PRACTICAL CHEMISTRY
Identification of cations using sodium hydroxide and ammonium hydroxide
Cations Action of NaOH Action of NH4OH
2+ Blue ppt. Insoluble in Blue ppt. Dissolves in
Cu
excess excess NH4OH to give
deep blue solution
2+ Green ppt. Insoluble in Green ppt. Insoluble in
Fe
excess excess
3+ Reddish brown ppt. Reddish brown ppt.
Fe
Insoluble in excess Insoluble in excess
2+ Gelatinous white ppt. Gelatinous white ppt.
Zn
dissolves in excess Dissolves in excess
NaOH to give NH4OH to give
colourless solution colourless solution
2+ Chalky white ppt. Chalky white ppt.
Pb
dissolves in excess Insoluble in excess
NaOH to give ammonium hydroxide
colourless solution.
2+ White ppt. Insoluble in No ppt.
Ca
excess NaOH
Few examples of double decomposition precipitation reactions
FeSO4 + 2NaOH → Fe(OH)2 ↓+ Na2SO4
ZnSO4 + 2NH4OH →Zn(OH)2 ↓+ (NH4)2SO4
CaCl2 + 2NaOH →Ca(OH)2 ↓ + 2NaCl
PbSO4 + 2NH4OH →Pb(OH)2 + (NH4)2SO4
Flame test - A small amount of salt is taken in a watch glass.Add few drops of
Con.HCl and make a paste using Metal or Platinum wire.Tale a small amount
of the paste in the wire and show it to the bunsen burner.
Sodium ions - Golden yellow
Potassium ions - Lilac
Calcium ions - Brick red
Copper ions - Blue or green
Test for gases
1. Test for H 2 gas
Metal + dil. Acid → Salt + Hydrogen gas
Colourless, odourless gas giving bubbles and pop sound when a burning
splinter is introduced.
2. Test for O 2 gas
2 H2O2 → 2H2O + O2
Colourless, odourless gas giving bubbles and rekindles or relights a glowing
splinter.
3. Test for NH 3 gas
NH4Cl + NaOH →NaCl + H2O + NH3
Colourless, pungent smelling gas producing dense white fumes when a glass
rod dipped in con.HCl is introduced.
4. Test for HCl gas
H2 + Cl2→ 2HCl
Colorless gas with a pungent odour producing dense white fumes when a glass
rod dipped in con.NH3 is introduced.
5. Test for Cl 2
Greenish yellow gas with a pungent smell and turns moist starch iodide paper
into blue-black.
6. Test for CO 2
CaCO3 + 2HCl→ CaCl2 + H2O + CO2
Colorless odourless gas with brisk effervescence and turns limewater milky.
The gas has no effect on potassium permanganate solution or potassium
dichromate solution.
7. Test for SO 2
MgSO3 + 2HCl → MgCl2 + H2O + SO2
Colourless gas with suffocating odour,turns limewater milky.The gas
decolourises potassium permanganate solution and potassium dichromate
solution colour changes from orange to green.
8. Test for NO 2
2Pb(NO3)2 → 2PbO + 4NO2 + O2
Reddish brown gas with irritating odour and turns moist potassium iodide
paper into brown.
9. Test for H 2S
Colourless gas with a pungent smell and turns moist lead acetate paper into
silvery black.
10. Test for water vapour
Turns white anhydrous copper sulphate into blue hydrated copper sulphate
and turns blue anhydrous cobalt chloride into pink hydrated cobalt chloride.
