of iron you have prepared in dilute sulphuric acid, and heat; add a little concentrated nitric acid to the hot liquid, a few drops at a time, till the colour is reddish-yellow; now evaporate to dryness over a low flame. Collect the yellowishwhite solid that remains; free it from adhering liquid by repeated pressure between porous paper; prove (with a small quantity) that it is (1) a compound of iron (use the borax-bead test), (2) a sulphate.

[The action of nitric acid was to supply oxygen; when concentrated nitric acid is heated it decomposes, giving oxygen, water, and oxides of nitrogen (2HNO1 = H2O + 2NO, + O).]

Dissolve the remainder of this sulphate of iron in dilute sulphuric acid; warm the liquid; add some iron filings; warm for some time; if all the iron dissolves add more iron, and continue to warm until the liquid is near its crystallising point; then filter; as the filtrate cools it deposits crystals of the green sulphate of iron.*

There are therefore two sulphates of iron; one is composed of more acid radicle relatively to metal than the other.

Exp. 3. You are asked to determine whether, when zinc and sulphuric acid react, more than one zinc sulphate is produced. The details given in Exp. 2 will be a sufficient guide to indicate how you ought to proceed.

Note that the only conclusive proof of the production or non-production of more than one sulphate would be quantitative analyses of the solids produced by the interaction of zinc with sulphuric acid under varying conditions.

You can prove by qualitative, and very roughly quantitative, experiments that whether much or little acid is used, the product is a white crystalline zinc sulphate the aqueous solution of which is neutral to litmus. Hence it is probable that only one sulphate of zinc is produced by the interaction of zinc and sulphuric acid.

Exp. 4. You are given solutions of (1) stannous chloride, SnCl,; (2) stannic chloride, SnCl.

To a portion of each solution add mercuric chloride solution and warm; a dark grey pp. (finely divided mercury) is produced in the solution of stannous, but no pp. in the solution of stannic, chloride.

gas

Into a portion of each solution pass sulphuretted hydrogen and warm; a brown pp. (stannous sulphide SnS) is formed

* See Chapter XVII. on Reduction.

in one solution, and a yellow pp. (stannic sulphide SnS,) in the other solution.

Into a portion of the solution of stannous chloride pass chlorine gas until the liquid is yellow; warm for some time, and again pass in chlorine; warm the liquid until the smell of chlorine is removed. Now prove that the solution gives (a) no pp. with mercuric chloride, (b) a yellow pp. with sulphuretted hydrogen.

You have converted stannous chloride into stannic chloride, by combining the former with chlorine.

Boil a portion of the solution of stannic chloride with copper turnings for 5 or 10 minutes; divide the liquid into three parts; to one part add mercuric chloride; into another part pass sulphuretted hydrogen gas; from the results obtained you conclude that stannous chloride was present in the liquid. To the third part add ammonia solution until the liquid smells strongly of ammonia; a white gelatinous pp. is formed, and the liquid is coloured azure blue :—the pp. is stannous hydroxide (Sn,O,H,), the blue colour shews the presence of a compound of copper.

Hence you conclude that stannic chloride has reacted with copper to form stannous chloride, and that some of the copper at the same time has dissolved. As you know that the compositions of stannous and stannic chlorides are SnCl, and SnCl,, respectively, you conclude that, probably, the chlorine removed from the stannic chloride by the reaction with copper has combined with copper, and that the copper chloride thus formed has dissolved in the water present. Accurate quantitative investigation confirms this supposition, and shews that the reaction which occurs between a solution of stannic chloride and copper may be thus represented in an equation ;—

SnCl Aq+ Cu = CuCl Aq+SnCl Aq.

The relations between the compositions of these two chlorides of tin and the acid from which both are derived, are similar to the relations between the compositions of the two sulphates of iron and the acid from which they are both derived. Each is a compound of a metal with an acid radicle; in the case of the iron salts this acid radicle is itself composed of two elements (sulphur and oxygen), in the case of the tin salts the acid radicle is a single element (chlorine). The salt whose name ends in ic is a compound of more of the acid radicle, relatively to a fixed mass of the metal, than the salt whose name ends

in ous. The -ous salt was changed to the -ic salt by combining with it more of the acid radicle; the ic salt was changed to the -ous salt, in one case by combining with it more of the metal, in the other case by removing part of the acid radicle.

2

A salt which is formed from acids by replacing the whole of the replaceable hydrogen by a metal is usually called a normal salt; a salt which is formed from an acid by replacing a portion of the replaceable hydrogen by a metal is usually called an acid salt. The salts FeSO4, Fe,(SO4), SnCl, SnCl, K.SO, K,C,O,, K,CO,, are normal salts. The salts KHC,O, KHSO, KĤCO, are acid salts. It is to be noted that most acid salts shew an acid reaction towards litmus, but that some acid salts (as defined above) do not exhibit an acid reaction some towards litmus. Most normal salts are neutral to litmus however are acid and some are alkaline. (s. Exp. 1 of this Chap.) Many normal salts may be formed by the interaction of an acidic with a basic oxide.

