The liquid at the same time becomes blue from the in it of Cu(NO3)2:

8HNO3 + 3Cu=2NO +3CuN2O¿+4H2O .

presence

NO is itself a colourless gas, but yields on mixture with air N2O, and N2O4 which are reddish-brown.

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249. If some cold dilute indigo-solution be poured into cold KNO, solution until it is decidedly blue, and then H2SO, be added, the blue colour remains unchanged. (Difference from chlorates).

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250. If any solid nitrate be fused for some time at a red heat with fusion mixture on platinum foil, oxygen gas is given off slowly with effervescence* and KNO2 is formed; if the foil after cooling be boiled in water, a portion of the solution may be proved to contain KNO, by the test in paragraph (245); another portion, if acidified with HNO3, will be found to give no precipitate with AgNO. (Difference from chlorates).

A solution of a nitrate may also be reduced to nitrite by nascent H. Add to a little KNO, solution some dilute H2SO4 and starch paste with KI, no colour will be produced, but on dropping in a fragment of Zn a deep blue colour is seen (245).

251. Solid KNO, fused on charcoal in the blowpipe flame "deflagrates," that is to say the surface of the charcoal burns rapidly like gunpowder.

Note. Many metallic nitrates [e.g. Pb(NO3)2] when heated in a glass tube closed at one end, evolve reddish-brown fumes with a characteristic smell (N204) and oxygen-PьN2Oq=N2O4+O+PbO. The fumes are recognised by their colour and smell, the O by its inflaming a glowing splinter of wood.

Hydrogen nitrate, or nitric acid (HNO3), has a strong acid reaction, it gives with FeSO4 solution a brown ring (247), and with Cu brown fumes (248) without addition of H2SO4. Evaporated with quill-clippings, it stains them bright yellow.

*If this experiment is performed in a small hard glass tube as described in 254, the oxygen can rarely be detected since it is evolved very slowly: hence the student may perform the ignition on platinum foil and neglect to test for O.

CHLORATES ('ClO3).-Use KClO3.

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252. If to a cold solution of KCIO, some cold dilute indigo solution be added drop by drop till the liquid is coloured faintly but distinctly blue, then some H2SO, or NaHSO be poured in and the mixture shaken, the blue colour of the indigo is at once destroyed. (Difference from nitrates).

253. Strong H2SO4 poured upon a little solid KClO3 becomes orange-red in colour, and evolves when shaken a bright yellow gas (C1204) :—

3KClO3 + H2SO4 = C12O4 + KC1O4 + K2SO4 + H„O.

This gas has a smell somewhat resembling that of chlorine; it explodes readily when gently heated, hence on warming the mixture in the tube crackling sounds or small explosions will be produced. This experiment may be dangerous, unless the KClO3 is used in very small quantity and heated gently with the mouth of the test-tube turned away from the operator.

254. AgNO, produces no precipitate with KCIO, solution. if it is free from KCl. But if some solid KClO3* be heated in a test-tube or better in a small ignition tube (10), after melting it begins to give off oxygen with effervescence; this gas may be recognised by placing in the tube a slip of wood with a spark at its end, which is easily obtained by kindling a match and suddenly blowing out the flame, the spark is caused to burst into a flame. If the tube is then allowed to cool and the residue dissolved by boiling with some water; a portion of the solution will be found to give no reaction for HNO, (245); but another portion will give a white precipitate (AgCI) with AgNO,, which does not dissolve in HNO, even on boiling, but is easily soluble in AmHO (difference from HNO,): this precipitate is caused by KCI :

KC1O3 = KCl+Og.

* If the chlorate used for this test is not an alkaline chlorate, fusion mixture must be mixed with it before heating, in order to obtain a soluble chloride in the residue.

255. Blowpipe Test.-KCIO, if heated on charcoal in the blowpipe flame, causes the charcoal to "deflagrate."

Hydrogen chlorate, or chloric acid (HClO), is a colourless, odourless liquid, which first reddens and then bleaches litmus-paper; on being kept for some time it changes into O,C1, HCIO, and H2O; the same change occurs rapidly when it is heated.

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256. As will be seen, there is little difficulty in distinguishing a nitrate from a chlorate. When present together they more or less interfere with one-another's reactions; but they may be detected, in the absence of chlorides and nitrites, by heating strongly for a short time and testing the residue for nitrite and chloride (250 and 254).

GROUP IV.-CHLORIDE GROUP.

Chlorides, bromides, and iodides closely resemble one another; they are all precipitated by AgNO, from solutions to which HNO, has been added in excess; in this respect they differ from all salts except cyanides, ferrocyanides, and ferricyanides, and these are easily distinguished by other

means.

