CHAPTER XXX

THE MAGNESIUM FAMILY

181. Magnesium carbonate. Place 2 or 3 g. of magnesium carbonate (preferably the mineral magnesite) in a hard-glass tube fitted with a delivery tube, and gently heat in the Bunsen flame, passing the gas evolved through clear limewater. What would you judge as to the ease of decomposition of magnesium carbonate in comparison with that of calcium carbonate? Increase the heat, and when the evolution of gas becomes slow, cool the solid product and shake it with water, testing the reaction toward litmus paper.

182. Magnesium chloride. a. What is the formula of crystallized magnesium chloride? Place a little of the solid in a test tube and heat it gently. Is water given off easily? With blue litmus paper keep testing the drops which condense on the sides of the tube. How do you account for the reaction (R)? After most of the moisture is driven off, add water to the residue. Is it soluble? What is it? What is the industrial importance of this fact?

b. Add a few drops of ammonium hydroxide to a solution of magnesium chloride (R). Repeat the experiment, first diluting the magnesium chloride with an equal volume of a solution of ammonium chloride. Does a precipitate form? Since there are many magnesium ions present, what other ion must have largely disappeared? Can you account for this by mass action? To the solution add disodium phosphate. The precipitate has the formula MgNH,PO (R).

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183. Weight of magnesium sulfate obtained from a known weight of magnesium oxide. Weigh accurately a small evaporating dish; then introduce from 0.3 to 0.5 g. of magnesium

oxide and accurately weigh the dish and contents. Add a little more than enough dilute sulfuric acid to dissolve the oxide (1 part acid to 3 parts water), and carefully evaporate to dryness, completing the operation under the hood. Moisten the powder with 1 or 2 drops of the acid and again evaporate, finally heating the product to a low red heat. Allow the crucible to cool, and weigh accurately the resulting magnesium sulfate. Compare your results with the theoretical results.

184. Zinc. a. Place a small piece of zinc on charcoal and heat it in the oxidizing-flame of the blowpipe (R). What is the color of the product while hot? Does its color change on cooling?

b. Try the solubility of zinc oxide in sodium hydroxide. Could a film of oxide remain on a piece of zinc in a solution of this reagent? If zinc is perfectly clean, how would you expect it to act with water? (See electromotive series.) Should zinc be soluble in a solution of sodium hydroxide? Try it.

c. Repeat § 182, b, substituting zinc sulfate for magnesium chloride.

185. Combining weight of zinc. Repeat § 183, substituting for the magnesium oxide about 1 g. of pure zinc, accurately weighed, and for the sulfuric acid, dilute nitric acid. What is the product first formed? What is obtained on heating to a low red heat? From the values obtained, calculate the combining weight of zinc referred to oxygen. How does this compare with the combining weight of zinc referred to hydrogen as unity as determined in § 85?

186. Cadmium. Obtain about 5 cc. of a solution of a salt of cadmium, and add to it a little dilute hydrochloric acid. Then pass hydrogen sulfide into the solution or add a solution of the reagent to it (R). What is the color of the precipitate? Is it soluble in dilute hydrochloric acid? Test its solubility in concentrated hydrochloric acid (?). For what is it used?

187. Analytical reactions. Pour into separate test tubes a solution of a compound of magnesium, zinc, and cadmium. Add a few drops of hydrochloric acid to each solution and then pass in hydrogen sulfide. Note the result (R). What would you infer as to the solubility of the sulfides of these metals in dilute acids? Add ammonium sulfide to separate solutions of compounds of magnesium and zinc. Repeat, adding an equal volume of ammonium chloride solution before adding the ammonium sulfide. Explain (R). How could you detect the three elements in the presence of each other?