place from the escape of a mixture of CO and CO2, and the former may be kindled at the mouth of the test tube, and will burn with a pale blue flame. No blackening of the mixture occurs as in the case of organic acids, which yield CO on heating with H2SO.

101.

HYDROFLUORIC ACID. HF, c.w. 20.

1. BaCl, produces a white precipitate of BaF2, ble in HCl, and sparingly in NHCl.

solu

2. CaCl, produces a gelatinous and almost transparent precipitate of CaF2, difficult to discern in the fluid, but made more apparent on addition of (NH1)HO. The precipitate is very difficultly soluble in HCl, even on boiling, and is nearly insoluble in acetic acid.

3. Heated with H2SO4, all fluorides are decomposed with evolution of HF, which is recognized by its power of etching glass. (See 47.)

This is best done by placing the fluoride in a platinum crucible, or small leaden cup, along with strong H2SO4, and covering the mouth with a waxed watchglass, convex side downwards, on which a few scratches have been made with a needle. The concave side of the watch-glass is filled with water to prevent the wax on the other side from melting, and the crucible or cup is then gently heated. On removing the glass and melting off the wax by gentle warming, the glass will be found etched at the unprotected parts.*

* If the fluoride contain much silicia, Si F4 is evolved instead of HF, and is detected by heating the substance with H2SO, in a test tube, and leading the evolved gas into water. Silica will separate out in flocculent tufts, and H2SiF6 will be found. in solution.

4. Heated with a mixture of borax and HKSO4, on a loop of platinum wire in the non-luminous gas flame, BF3 is produced, which momentarily colours the flame green.

102. CARBONIC ACID. H2CO3 or H2O + CO2

1. BaCl2 produces in neutral solutions a white precipitate of BaCO3, soluble in acids with effervescence.

2. Treated with dilute HCl, all carbonates at once evolve CO2 with effervescence, and if this gas be conducted into lime-water it produces a turbidity from formation of CaCO3. (The experiment may be conveniently performed by placing the carbonate and dilute acid in one test tube and the lime-water in another. As soon as the CO2 has collected, it may be decanted into the lime-water tube-care being taken to prevent any liquid from being decanted with it—and on shaking the latter the lime-water will become turbid.)

103. SILICIC ACID. Si(HO), c.w. 96.

1. BaCl, produces a white precipitate of SiBagO4) which is decomposed on addition of HCl, and Si(HO), separates out as a gelatinous precipitate.

2. HCl, added drop by drop to a strong solution of a silicate, produces a gelatinous precipitate of Si(HO)4, but if added to a dilute solution or in large excess, no precipitate is obtained until the mixture has been evaporated to dryness and ignited, when SiO, separates out, and this is not re-dissolved on addition of HCl.

3. Fused with Na,CO, in a loop of platinum wire in

the non-luminous gas flame, effervescence occurs from the disengagement of CO2, and the bead is transparent on cooling, unless the Na2CO3 be in excess.

4. Fused with microcosmic salt on a loop of platinum wire in the non-luminous gas flame, solution does not take place, but the silica floats about on the bead undissolved.

104. The remaining six acids of this group are precipitated or decomposed by one or other of the group reagents for bases, and are therefore precipitated in the course of examination for bases, or expelled on the addition of HCl. The action of the base groupreagents is as follows :

Ag Group.

As Group.

:

H2SO, decomposed by HCl, with evolution of
SO2

H2S2O3 decomposed by HCl, with evolution of
SO, and separation of S.

HASO, precipitated by HS as As2S (yellow).
H2ASO4

HIO, decomposed by H,S, with formation of an iodide and separation of S.

HCrO4 precipitated by (NH4)2S as Cr2(HO)。.

The following are additional tests for these acids :

105.

SULPHUROUS ACID. H2SO, c.w. 82.

1. BaCl, produces a white precipitate of BaSO3, soluble in HCl. This solution, on addition of chlorine

water, yields a white precipitate of BaSO4, the sulphite being oxidized to sulphate.

2. AgNO, produces a white precipitate of Ag2SO3, which darkens on heating, from precipitation of Ag.

3. Added to a mixture of zinc and HCl, H2S is produced and recognized by its smell and blackening action on paper moistened with solution of a lead salt.

4. H2S decomposes free H2SO, with separation of sulphur.

106. HYPOSULPHUROUS ACID. H2S2O3, c.w. 114. 1. BaCl2 produces a white precipitate of BaS2O3, soluble in HCl, with formation of sulphur as a yellow precipitate.

2. HCl or H2SO4 produces no immediate precipitate, but on standing a short time sulphur is precipitated (yellow), and simultaneously SO2 is evolved.

3. AgNO, produces a white precipitate of Ag2S2O3, which rapidly darkens in colour and becomes ultimately black from formation of Ag2S. These changes are hastened by heat.

4. FeCl produces a reddish coloration, but on heating it is decolorized, the ferric being reduced to ferrous chloride.

107.

ARSENIOUS ACID.

HASO3, c.w 126.

1. AgNO3 produces in neutral solutions a yellow precipitate of Ag AsO,, soluble in (NH)HO. If no precipitate appear at first owing to the solution not

being neutral, add a few drops of a very dilute solution of (NH)HO until it appears. (See 75, 2.)

2. MgSO4+NH,C1+(NH,)HO produce no precipitate.

(See also reactions for Arsenic, 75.)

108. ARSENIC ACID. HASO4, c.w. 142.

1. AgNO3 produces in neutral solutions a light brown precipitate of Ag,AsO. If necessary, add very dilute ammonia, as in the preceding case.

2. MgSO4 + NH1C1+ (NH1)HO produce a white precipitate of MgNH1AsO.

(See also reactions for Arsenic, 75.)

109.

IODIC ACID. HIO3, c.w. 176.

1. BaCl, produces a white precipitate of Ba(IO3)2, soluble in HNO3.

2. AgNO3 produces a white crystalline precipitate of AgIO, readily soluble in (NH4)HO, but sparingly soluble in HNO3.

3. SO2 produces at first a precipitate of I, which is converted into HI on addition of excess of the re-agent.

4. On heating, iodates are decomposed, oxygen being evolved, and in some cases iodine is also given off in violet vapours.

110.

CHROMIC ACID. H2CrO4, c.w. 118°2.

1. BaCl, produces a yellow precipitate of BaCrO4, soluble in HCl and HNO3, but insoluble in acetic acid.