;

Exp. 5. Pass a stream of dry carbon dioxide over some warm powdered lime (calcium oxide) in a glass tube (Fig. 18). After a time stop the current of the gas, remove the white

solid from the tube and prove that it is (1) a carbonate, (2) a compound of calcium.

Calcium compounds when volatilised in a non-luminous flame give a red colour to the flame. Solutions of calcium compounds neutralised with ammonia, give a white pp. (calcium oxalate) on addition of a solution of ammonium oxalate, which pp. is insoluble in acetic acid.

The chemical change which you have conducted is represented in an equation thus, CaO + CO2 = CaCO ̧ Calcium carbonate is a salt, calcium oxide is a basic oxide, and carbon dioxide is an acidic oxide.

Exp. 6. Melt some solid potassium oxide in a silver (or nickel) basin; add a little powdered manganese dioxide, and continue to heat for some minutes; then allow to cool and dissolve the greenish solid in the basin in cold water. A deep green solution is obtained. This solution contains the salt potassium manganate (KMnO4). The reaction which occurred between the potassium oxide—a basic oxide—and the manganese dioxide--an acidic oxide is thus represented in an equation;

K2O + 3MnO2 + O (from the air) = KMnO4 + Mn ̧O̟ ̧ + H2O.

2 3

Exp. 7. Potassium oxide is a basic oxide; chromium trioxide is an acidic oxide. Heat together, in a crucible, potassium oxide and chromium trioxide (CrO); when the whole mass has been molten for a few minutes allow to cool, and dissolve in water; evaporate the yellow solution to the crystallising point; cool; collect and purify the yellow crystals which form. Set these crystals aside; call them A.

To a solution in water of chromium trioxide add a solution in water of potassium oxide till the liquid is as nearly as possible neutral; evaporate, collect and purify the yellow crystals which form; call these crystals B.

Prove that A and B are both (1) compounds of potassium, (2) chromates.

The production of a red pp. of silver chromate (Ag2CrO4), soluble in hot concentrated nitric acid, when a solution of silver nitrate is added to an aqueous solution of a salt, is a test for a chromate.

As far as your experiments indicate, A and B are the same compound. Quantitative analysis establishes this conclusion. The composition of A and B is represented by the formula K2CrO4: the reactions by which you have prepared this salt may be thus represented:

(1) K,0 + CrO3 = K2CrO4.

(2) 2KOH *Aq + H2CrO, *Aq = K2CrО ̧Aq + 2H2O.

An aqueous solution of K2O contains the alkali KOH; an aqueous solution of CrO, contains the acid HCrO4.

Many normal salts may then be regarded as, (1) metallic derivatives of acids, (2) compounds of basic with acidic oxides. Some salts are formed by the combination of basic oxides, or hydroxides, with normal salts: such salts are usually called basic salts.

Exp. 8. The compound bismuth chloride (BiCl ̧) is a normal salt; bismuth oxide (Bi̟O̟,) is a basic oxide. Melt some bismuth chloride in a basin over a Bunsen-lamp, and add little by little some powdered bismuth oxide. The white solid thus formed is bismuth oxychloride; its composition is expressed as that of a compound of the normal salt bismuth chloride with the basic bismuth oxide;-BiCl,. Bi,O,. This compound is a basic salt.

Exp. 9. Lead acetate [Pb(C,H2O2)1⁄2] is a normal salt; lead oxide (PbO) is a basic oxide. Dissolve about 10 grams lead acetate in about 50 c.c. water; boil; add about 7 grams lead oxide; keep boiling till the greater part of the lead oxide has dissolved; then filter, and allow filtrate to cool. The basic salt Pb(C,H2O2)2. PbO is deposited as the liquid cools.

In this chapter you have learned more concerning the relations between the composition of salts, and the composition, on the one hand, of the acids from which salts are derived, and on the other hand, of the oxides by the combination of which salts are frequently produced.

You have gained some knowledge of the meaning of the terms used in the classification of salts, normal, acid, basic salts. You have found that some metals form two series of salts by interacting with one and the same acid; and that, in these cases, the ratio of acid radicle to metal is different in the two series of salts. You have also learned how to pass from one of these series to the other, in the cases of some iron and tin salts.

Incidentally you have performed tests by which the presence in liquids of compounds of (1) iron, (2) copper, (3) calcium, and (4) the presence of a chromate, may be proved.

Reference to "ELEMENTARY CHEMISTRY." Chaps. IX. and XI.