CHLORIDES ('Cl).-Use NaCl.

257. AgNO, added to a little NaCl dissolved in water, gives a pure white precipitate (AgCl), which collects into curdy masses when heated or shaken, and quickly turns violet in sunlight or more slowly in daylight. Decant the water; to one portion of the precipitate add HNO, and warm, it does not dissolve; to another portion add AmHO, it readily dissolves. Decant the liquid from a portion of the precipitate after shaking and letting the precipitate settle, and warm the precipitate with a little strong H2SO4, the acid remains colourless and no coloured vapour is given off.

258. Solid NaCl warmed with strong H2SO4 gives off colourless HCl-gas, which is recognised by fuming in the air, reddening moistened blue litmus-paper, and making a drop of AgNO, solution on the end of a glass rod milky.

259. If solid NaCl be powdered and mixed with finely powdered MnO,, then strong H,SO, added and the mixture warmed, chlorine gas comes off :

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4NaCl + MnO2+4H2SO4 = 2C1 + MnCl2 + 4NaHSO4 + 2H2O.

This gas is recognised by its property of bleaching a piece of moist blue litmus-paper, introduced into the tube on a glass rod. The most delicate way of testing is to warm the mixture in a small beaker covered with a watch-glass which bears on its under surface a piece of moist litmus-paper, and to let the apparatus stand for some time. Moist starch is not coloured when held in the air in the tube.

Note.-Many samples of commercial MnO, evolve Cl when heated with H2SO4; hence the MnO, unless specially prepared by precipitation, must be carefully tested before using it for this reaction; or the MnO2 and H2SO4 may be first boiled together as long as any bleaching action is produced when moist litmus is held in the tube, then the substance to be tested is added and Cl again tested for whilst heating the mixture.

260. If an intimate mixture of solid NaCl with three or four times as much K2Cr2O, in fine powder be made by rubbing the two substances together in a mortar, and this mixture be then poured into the tubulated flask shown in fig. 38* (229) (see note below), and mixed with strong H2SO4 by pouring in the acid and shaking it round in the flask, on warming the flask reddish-brown vapour (CrO2C12) will be evolved:

:

4NaCl + K2Cr2O7+6H2SO4 = 2CrO2Cl2 + 4NaHSO4+

2KHSO4+3H,O.

If the neck of the flask be closed by an india-rubber or a glass stopper and the delivery tube be made to dip into water in a test-tube, the vapour on passing into the water will impart to it a reddish-yellow colour owing to the formation of HCrO4:

*

CrO2Cl2+ 2H2O = H2CrO4 + 2HC1.

The test-tube fitted as shown in fig. 37 is much less suitable.

On adding to this yellow liquid excess of AmHO, the colour becomes pale yellow; on now adding excess of HÃ, the original reddish-yellow colour is reproduced, and in this liquid the presence of HCrO4, and therefore indirectly that of HCl, may be shown by the formation of a yellow precipitate on addition of PbA,-solution.

Note. Since by this method the detection of a chloride depends on the formation of HCrO4 in the liquid in the test-tube from the vapour or CrO2Cl2, great care must be taken that no H,CrO4 is introduced into that liquid from anyother source. Now, since the mixture introduced into the flask contains a chromate, the greatest precaution must be taken that none of it is allowed to get into the bent delivery-tube, and thence into the test-tube. In introducing the powder into the flask, it must be poured down the side of the perfectly dry neck opposite to that at which the delivery-tube enters; also whilst heating the mixture it must not be allowed to rise or splash into the neck of the flask. If these precautions are not taken, and HCrO, as such is introduced into the water in the test-tube, the test is worthless as an indication of the presence of a chloride.

Care must also be taken that the liquid is not sucked back from the test-tube into the flask: this is prevented by raising the deliverytube out of the water in the test-tube as soon as the heating is stopped.

Hydrogen chloride or hydrochloric acid (HCl) is a colourless gas which fumes in the air, and dissolves very readily in water; both the gas and its solution render AgNO, solution, which has been acidified with HNO3, milky. Heated with MnO2 the liquid acid evolves chlorine gas.

BROMIDES ('Br).—Use KBr.

261. AgNO, added to a little KBr solution gives a yellowish-white precipitate (AgBr), which is easily coagulated by heating or shaking the liquid; it is insoluble in HNO3, but rather soluble in AmHO, thus somewhat resembling the AgCl precipitate from which it is distinguished by its colour. By shaking well, allowing to settle, decanting the liquid, and heating the precipitate with strong H2SO4, no violet vapour is evolved, the precipitate thus differing from AgI.

262. Solid KBr when heated with strong H2SO4 gives