case a red pp. of silver chromate, Ag,CrO4, and in the other a yellow pp. of lead chromate, PbCrO4, is obtained.

To obtain these salts of chromic acid (HCrO) from the basic oxide Cr2O, it is only necessary to oxidise this compound in presence of a strongly basic oxide or hydroxide.

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Exp. 11. Melt some caustic potash in a crucible over a Bunsen-lamp; add a few crystals of potassium nitrate (KNO2), and then a little Cr2O. The KNO, is decomposed, giving off oxygen which oxidises the Cr2O, to 2CrO, and this reacts with the molten KOH to form K,CrO,. On cooling a yellow mass of K,CrO, (mixed with unchanged KOH) is obtained.

The production of yellow potassium chromate, K,CrO4, in this way is an extremely good test for the detection of chromium in salts of this metal.

Besides the chromates there is known another series of salts called dichromates.

Exp. 12. To a saturated solution of potassium chromate add dilute sulphuric acid so long as the colour of the liquid changes; then evaporate and allow to crystallise. Red-yellow crystals of potassium dichromate, K,Cr,O,, separate out. (2K,CrO1Aq + H2SO1Aq = H2O + K„Cr2O„Aq + K2SO1Aq.)

In order to convert a chromate or dichromate into a salt of chromium it is necessary to heat with excess of a concentrated acid, or to treat with a reducing agent*.

Exp. 13. To a concentrated solution of potassium chromate or dichromate, add a considerable excess of concentrated sulphuric acid, and evaporate the solution until the colour becomes deep green. Now prove that this solution gives the ordinary reactions of a chromic salt; (1) potash forms a greenish pp. of Cr20.3H0, soluble in considerable excess of the precipitant, [compare this reaction with that between KOHAq and CrO Aq (Exp. 10)]; (2) ammonia precipitates the same Cr2O.3H0, it is only slightly soluble in excess of ammonia. The changes carried out in this Exp. may be thus represented in equations;

2K,CrО ̧Aq + 5H2SO1=2K„SO1Aq + Cr2(SO1) ̧Aq + 5H2O + 30: K,Cr,O,Aq+4H2SO4 = K2SO,Aq + Cr2 (SO,),Aq + 4H2O + 30. A mixture of K,CrO, or K,Cr2O, and H2SO, will evidently act as an oxidising agent.*

Exp. 14. To a solution of potassium chromate or dichromate add a little sulphuric acid, and then pass in sulphur dioxide (made by heating concentrated sulphuric acid with copper) until the liquid is green; and prove that the solution now gives the reactions of a chromic salt. The change which has occurred may be represented thus; 2K ̧CrO,Aq + 2H2SO1Aq + 3SO2 = 2K,SO,Aq+Cr,(SO), Aq + 2H ̧Ŏ.

Two series of salts, manganates e.g. K.MnO,, and permanganates e.g. KMnO,, may be obtained from the salts or the oxides of manganese. As MnO, is practically insoluble in water, these salts are obtained by fusing this oxide with strongly basic oxides or hydroxides in presence of oxygen or an oxidising agent. These salts do not correspond to the oxide MnO, but to a hypothetical oxide MnO,.

Exp. 15. Melt some potash in a crucible, add a very little nitre, and then a little manganese dioxide. Keep the whole melted for a little and then allow to cool. The green mass thus obtained contains potassium manganate, K,MnO1: the reaction may be thus represented in an equation; MnO, + 2KOH+O (from KNO)=K,MnO, + H2O.

The production of green potassium manganate, KMnO4, in this way is a delicate test for the presence of manganese in any compound.

When the fused mass is cold, dissolve it in cold water, and add a few drops of dilute sulphuric acid; the green colour changes to pink owing to the production of potassium permanganate.

Potassium permanganate interacts with sulphuric acid in the presence of an oxidisable body to produce potassium and manganese sulphates and oxygen; the oxygen combines with the oxidisable body.

Exp. 16. Make a solution of ferrous sulphate in cold water; add some sulphuric acid and heat; then add potassium permanganate solution (KMnO,Aq) drop by drop with constant stirring until the liquid is coloured very faintly pink. The pink colour is due to the presence of a minute trace of unchanged permanganate; the permanence of this colour shews that the chemical change which has been occurring in the liquid is completed. Now divide the liquid into two parts; to one add a few drops of potassium ferricyanide solution; the fact that no pp. is produced but only a slight brownish coloration shews that the liquid contains a ferric salt but is free from ferrous compounds.

