compare with that of the original solution? This reaction is characteristic of copper compounds.

d. Recall the formation of cuprous oxide (Exercise 52). e. Moisten the end of a copper wire with hydrochloric acid and hold it in the edge of a Bunsen flame (?).

EXERCISE 94

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 0.5 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 onefourth 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?

EXERCISE 95

A STUDY OF SILVER AND ITS COMPOUNDS

Apparatus. 200-cc. beaker; funnel; blowpipe; 3 test tubes.

Materials. 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.).

a. 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. When the solution is complete, dilute the product with about 25 cc. of water. Account for the color of the liquid. Now add a solution of 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 (c, Exercise 93). 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.

b. 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?

c. Recall the formation of silver sulfide (Exercise 33).

SOME PROPERTIES OF TIN

Apparatus. 100-cc. beaker; piece of charcoal; blowpipe; ring stand and burner.

Materials. 2 pieces of tin (size of a pea); hydrochloric acid; mercuric chloride solution (R. S.).

Note the physical properties of tin (?). Heat a bit of it on charcoal.

Dissolve a small piece of the metal in hydrochloric acid (R). Cool, dilute with a little water, and add 1 or 2 drops of the solution to 3 cc. of mercuric chloride solution. A white precipitate of mercurous chloride forms:

Now add a few drops more of the stannous chloride solution and heat the mixture gently. The mercurous chloride is reduced to metallic mercury, which forms a dark-gray precipitate:

SnCl2+2 HgCl-SnCl ̧ + 2 Hg

EXERCISE 97

A STUDY OF LEAD AND SOME OF ITS COMPOUNDS

Apparatus. Ring stand and burner; blowpipe; 200-cc. beaker; 5 test tubes.

Materials. 2 g. lead (obtain some scrap lead from a plumber); piece of charcoal; nitric acid; ammonium sulfide (R.S.); sulfuric acid; potassium chromate (R.S.); hydrochloric acid; strip of zinc.

a. Note the physical properties of the metal. Heat a small bit on charcoal. Is it easily melted? Note the coating formed on the charcoal (?).

b. Place about 1g. of the metal in a beaker (hood) and add 20 cc. of water and 5 cc. of nitric acid. Heat gently until the metal is dissolved (?). Dilute to 100 cc. and filter, if necessary, to obtain a clear solution. Call this solution "A." Now test small portions of this solution with ammonium sulfide, sulfuric acid, and potassium chromate (K,CrO,) respectively (R). Note the color of the precipitates. Add a few drops of hydrochloric acid to a test tube one-fourth full of solution A. Lead chloride is precipitated. Heat the mixture to boiling, and if the liquid does not become clear, add just enough boiling water to dissolve the precipitate; then set it aside until cool and note the result. How can you distinguish between lead chloride and silver chloride?

In the remainder of solution A suspend a piece of zinc (?).

EXERCISE 98

DETECTION OF SILVER, LEAD, AND MERCURY WHEN PRESENT IN THE SAME SOLUTION (OPTIONAL)

Apparatus. 300-cc. beaker; ring stand and burner; stirring-rod; funnel.

Materials. Solutions of AgNO,, Pb(NO3)2, and HgNO, (R.S.); filter paper; potassium chromate (R. S.); sulfuric acid; ammonium hydroxide; nitric acid; hot water.

The detection of any one metal becomes more complicated when other metals are present in the same solution. As a rule it is necessary so to treat the mixture as to separate the metals from each other. The principle involved is illustrated in the following procedure, the solution containing the nitrates of silver, lead, and mercury (ous).

Prepare a solution containing 0.2 g. of each of the following compounds: AgNO, Pb(NO), HgNO, Dilute with water to about 200 cc. Precipitate with HCl (R). Filter and fill the paper with boiling water three or four times, collecting the

2

4

4

liquid as it flows from the funnel. This liquid contains the PbCl, which has been dissolved by the hot water. Its presence may be proved by testing separate portions of the filtrate with solutions of K,CrO, and H2SO, respectively (Exercise 97). To the residue on the filter paper (of what is it composed?) add 2 or 3 cc. of NH2OH and collect the liquid as it drops from the funnel. This liquid contains the AgCl dissolved from the residue by the NH OH. To prove its presence add HNO, to the liquid until just acid to litmus paper. The AgCl is precipitated.

4

What effect did the NH OH have upon the color of the residue on the filter paper? This change in color is due to the action of NH OH on the HgCl, and serves as a test for the presence of the latter.

Supposing that the original solution contained only one or two of the metals of the group, how would the absence of the remaining ones be indicated?

EXERCISE 99

A STUDY OF SOME OF THE COMPOUNDS OF MANGANESE

Apparatus. 6 test tubes.

(OPTIONAL)

Materials. 0.1 to 0.2 g. potassium permanganate (KMnO4); crystal of ferrous sulfate; sulfuric acid; ammonium sulfide (R. S.); ammonium carbonate (R. S.); sodium hydroxide; manganese chloride solution (R.S.).

a. Examine the physical properties of potassium permanganate (?). Dissolve about 0.1 g. of it in 5 cc. of water (?). Add a drop of the solution to a solution containing a small crystal of ferrous sulfate and 2 or 3 drops of sulfuric acid. The ferrous sulfate is changed to ferric sulfate, the oxygen in the reaction (see equation below) coming from the potassium permanganate, which is a good oxidizing agent.

2 FeSO4 + H2SO,+0-Fe,(SO), +H2O