CHAPTER XXXI ALUMINIUM 188. 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 ? 189. 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 of a salt of aluminium. Does a precipitate form? How could you test for zinc and aluminium in the presence of each other? How test for magnesium and aluminium? b. Obtain about 1 g. of some dry compound containing aluminium, and heat it on charcoal with the blowpipe (R). Moisten the residue with a drop or two of a solution of cobalt nitrate and heat it once more. Note the color of the residue. This constitutes a test for compounds containing aluminium. c. Repeat the experiment, substituting a compound of zinc for that of aluminium. The green product is called Rinmann's green. d. Mix a little dry alum and sodium bicarbonate and rub them together. Pour a little water on the mixture and note the result. 190. Aluminium nitride. Mix thoroughly 10 g. of fine aluminium powder with 1 g. of lampblack. Place the mixture in the form of a cone on a brick or iron plate. In the top of the cone introduce a piece of magnesium ribbon about 5 cm. long. Now light the magnesium ribbon. The combustion progresses safely and without explosion. When the mass is cooled, note the crystals of aluminium nitride mixed with crystals of aluminium oxide. Place some of the product in a test tube and cover it with a solution of caustic soda, heat gently, and note odor of gas evolved (?). What possible use does this suggest for aluminium? 191. Double salts. a. Alums. What is a double salt? What is an alum? Calculate the weight of aluminium sulfate (remember that it is a hydrated salt) and of ammonium sulfate required for the preparation of 25 g. of crystallized alum. Dissolve these separately in hot water so that the combined volume will be about 75 cc. Unite the hot solutions and set the product aside to crystallize. Can you make out the form of the crystals? Test the reaction to litmus paper of a solution of a few pure crystals. b. Carnallite. What is the formula of carnallite? Calculate the weight of the individual salts necessary to make 25 g. |