Note the change in the colour and appearance of the flame at a, and the garlic-like odour produced; the arsenious hydride is being burnt to oxide and water, thus

Bring a porcelain crucible-lid over the flame; the arsenious oxide is deposited on the lid as a white, nearly invisible, film.

Now bring a porcelain crucible-lid into the flame as shewn in fig. 28. The supply of oxygen is thus limited, and at the same time the flame is cooled; the change which now proceeds is chiefly this ;-2ASH, +30= 2As + 3H,0. The arsenic is deposited on the cool surface of the lid.

Now heat the tube through which the mixture of hydrogen and arsenious hydride is passing at about the point c; a shining metal-like deposit of arsenic is slowly formed nearer the open end of the tube than the heated part. This deposit is soluble in a solution of bleaching powder. The gas AsH, is decomposed by heat into its elements; the arsenic is deposited on the cold parts of the tube, and the hydrogen passes on.

Turn the tube c so that the open end points downwards; let this end dip into an aqueous solution of silver nitrate (AgNO3) in a test tube; after the gas has passed through this solution for a little, remove the tube, and pour the liquid through a filter; to the filtrate add one or two drops of dilute ammonia solution; a pale yellow pp. of silver arsenite is produced; add a little more ammonia, the pp. dissolves. Wash the black solid on the filter several times with hot water, then dissolve it in warm nitric acid and prove that the solution contains silver. The interaction which has occurred is represented thus;-12AgNO,Aq+xAgNO2Aq+2AsH2+3H2O=

xAgNO,Aq+As2O,Aq+12HNO2Aq+12Ag; the silver nitrate is reduced by the arsenious hydride. The addition of ammonia after filtering neutralises the nitric acid formed in the reaction, the arsenious oxide then interacts with the silver nitrate remaining in solution to produce silver arsenite; thus,

6AgNO,Aq+ As,O,Aq+3H,O= 2AgAsO+6HNO,Aq.

Exp. 8. Arrange another apparatus as shewn in fig. 29, and conduct a series of experiments as described under Exp. 7, but use a solution of an antimony compound (an aqueous solution of tartar emetic KSbC,H,O, will do well) in place of arsenious oxide.

Note (1) the colour of the flame of the mixture of hydrogen and antimony hydride (SbH);

(2) the appearance of the deposit of antimony on the porcelain lid;

(3) the position and appearance of the deposit of antimony on the tube, and its insolubility in bleaching powder solution;

(4) the fact that after passing the gas through silver nitrate solution, and filtering, no antimony remains in solution ;-prove this by adding hydrochloric acid to the filtrate, filtering off the pp. of silver chloride so produced, and passing a little sulphuretted hydrogen into the filtrate; no pp. is formed; had antimony been present orange-red Sb,S, would have been precipitated. The interaction between antimony hydride and silver nitrate in solution may be represented thus ;

3AgNO,Aq+xAgNO„Aq + SbH, = Ag2Sb +3HNO„Aq+

xAgNO,Aq.

Exps. 1 to 8 exhibit the methods of preparation and some of the important properties of the hydrides MH, when M = N, P, As, Sb.

The five elements, nitrogen, phosphorus, arsenic, antimony, and bismuth form each more than one oxide; we shall prepare some of the oxides having the composition M2O, and examine their properties.

The oxide N2O, is obtained by partially deoxidising nitric acid (HNO3).

Exp. 9. Place about 5 grams arsenious oxide in a test

Fig. 30.

C

tube arranged with a cork and exit tube passing into a U tube as shewn in fig. 30. Add about 20 c.c. of a mixture of equal volumes concentrated nitric acid and water. Surround the U tube with a mixture of ice and salt and gently heat the test tube. A blue-coloured liquid collects in the U tube; this is impure N,O,:

2HNO, Aq + 2H ̧0+ As2O2 = 2H ̧AsО ̧Aq + N2O ̧.

Put about 20 c.c. of water in a beaker surrounded by ice, and after a little time add 2 or 3 c.c. of the blue liquid from the U tube. Now prove (1) that the solution has an acid reaction to litmus paper, (2) that it contains nitrous acid.

(N2O2+ H2O=2HNO,Aq).

