Chemical lecture experiments . elenium separates out of the solution in theform of a fine, dark red powder. Se ; fuming H2SO4 ; K2Cr207 177 178 CHEMICAL LECTURE EXPERIMENTS (b) Selenium, when heated with fuming sulphuric acid,dissolves with a green color. If the solution is then dilutedwith concentrated sulphuric acid and heated to boiling for afew moments, it will finally be decolorized as the seleniumis oxidized by the sulphur trioxide. On pouring the de-colorized solution into water no precipitate of selenium willoccur. 2 S03 + Se = 2 S02 + Se02. Fuming H2S04; Se. SELENIUM DIOXID
Chemical lecture experiments . elenium separates out of the solution in theform of a fine, dark red powder. Se ; fuming H2SO4 ; K2Cr207 177 178 CHEMICAL LECTURE EXPERIMENTS (b) Selenium, when heated with fuming sulphuric acid,dissolves with a green color. If the solution is then dilutedwith concentrated sulphuric acid and heated to boiling for afew moments, it will finally be decolorized as the seleniumis oxidized by the sulphur trioxide. On pouring the de-colorized solution into water no precipitate of selenium willoccur. 2 S03 + Se = 2 S02 + Se02. Fuming H2S04; Se. SELENIUM DIOXIDE 4. Preparation. — Selenium burns in a current of oxygen toform selenium dioxide. A few lumps of selenium are placedin a bulb-tube, which is fitted to the mouth of a dry filter-flask, the side tube of which isconnected with a flue (Fig. 77).A current of oxygen is passedthrough the bulb- tube and theselenium heated to product condenses on thearm of the tube and in the re-ceiver as a white powder. Se + 02 = Fig. 77 Apparatus (Fig. 77) ; bulb-tubefilter flask ; O supply ; Se. 5. Green color of the vapor. — A small quantity of seleniumdioxide is heated in a test-tube. The greenish vapor is bestseen by holding the tube in front of a white background. SELENIOUS ACID 6. Oxidation of selenium with nitric acid. — When pow-dered selenium is heated in a test-tube with concentratednitric acid, it rapidly dissolves, with the liberation of nitrous SELENIC ACID 179 fumes, forming selenious acid. After neutralization withammonia the solution gives any of the reactions for seleni-ous acid. 7. By dissolving selenium dioxide in water. — Seleniumdioxide dissolves readily in water, forming selenious aqueous solution may be used in the following experi-ments. 8. Action with hydrogen sulphide. — Hydrogen sulphidesolution, when added to a solution of selenious acid, pro-duces a bright yellow precipitate of selenium sulphide. 9. Reduction by sulphurous acid. —
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