Page images
PDF
EPUB

moderately strong alcohol. Repeat the treatment in the case of the crystals obtained by using iodine until a fresh quantity of alcohol dissolves only a very little of the solid; then dry the residue, label it KIO,, and set aside.

Repeat the treatment with cold alcohol two or three times in the case of the crystals obtained by using bromine; then evaporate the alcoholic liquid on a water bath to about half its bulk; allow to cool; collect and dry the crystals which form, label them KBrO,, and set aside.

Examine the appearance of the three salts you have prepared; prove that each is soluble in a large quantity of cold water; that each is decomposed by heating in a dry tube, with evolution of oxygen, and production of a white residue which is potassium chloride, bromide, or iodide, according as it is obtained from potassium chlorate, bromate, or iodate.

The decomposition of the chlorate &c. by heat, is represented thus; KXO1 = KX + 30, where X = Cl, Br, I.

3

The three elements chlorine, bromine, and iodine evidently exhibit similar chemical properties; they form similar compounds under similar conditions.

Reference to "ELEMENTARY CHEMISTRY" Chap. XI, pars. 149-159.

CHAPTER XIII.

CLASSIFICATION OF ELEMENTS (continued).

THE elements magnesium, zinc, and cadmium are placed in the same class.

Exp. 1. Observe the prominent physical properties of the three elements; their hardness, malleability, lustre, and colour. Prove that each is a good conductor of electricity by placing each in a circuit including an electric bell, as shewn in fig. 24.

Fig. 24.

The ringing of the bell proves that the current is passing through the magnesium, zinc, or cadmium.

M. P. C.

5

еееееееее

Exp. 2. Pass an electric current through aqueous solutions of (1) magnesium sulphate, (2) cadmium sulphate, (3) zinc sulphate, to each of which you have added about volume of concentrated sulphuric acid.

or

of its

The arrangement of the experiment is shewn in fig. 25. Notice that a greyish white solid forms on the negative electrode in each case; these deposits are (1) magnesium, (2) cadmium, (3) zinc. Compare the result with that obtained in the electrolysis of hydrochloric acid (Exp. 15, Chap. XII.).

[blocks in formation]

Exp. 3. Place the end of a piece of magnesium ribbon or wire in the flame of a Bunsen-lamp; the magnesium is rapidly burnt to magnesia with production of brilliant white light. Heat some zinc in an iron spoon over a blowpipe until it is quite melted; then direct a fairly rapid stream of oxygen from a gasholder over the molten metal, keeping the spoon in the blowpipe flame. The zinc is burnt to zinc oxide. Repeat this experiment with cadmium in place of zinc; the cadmium is burnt to cadmium oxide. The reaction in each case is;M+O= MO where M = Mg, Zn, or Cd.

Exp. 4. Shake up a little magnesium oxide, zinc oxide, and cadmium oxide, with water for a few minutes; then divide each into two parts and add to these red and blue litmus, respectively. The red litmus is turned blue, but not deep blue, in the case of the magnesium oxide; the other oxides do not change the colour of either the red or the blue litmus.

Magnesium oxide is then slightly alkaline.

Add some of each oxide to a little warm sulphuric acid, continuing to add the oxide until some remains undissolved. Filter, and evaporate each filtrate to the crystallising point; Collect each crop of crystals, dry them, and purify them by recrystallisation from water. Prove that the crystals obtained by using magnesium oxide are (1) a compound of magnesium, (2) a sulphate; that the crystals obtained by using zinc oxide are (1) a compound of zinc, (2) a sulphate; and that the crystals obtained by using cadmium oxide are (1) a compound of cadmium, (2) a sulphate.

The production of a yellow pp. of cadmium sulphide (CDS), insoluble in warm ammonium sulphide, when sulphuretted

hydrogen is passed into an acidulated solution, is a test for the presence of cadmium in that solution.

The three oxides experimented with are therefore basic oxides; they interact with acids to produce salts. The change which occurred in the formation of the sulphates of magnesium, zinc, and cadmium, is represented thus;

MO+ H2SO1Aq = MSO1Aq + H2O, where M = Mg, Zn, or Cd. The crystals of the three sulphates have the compositions MSO..7H2O when M = Mg or Zn, and 3CdSO,.8H ̧0.

Exp. 5. To aqueous solutions of magnesium sulphate, zinc sulphate, and cadmium sulphate, add caustic potash; collect the white pps. which are produced, on filters, and wash each with cold water until the washings are neutral to litmus and free from sulphates. Dry the pps. at 100°. These pps. are hydroxides having the composition MO2H, where M = Mg, Zn, or Cd. Prove that zinc hydroxide is soluble in excess of potash, but the others are insoluble. Place a part of each dried pp. in a large, clean, dry, test tube, and heat the lower part of the tube. Water is given off in each case. The hydroxides are changed to oxides and water; thus, MO2H2 = MO + H2O.

Exp. 6. Repeat last exp. but use sodium carbonate in place of caustic potash; pps. of magnesium, zinc, and cadmium, carbonates are produced, with evolution of carbon dioxide in each case; boil the liquids holding the pps. in suspension for some time; then filter, wash the pps. with hot water until the washings are free from sulphates and carbonates, and dry the pps. at 100o. You thus obtain the carbonates MCO, combined with more or less of the hydroxides MO2H2, where M = Mg, Zn, or Cd. Place a small quantity of each carbonate in a crucible and heat strongly over a blowpipe flame until a very little of the solid taken out and added to a few drops of dilute hydrochloric acid in a test tube ceases to evolve any gas as it dissolves in the acid. The residues are oxides MO: the change which has occurred is MCO2 = MO + CO2.

These experiments afford reasons for placing the three elements magnesium, zinc, and cadmium in the same class ; they also shew that there are slight differences in the chemical behaviours of the three elements under similar conditions.

Reference to "ELEMENTARY CHEMISTRY." Chap. XIX.

CHAPTER XIV.

CLASSIFICATION OF ELEMENTS (continued).

THE elements nitrogen, phosphorus, arsenic, antimony, and bismuth are placed in the same class.

Compare the prominent physical properties of the five elements.

Nitrogen: a colourless, odourless, gas.

Phosphorus: a soft, waxy, crystalline, solid; without metallic lustre; low spec. gravity.

Arsenic and antimony: lustrous, grey, brittle, crystalline, solids.

Bismuth heavy, brittle, lustrous, crystalline, solid. Nitrogen, phosphorus, arsenic, and antimony form compounds with hydrogen, MH,.

Exp. 1. Arrange an apparatus as shewn in fig. 26. A is a dry flask containing a mixture of about 20 grams powdered ammonium chloride (NHCl) with an equal weight of lime (CaO); this mixture is covered with a layer of lime; B is a tower containing loosely packed pieces of lime; c is a dry bell-jar. Have ready another dry jar, and a large dry flask to which you have fitted a good caoutchouc cork carrying a piece of rather wide glass tube narrowed at the end which goes into the flask, with a caoutchouc tube attached and a pinchcock on the caoutchouc tube (Fig. 27). Heat the flask A on a sand-tray; ammonia, NH,, comes off, is dried by passing through the lime in B, and collects in the jar c. The reaction is 2NHCl + CaO = 2NH ̧ + CaCl, + H2O; the layer of lime absorbs the water produced and prevents it from trickling down and so probably breaking the flask.

« PreviousContinue »