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HIGH-SCHOOL CHEMISTRY
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Compounds of Lead A Project CHARLES H. STONE
I44 Wall Street, Orlonrlo, Florida
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HEN the student is directed to treat a solid compound with an acid, he may occasionally encounter a pretty problem. Dilute acids contain much water. Action between the acid and the solid can take place only a t the points of contact-that is, the surface of the solid. If the product of the reaction is soluble in water it is continually removed, leaving the surface of the solid exposed to the continued action of the acid. If, on the other hand, the reaction product is not soluble in water, then it remains in situ, and a gradually thickening product is formed over the entire surface of the solid which, presently, becomes thick enough to shut off all action of the acid upon the solid. This general condition is not often stressed sufficiently in our texts but is easily demonstrated. To three test tubes, each containing 2 to 3 g. of marble chips, are added, in order, dilute sulfuric, nitric, and hydrochloric acids. The action starts off briskly enough in all three tubes; it continues unabated in the last two tubes since the nitrate and chloride of calcium are soluble. In the first tube the action soon slows down and presently stops, since calcium sulfate soon reaches its limit of solubility. The above applies in what follows. The compounds of lead are of interest since they display a variety of colors and, in one instance, a different crystal form than has previously been produced in the ~ u ~ i lwork. 's A
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PROCEDURE
In a clean, dry iron mortar one powders 50 g. of good quality cemssite (PbCOd ore, or more if the ore is of low quality. The powder is transferred to a beaker or evaporating dish and treated with small portions of warm nitric acid, stirring until it appears that the reaction is ended. The pupil may be asked why hydrochloric or sulfuric acid may not be used. Water is added to the material and heat is applied to expel carbon dioxide. Filtration follows; the residue on the filter is wasbed to remove all trace of soluble material and is then discarded. The clear filtrate is now to be divided into several portions to be treated, respectively, with such reagents as solutions of potassium iodide, potassium chromate, sodium oxalate, carbonate, sulfate, and others to produce a series of insoluble lead compounds. These are filtered and washed, remembering that the iodide of lead is rather soluble in hot water. Each ~ r o d u c tis drained, dried, powdered, and Preserved. An evaporating dish containing 5 to 10 g. of lead
carbonate is heated moderately until the change is complete and lead oxide remains. Here i t may be well to remind the student that this is a general reaction, for many metal carbonates act similarly when heated. When lead oxide is placed on a thin iron plate and heated moderately to dull redness, the powder being often turned over with a spatula, red lead or minium results. This substance has a peculiar composition: two parts of lead oxide and one part of lead (PbO.PbO. PbOz). The lead oxide is acted upon readily by nitric acid; the lead dioxide is unaffected. Ten grams or more of the red lead are treated in an evaporating disb with warm dilute nitric acid with stirring until the last trace of red disappears and only a brown mud remains. Hot water is added and the whole is transferred to a filter. The brown residue is washed first with a little warm nitric acid to remove any remaining minium, and then with hot water. The brown product is dried and preserved. This is the substance used on the anode plates of storage batteries. The filtrate may be set aside in a crystallizing disb for some days until six-sided crystals form with sides alternately long and short. To 50 cc. of water in a beaker add about a gram of powdered lead iodide and heat to boiling. Meanwhile warm a filter standing in a graduate or vertically supported larce test tube; this is to prevent too rapid cooling o f t h e liquid later tg mn through the filter. Pour the hot solution of lead iodide into the filter and let stand. As the liouid cools. brilliant s.~ a n d e of s lead iodide willappear. ' A lead tree, arbor suturnii, is easily prepared. Dissolve a gram of powdered lead acetate or nitrate in 25 cc. of normal acetic acid (60 g. per liter). Add this to 25 cc. of water glass of sp. gr. LOG and stir well; pour into a 50-cc. graduate or large test tube and let stand overnight. A thick gel forms. Now press into the top of the gel a clean piece of zinc and stopper tightly. The zinc gradually displaces the lead, forming a beautiful lead treewhich is quite insensitive to shock since it is supported by the gel. A, iron nail as and a piece of clean lead as anode may be inserted in a solution of lead acetate and a direct current passed, with formation of spongy lead on the cathode, l-here are various other lead compounds, lead tetra&loride, lead tetraethyl (used in gasoline to prevent
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COMPOUNDS OF LEAD knocking), for example, but these are less easily prepared. Work of the above character may be assigned to the occasional able student as a pleasing variation from the usual work of the laboratory. The experimental
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work called for is simple, interesting, and absolutely devoid of danger. The student may prepare an exhibit of his work and attach his snapshot picture, forming a display of interest to his fellow students and to parentsandfriends.
