Page 53 - Chemistry - XI

(A) PRELIMINARY TESTS FOR ANIONS (WET TESTS)

S. No. Experiment Observation Inference
1. Dilute H SO test
2 4
Take a little salt in a (i) Colourless, odourless gas with eff ervescence CO 2−

dry test tube and add turns lime water milky. 3
dil. H SO . Warm if (ii) Colourless gas with suff ocating smell of 2−
4
2
no reaction occurs. sulphur which turns acidifi ed K Cr O paper SO 3
2
7
2
green.
(iii) Colourless gas with having smell of rotten S 2–
eggs which turns lead acetate paper black.
(iv) Brown gas having pungent smell which turns
FeSO black. NO − 2
4
Chemical Reactions

When salts of weak and volatile acids are treated with dil. HCl or dil. H SO , they decompose with the
4
2
evolution of gases. The identifi cation of the gas evolved indicates the nature of the acid radical.
1. Carbonates:
CO +H SO SO +H O+CO
2
2
3 2 4 4 2 2
CO thus produced turns lime water milky due to formation of insoluble CaCO .
2 3

Ca OH +CO CaCO+ HO
2 2 3 2
2. Sulphites: All sulphites liberate SO gas with dil. H SO which turns acidifi ed K Cr O paper green due
2
2
2
4
7
2
to formation of chromium sulphate.
SO +H SO SO +H O+SO
2
2
3 2 4 4 2 2

KCrO +H SO +3SO KSO+Cr SO +H O
2 2 7 2 4 2 2 4 2 4 3 2
Green
3. Sulphides: A sulphide on treatment with dil. H SO or dilute HCl yields H S which has the smell of
4
2
2
rotten eggs and turns lead acetate paper black.
S+ HSO H S+SO 2
2
2 4 2 4
(Smell
of rotten
eggs)

Pb CH COO+ HS PbS+ 2CHCOOH
3
2 2 3
(Black ppt.)

4. Nitrites: All nitrites produce NO with dilute HSO acids. NO turns ferrous sulphate solution black.
4
2
2
2
3NO+3H SO 3HSO +3HNO

2 2 4 4 2
3HNO → HNO +H O+ 2NO
2 3 2
FeSO +NO → FeSO. NO
4 4
(Brownish
black)
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