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 used in making indelible ink.

b. To 5 cc. of a solution of silver nitrate add sodium hydroxide in excess (R). Wash the black precipitate repeatedly with hot water. Test its solubility in nitric acid (R); in ammonia water (R).

c. To 2 or 3 cc. of a solution of silver nitrate add hydrochloric acid (R). Is the chloride soluble in nitric acid? in ammonium hydroxide (R)? in sodium thiosulfate? Prepare sufficient silver sulfide to make note of its color and solubility in hydrochloric acid.

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

207. Preparation of pure silver. Place 2 or 3 g. of an alloy of copper and silver 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 sodium chloride until a precipitate ceases to form. With stirring, the precipitate (what is it?) settles to the bottom of the beaker. Carefully decant the clear, supernatant liquid and test it for the presence of copper (§ 201, c). Wash the precipitate two or three times by pouring hot water over it and decanting. Finally, remove any remaining water by filtration. Mix the moist precipitate with an equal amount of sodium carbonate, transfer the mixture 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.

CHAPTER XXXIV

TIN AND LEAD

208. Stannous compounds. a. Dissolve about 0.5 g. of tin in hydrochloric acid (R). Why should this metal dissolve so much more slowly than zinc ? Cool, dilute to 10 cc. with water, and use for the experiments below.

b. Place about 2 cc. of a solution of mercuric chloride in a test tube and add stannous chloride drop by drop (R). Does the color of the precipitate change with the continued addition of the reagent (R)?

c. To 1 or 2 cc. of the solution add a little aqua regia, boil until the acid is expelled, and repeat the experiment with mercuric chloride. Why does no precipitate form?

d. Pour a few drops into a solution of hydrogen sulfide (R). e. Add a few drops of a solution of gold chloride to about 5 cc. of water. To 50 cc. of water add 1 or 2 drops of the solution of stannous chloride and 1 or 2 drops of a solution of sodium hydroxide. Add a little of this solution, drop by drop, to the solution of gold chloride. A deep rose-purple color slowly develops, known as the purple of Cassius. It is due to finely divided (colloidal) gold (R).

209. Stannic compounds. a. Add a few drops of concentrated nitric acid to a small piece of tin, heating gently if necessary. What is the compound formed? Is it soluble in water? in hydrochloric acid?

b. Dissolve a small piece of tin in aqua regia. What compound is formed (R)? Pour a few drops of the solution into a solution of hydrogen sulfide (R). Treat a little of the solution with a few drops of sodium hydroxide (R). Is the precipitate soluble in excess of the reagent?