similar volume of a solution of mercurous nitrate. Now add 2 or 3 drops of hydrochloric acid to each test tube (R). What conclusions do you draw in reference to the solubility of the two chlorides of mercury? c. Prepare some mercuric sulfide (?).

EXERCISE 83

A STUDY OF SILVER AND ITS COMPOUNDS

Apparatus. 200-cc. beaker; funnel; blowpipe; 4 test tubes; burner; ring stand.

Materials. Piece of cotton cloth 2 or 3 cm. square; silver dime; nitric acid; hydrochloric acid; ammonium hydroxide; filter paper; hot water; 2 or 3 g. sodium carbonate; piece of charcoal; 10 cc. silver nitrate solution (R. S.); solutions of potassium bromide and of potassium iodide (R. S.); 1 cc. formaldehyde solution.

a. Place a drop of silver nitrate solution on a piece of cotton cloth and warm gently. Can you wash the stain away?

What is it? Try ammonia water. Owing to the permanence of this stain, silver nitrate is sometimes used in making indelible ink.

When the solution is about 25 cc. of water. Now add a solution of

b. Place a silver dime in a small beaker and add (hood) sufficient nitric acid to dissolve it. The solution may be hastened by applying a gentle heat. complete, dilute the product with Account for the color of the liquid. hydrochloric acid until a precipitate ceases to form. On being stirred, the precipitate (?) settles to the bottom of the beaker. Carefully decant the clear supernatant liquid and add ammonium hydroxide until the solution becomes alkaline (?) (d, Exercise 81). Wash the precipitate two or three times by pouring hot water over it and decanting. Finally, remove any remaining water by filtration. Mix the product with an equal bulk of sodium carbonate, transfer to a small cavity in a piece of charcoal, and heat it with a blowpipe. The silver salt is gradually reduced to metallic silver, which will fuse into a globule if sufficient heat is applied. How does this differ in composition from the metal constituting the silver dime?

c. Prepare small amounts of the chloride, the bromide, and the iodide of silver (R). Expose to the sunlight the test tubes containing the precipitates and note any changes. For what are these compounds used?

d. Recall the formation of silver sulfide (Exercise 39).

e. Thoroughly clean a test tube by rinsing it with a few drops of nitric acid and then with distilled water. Pour into the tube 2 cc. of silver nitrate solution and dilute with an equal volume of water. Now add ammonium hydroxide, a drop at a time (shake the tube after addition of each drop), until the precipitate which forms at first redissolves, leaving a clear liquid. Next add 2 drops of a solution of formaldehyde, mix thoroughly, and place the tube in a beaker of cold water. Gradually heat the water to boiling (?).