weighing the dish. 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. What is the product? Assuming the atomic weights for oxygen and sulfur found in the table of atomic weights, what will your results require the weight of magnesium to be? 175. 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 § 173, b, substituting zinc sulfate for magnesium chloride. 176. Oxygen equivalent of zinc. Repeat § 174, 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 equivalent weight of zinc referred to oxygen. How does this compare with the hydrogen equivalent determined in § 79? What is the difference between the oxygen equivalent and the hydrogen equivalent? 177. 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? in more concentrated acid? For what is it used? 178. 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? CHAPTER XXIII ALUMINIUM 179. Aluminium and its hydroxide. a. Dissolve a small piece of aluminium in hydrochloric acid. Where must the metal stand in reference to hydrogen in the electromotive series? Add sodium hydroxide, a drop at a time, until a precipitate is produced (R). Continue the addition with frequent stirring (R). When solution has been effected, add hydrochloric acid, a drop at a time (R). By what name would you designate a hydroxide with such properties? b. Try the action of aluminium on boiling water. From the place of the metal in the electromotive series would you expect it to decompose water? Now try the action of the metal on a solution of sodium hydroxide (R). Does the fact that the hydroxide is soluble in alkalies suggest a reason, for the fact that, while the metal is not acted upon by water, it dissolves in alkalies? Polish the surface of a piece of aluminium and dip it into a solution of mercuric chloride. What becomes of the mercury (electromotive series)? The mercury keeps the hydroxide from sticking to the aluminium. Does water now attack the metal? c. Try the action of a solution of ammonium hydroxide on a solution of aluminium chloride (R). Is the hydroxide dissolved by an excess of ammonium hydroxide ? 180. Aluminium salts. a. To a solution of aluminium sulfate (or any other soluble salt of the metal) add a solution of sodium carbonate. What gas is evolved? What solid is precipitated? How could you prove it is not a carbonate? Repeat the experiment, using ammonium sulfide in place of sodium carbonate (R). Pass hydrogen sulfide into a solution |