To test for nitrous acid or nitrites, add a few drops of an aqueous solution of potassium iodide (KI) and a very little starch paste (prepared by boiling a very small quantity of starch with water and allowing to cool) to the acidulated liquid; a blue colour, due to "iodide of starch," is produced. The reaction is 2KNO,Aq+2KIAq + 4HCl = 4KCIAq + 2H2O + 2NO + I ̧.

Exp. 10. The oxide P2O, is prepared by oxidising phosphorus in a limited supply of air. This oxide interacts with water to form a solution of phosphorous acid, H,PO,Aq. The apparatus represented in fig. 31 consists of a piece of

hard glass tubing, a, narrowed at one end b, and fitted at the other end with a cork carrying a tube which passes into a dry two-necked bottle, c, this bottle communicates with a large bottle or jar, d, as shewn in the figure; the tube e acts as a syphon, it is furnished with a stopcock f.

Fill the bottle d with water; arrange the apparatus as shewn; then cut three or four small pieces of phosphorus from the middle of a stick of phosphorus and place them in the tube a. Partly open the stopcock f; a slow current of air is sucked through the entire apparatus. Now gently heat the phosphorus; it is burnt in the limited air-supply; the chief product of the burning is P,O,, which collects, as a white solid, on the cool parts of a and in the bottle c. Stop the process before the whole of the phosphorus is burnt; prove that the white solid obtained dissolves in water and that this solution contains an acid.

2

The

The oxides As,O,, Sb,O,, and Bi̟,O,, may be prepared by burning the elements As, Sb, or Bi, in air or oxygen. oxides As,O, and Sb,O, are slightly acidic, and Sb., is also feebly basic; Bi,O, is basic.

3

2

3

Exp. 11. Prove (1) that arsenious oxide (As,O,) is very slightly soluble in water and that the solution scarcely affects the colour of blue litmus paper; (2) that antimonious oxide (Sb,O) is almost insoluble in water; (3) that bismuthous oxide (BiO) is quite insoluble in water. Dissolve a little Sb,O and Bi,O,, separately, in concentrated sulphuric acid, evaporate, and allow to crystallise; in each case crystals are obtained; these crystals have the composition M.3SO, where M=Sb or Bi.

2

To a little antimonious oxide add some concentrated nitric acid, and heat; the oxide does not dissolve; the white solid which remains is a mixture of Sb., and Sb,O,.

To a little bismuthous oxide add some concentrated nitric acid, and heat; the oxide dissolves; evaporate the solution, and allow to crystallise. The crystals which separate are bismuth nitrate, Bi3NO..

3.

=

The oxides M,O,, when M = N, P, As, or Sb, are acidic oxides when M Bi the oxide is a peroxide possessed of exceedingly feeble acidic functions; this oxide interacts with acids to form the same salts as are obtained from Bi2O, NO, is prepared by removing water from HNO; PO by burning P in excess of air or oxygen; As,O, and SbO, by

5

5

5

2

5

2

evolving oxygen in contact with the elements, or the oxides M,O,, by the decomposition of concentrated nitric acid; and Bi O, by evolving oxygen in contact with Bi,O, in presence of much potash or soda.

Exp. 12. Burn a small piece of phosphorus in a large bottle full of air: dissolve the white solid which is formed (PO) in water and prove that the solution turns blue litmus red. (s. also Exps. 15 and 16.)

To a small quantity of arsenious oxide add some concentrated nitric acid, heat for some time, then pour off the remaining acid, add more concentrated nitric acid and heat; then evaporate in the draught place just to dryness, and raise to a low red heat. Collect the As,O, which remains and then dissolve it in water; prove that this solution contains an acid, and that this is arsenic acid.

5

To test for arsenic acid (H,ASO,) add silver nitrate, when a red pp. of silver arsenate, Ag, AsO, forms; this pp. is soluble in hot concentrated nitric acid, and is reprecipitated on concentration and cooling.

Arrange an apparatus as shewn in fig. 32. Into the flask

a

d

B

Fig. 32.

a put several lumps of manganese peroxide (MnO,); put dilute potash solution in the bottle b; the tube c is wider than the delivery tube from a; d is a tube about 10 mm. wide at the