EXERCISE 84

PROPERTIES OF TIN AND ITS COMPOUNDS

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.).

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

b. 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 (R).

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 (R).

EXERCISE 85

A STUDY OF LEAD AND SOME OF ITS COMPOUNDS

Apparatus. Ring stand and burner; blowpipe; 200-cc. beaker; 5 test tubes; 100-cc. beaker; funnel and filter paper.

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; 2 g. red lead.

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 1 g. 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 (?). c. Introduce 1 g. of red lead into a 100-cc. beaker and add 5 cc. of water. Mix well and add 1 cc. of nitric acid. Stir the mixture and heat it gently. Notice the change in color (?). Now add 25 cc. of water, stir the mixture, and filter. Test the filtrate for the presence of lead (?). Place the filter paper and contents in a small beaker, add 2 or 3 cc. of hydrochloric acid, and stir with a glass rod so that the acid will come in contact with the solid on the paper. Heat the beaker gently. What gas is evolved (color and odor)?

EXERCISE 86

* DETECTION OF SILVER, LEAD, AND MERCURY, WHEN PRESENT IN THE SAME SOLUTION

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

funnel.

Materials. Solutions of silver nitrate, lead nitrate, and mercurous nitrate (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: silver nitrate, lead nitrate, and mercurous nitrate. Dilute with water to about 200 cc. Precipitate with hydrochloric acid (R). Filter and fill the paper with boiling water three or four times, collecting the liquid as it flows from the funnel. This liquid contains the lead chloride which has been dissolved by the hot water. Its presence may be proved by testing separate portions of the filtrate with solutions of potassium chromate and sulfuric acid respectively.

To the residue on the filter paper (of what is it composed ?) add 2 or 3 cc. of ammonium hydroxide and collect the liquid as it drops from the funnel. This liquid contains the silver chloride dissolved from the residue by the ammonium hydroxide. To prove its presence add nitric acid to the liquid until just acid to litmus paper. The silver chloride is precipitated.

What effect did the ammonium hydroxide have upon the color of the residue on the filter paper? This change in color is due to the action of ammonium hydroxide on the mercurous chloride, 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 87

A STUDY OF SOME OF THE COMPOUNDS OF

Apparatus. 6 test tubes.

MANGANESE

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 FeSO+H2SO, +0→→ Fe(SO), +H2O

b. In potassium permanganate the manganese acts as an acid-forming element. It also acts as a base-forming element in certain compounds. Try the action of ammonium sulfide, ammonium carbonate, and sodium hydroxide, respectively, on a solution of manganese chloride (?).

EXERCISE 88

A STUDY OF SOME OF THE COMPOUNDS OF CHROMIUM

Apparatus. 6 test tubes.

Materials. Solution of potassium chromate (R. S.); lead acetate solution (R. S.); barium chloride solution (R. S.); ammonium sulfide (R. S.); sodium carbonate solution (R. S.); sodium hydroxide; 0.5 g. chromium chloride or chromium sulfate dissolved in 25 cc. of water.

a. Chromates and dichromates. Write the formula for potassium chromate; for potassium dichromate. Is the chromium an acid-forming or a base-forming element in these compounds? Add 2 or 3 drops of sulfuric acid to a little potassium chromate solution (?).

b. Try the effect of a solution of potassium chromate on a solution of a compound of lead (R); also on a compound of barium (R).

c. Add a few drops of hydrochloric acid to a little solid potassium chromate and explain the results (R). Repeat, using potassium dichromate (R).

d. Salts of chromium. Try the effect of the following reagents on a solution of a salt of chromium: ammonium sulfide, sodium carbonate, sodium hydroxide. Write all the equations.

EXERCISE 89

*BORAX-BEAD TESTS

Apparatus. Platinum wire; burner.

Materials. Borax (R. S.); 1 cc. of a solution of a compound of each of the following metals: nickel, iron, manganese, copper.

Recall the effect of adding a trace of cobalt nitrate to a borax bead (b, Exercise 62). Repeat the experiment, substituting for the cobalt nitrate, salts of the following metals: nickel, iron, manganese, copper (?).