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Chemistry

State one relevant observation for each of the the following reactions.

(a) When hydrogen chloride gas is passed through a solution of sodium thiosulphate.

(b) Concentrated sulphuric acid is added to a solution of potassium nitrate.

(c) When fused lead bromide is electrolysed.

Hydrogen Chloride

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Answer

(a) When hydrogen chloride gas is passed through a solution of sodium thiosulphate a yellow precipitate of Sulphur can be seen along with water, Sodium chloride, and Sulphur dioxide gas which has a burning smell.

Na2S2O3 + 2HCl ⟶ 2NaCl + H2O + SO2 + S ↓

(b) Nitric acid is released with a pungent odour when concentrated sulphuric acid is added to a solution of potassium nitrate.

KNO3 + H2SO4 (conc.) <200°C\xrightarrow{\lt 200 \degree\text{C}} KHSO4 + HNO3

(c) At anode, reddish brown fumes of bromine vapours are seen. At cathode, silver grey deposit of lead metal is observed.
At cathode:
Pb2+ + 2e- ⟶ Pb
At anode:
Br- ⟶ Br + e-

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