But from the condition of the Exp. the only ferric salt that can be present is ferric sulphate. To the other part of the liquid add an excess of ammonia; the brown-red pp. which forms is a mixture of ferric and manganic hydrates; prove the presence of a manganese compound in this pp. by applying the fact that when these compounds are heated with molten potash and a very little nitre, green potassium manganate is produced (s. Exp. 15.) Exp. 2 taught us that manganic hydrate Mn,O,. H2O is pptd. when excess of ammonia is added to the solution of a manganese salt in presence of air and ammonium chloride; therefore the reaction with ammonia just concluded proves that the liquid obtained by adding the slightest possible excess of KMnO1Aq to FeSOAq in presence of H,SO Aq contained a salt of manganese; but from the condition of the Exp. this salt must almost certainly have been manganese sulphate (MnSO4).

We have therefore proved, as far as can be satisfactorily proved by qualitative experiments, that when potassium permanganate and ferrous sulphate interact in presence of sulphuric acid and water, ferric sulphate and manganese sulphate are produced. The quantitative study of the chemical change has shewn that it may be represented by the equation 10FeSO Aq + 8H2SO,Aq + 2KMnO ̧Aq = 5Fe„3SOAq + 2MnSO,Aq + K.SO1Aq + 8H2O.

Exp. 17. To a solution of oxalic acid (H,C,O,) add sulphuric acid; warm, and add potassium permanganate solution, drop by drop, until a very slight pink colour remains in the liquid; now prove that the solution contains a manganese salt. To another quantity of warm oxalic acid solution mixed with sulphuric acid in a test tube arranged with a cork, exit tube, and funnel tube as shewn in fig. 35, add some KMnO̟Aq and allow the escaping gas to pass into clear lime water; the production of a white pp. in the lime water proves that the gas evolved is carbon dioxide. The reaction which has occurred may be thus represented; 5H,C,O,Aq+3H2SO,Aq+2KMnO1Aq = K2SO1Aq + 2MnSO1Aq + 8H2O + 10CO,.

In Exps. 16 and 17, a salt of a manganese-containing acid (KMnO1) has been changed to a salt of manganese (MnSO4); the reverse change was accomplished in Exp. 15, taken in conjunction with Exp. 6. Compare the corresponding changes in the case of chromium compounds carried out in Exps. 11, 13, and 14.

The experiments performed in this chapter shew that the elements chromium manganese and iron are properly placed in

Fig. 35.

the same class; and that chromium and manganese are distinctly more negative and less metallic in their chemical reactions than iron.

Reference to "ELEMENTARY CHEMISTRY." Chap. XI. pars. 194-203.

CHAPTER XVI.

CONDITIONS WHICH MODIFY CHEMICAL CHANGE.

THE products of a chemical change are often different at different temperatures.

Exp. 1. Dissolve two equal quantities of powdered ferrous sulphate in equal quantities of water, in one case using cold, and in the other hot, water. When solution is complete prove that the solution in cold water gives the reactions of a ferrous compound, but that the solution in hot water gives the reactions both of a ferrous and a ferric compound; part of the ferrous sulphate has been changed to (a basic) ferric sulphate.

Exp. 2. To two quantities of a solution of copper sulphate, one at the ordinary temperature, the other kept boiling, add a slight excess of caustic potash solution. In the cold solution a pp. of hydrated copper oxide (CuO. H ̧O) is produced, but a pp. of copper oxide (CuO) is formed in the hot solution.

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Exp. 3. To a cold solution of formic acid add a little mercuric oxide (prepared by precipitation); the oxide dissolves with formation of mercuric formate. To a hot solution of formic acid add mercuric oxide; a black pp. of mercury is produced.

In Chap. XII. Exps. 19 and 22, you learned that when chlorine and aqueous potash interact at ordinary temperatures potassium chloride and hypochlorite are produced, but that potassium chloride and chlorate are formed when the same bodies-chlorine and potash-interact at about 100o.

Sometimes the rate at which a chemical change proceeds