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In Inorganic Chemistry two types of test are observed :
cationic and anionic.
Anionic Test : In case of anionic test we use the following reagents.
— H2SO4/HCl dil. — H2SO4 conc.
H2SO4/HCl (dilute)
On adding dil. H2SO4 or dil. HCl the observation is (1) A brown colour gas is coming out.
Gas ⇒ NO2, Radical ⇒ NO2–
(2) A gas is coming out with vinegar smell. Gas ⇒ CH3COOH, Radical ⇒ CH3COO–
Note :
(3) A gas comes out which turns lead acetate paper black. Gas ⇒ H2S, Radical ⇒ S– –
Smell of rotten eggs.
(4) A gas comes out which is having suffocating smell and turns K2Cr2O7 (orange) into green.
Gas ⇒ SO2, Radical ⇒ SO3– –
(5) If a gas comes out with brisk effervescence. Gas ⇒ CO2, Radical ⇒ CO3– –
Example : Find out all the given in the reaction.
Ans. A = Na2S, C = H2S. Note :
S2O3– – Thiosulphate
Na2S2O3 Hypo (Sodium Thio Sulphate)
If a compound remove the color of violet iodine then it is Na2S2O3.
Na S O2 2 3+2 I2 NaI+Na S O
Violet Disappear2 4 6 +2.5
(Iodomatic Test)
Na2S2O3 on reaction with AgBr gives white ppt. this white ppt. become black on standing and gives a compound Na3[Ag(S2O3)2] which is used as fixer in photography.
Test By Conc. H2SO4
If conc. H2SO4 is added.
(1) Brown colour gas with brown ring Test. Gas = NO2, Radical = NO3–
(2) A dark brown colour gas comes out without brown ring test.
Gas = Br2, Radical = Br– (3) A violet coloured gas comes out
Gas = I2, Radical = I– (4) If HCl comes out ⎯→ Cl–
Note : In case of F– and Cl–. Gases F2 and Cl2 are not coming because F2 and Cl2 are good oxidiser due to this F– and Cl– are not oxidised by H2SO4. But in case of Br– and I– oxidation is easily possible.
(5) If a gaseous mixture comes out which burns with blue flame. Gas = CO + CO2 Radical = • C2O4– – + Fe+2 ⎯→ [Fe(C2O4)2]2– Yellow Complex • + H2SO4 ⎯→ CO + CO2 + H2O
Chromyle Chloride Test
It is given by only chloride ion of a metal. It is not given by chlorides of Hg, Sn, Ag, Pb, and Sb.
Some Important Tests (Anionic)
(i) I2 gives violet layer, with chloroform. (ii) F– test NaF +H SO NaHSO HF HF + SiO SiF H O SiF H O 2 4 (Conc.) 4 2 4 2 4 2 H [SiF2 6 H SiO Waxy, deposite 4 4 ]
(iii) Borate (BO3– –)
(iv) Sulphate (SO4– –)
Note : For Cromyl chloride test :
(i) The test tube must be dry otherwise chromyl chloride may hydrolyse.
CrO2Cl2 + 2H2O ⎯→ H2CrO4 + 2HCl
(ii) Bromides and iodides do not give chromyl chloride test and in this test they evolve Br2 and I2.
(iii) If NaOH solution is incompletely acidified, then white ppt. of lead hydroxide may be obtained.
(iv) In the organic layer test of Br–, chlorine water must not be used in excess, otherwise layer attains pale yellow colour due to formation of bromine monochloride or it may be colourless due to formation hypobromous acid.
Br2 + Cl2 ⎯→ 2BrCl (Bromine monochloride) Br2 + 2H2O + Cl2 ⎯→ 2HBrO + 2HCl
Colourless
(v) In the organic layer test of I–, excess of chlorine water must be avoided otherwise the layer becomes colourless due to formation of iodic acid.
I2 + 5Cl2 + 6H2O ⎯→ 2HIO3 + 10HCl
(vi) Ring test is not reliable in presence of NO2–, Br– and I–.
(vii) Smell of NH3 also indicates the presence of NO3–, (if nitrate salt is boiled with Zn/NaOH or Al/NaOH.
Zn + 2NaOH ⎯→ Na2ZnO2 + 2(H) Al + NaOH + H O ⎯→ NaAlO + 3[H]
(x) During testing with conc. H2SO4, the solution must not be heated to boiling, otherwise H2SO4 will get decomposed and evolution of SO2 will make identification of other gases difficult.
(xi) In the organic layer test of Br– and I–, HNO3, CaOCl2, KMnO4, NaOCl or chloramine may also be used in place of chlorine water.
1. Which will not give cromyl chloride test
(a) AgCl (b) NaCl (c) NH4Cl (d) CaCl2
2. A gas is coming out by conc. H2SO4 reacting with salt 'X'. Which burns with blue flame and this salt can be converted by the heating of 'Y'.
X Y Gas
(a) HCOONa CO + CO2
(b) HCOONa CO + CO2 (c) Na2CO3 HCOONa CO + CO2 (d) HCOONa Na2CO3 CO + CO2
3. Which will not evolve gas with conc. H2SO4
(a) NaCl (b) NaF (c) NaBr (d) a and b
4. H2SO4 + K2Cr2O7 gives brown color gas with 'X' then this 'X' may be (a) NaCl (b) NaBr (c) Both (d) None
5. If 'A' on reaction with BaCl2 gives 'B' and this 'B' is insoluble in water then 'A' is containing
(a) NaNO3 (b) Na2SO4 (c) NH4NO3 (d) NH4NO2
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(1) When HgS reacts with Nascent Hydrogen it give Hg. (2) When sulphide reacts with Nitroprosside then it gives
another complex.
(3) Ring test is not possible in presence of bromide, iodide and nitride.
Substance sublime of Heating and colour of sublimate
Colour Substance Greenish-Black HgS Yellow S, HgI2, As2S3 Yellow-Blue/violet I2 Gray As White Sb2O3, HgCl2, Hg2Cl2, As2O3, AlCl3, Amm. Halides.
