Chemistry
Distinguish between the following as directed:
(a) Sodium sulphite solution and sodium sulphate solution. [using dilute H2SO4]
(b) Lead salt solution and zinc salt solution. [using NH4OH solution in excess]
Practical Chemistry
ICSE 2022
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Answer
(a) On adding dilute sulphuric acid to sodium sulphite solution and warming, a colourless gas with suffocating smell of burning sulphur is evolved. The evolved gas turns lime water milky and changes the colour of acidified K2Cr2O7 solution from orange to green suggesting that it is SO2.
Na2SO3 + H2SO4 (dil.) ⟶ Na2SO4 + H2O + SO2↑
Sodium sulphate does not react with dilute sulphuric acid as an acid does not react with its own salt due to the presence of same anion.
(b) A lead salt gives a chalky white ppt. on reaction with ammonium hydroxide that is insoluble in excess of ammonium hydroxide. For example:
Pb(NO3)2 + 2NH4OH ⟶ 2NH4NO3 + Pb(OH)2 ↓
On the other hand, zinc salt forms a white gelatinous ppt. which dissolves when excess of ammonium hydroxide is added. Hence, the two can be distinguished.
ZnSO4 + 2NH4OH ⟶ (NH4)2SO4 + Zn(OH)2 ↓
Zn(OH)2 + (NH4)2SO4 + 2NH4OH ⟶ [Zn(NH3)4]SO4 + 4H2O
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