Explanation
Here, Cu2+ belongs to Group II, cations produce very insoluble sulfides (Ksp values less than 10-30) So they can be precipitated by low amounts of sulphide ion . This can be achieved by adding an acidic solution of H2S, Hence Cu2+ will dissolved. Where as Ni2+, CO2+ and Mn2+ belongs to Group II. Cations produce slightly soluble sulphide ions so they can be precipitated by relatively high amount of sulphide ion. This can be achieved by adding a basic solution of H2O.
A solution of mercey metal ion (Hg2+) when treated with KI gives red precipitate, which dissolves in in excess KI to give a colorless solution. The solution of metal ion on treatment with a solution of cobalt(11) thiocyanite gives a mercury(11) thiocyanite which is a deep blue crystalline precipitate.
i.e
Hg2+ + 2I- → HgI2 ↓ ,
HgI2 + 2I- → [HgI4]2-
(excess) Soluble
Hg2+ + CO(SCN)2 → Hg(SCN)2
Blue crystalline ppt
Prussian blue – A deep blue precipitate obtained by mixig together aqueous solution of Fe3+ salt and ferroyanide. 4Fe3+ + K4[Fe(CN)6] → Fe4[Fe(CN)6]3+12K+ (Prussian blue )
CuSO4 reaction with KCN gives a white ppt of Cuprous Cyanide (CUCN) and cyanogens gas. The cuprous cyanide dissolves in excess KCN forming K3 [Cu (CN)4].
CuSO4 + 2KCN → K2SO4 + Cu(CN)2
2 Cu(CN)2 →2CuCN + (CN)2
(cuprous cyanide) (cyanogens gas)
CuCN + 3KCN → K3[Cu(CN)4]
Fe2+, Cu2+ and Zn2+ belongs group – III and Group IV, Hence they precipitate as hydroxides or sulphide and they are soluble in base.
But Cr3+ belongs to Group II, it is insoluble in base but soluble inpresence of .
H2O2 :
Cr(OH)3 + 4 NaOH + 3[0] → 2Na2CrO4+5H2O
Hence the colored solution S contains Na2CrO4
In III group NH4Cl decreases the concentration of OH- ions due to common ion effect.
NH4NO3 → N2O + 2H2O
CdS is yellow in colour
Ba gives apple green colour to the bunsen flame
On adding NH4Cl to NH4OH solution, the concentration of decreases due to common ion effect.
NH4NO3 can be used in place of NH4Cl for the precipitation of III group basic radicals but the possibility of Mn3+ getting precipitated as Mn(OH)3 in III group will be increased
NiCl2 solution is green in colour
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