The reaction between two compounds in aqueous state to give two new compounds out of which one is a precipitate is known as precipitation reaction.

This is used in analytical chemistry for identifying the positive radical in a salt.

• 1. Iron Sulphate and Sodium Hydroxide

  
  

  FeSO₄ (aq) + NaOH (aq) = Fe(OH)₂   + Na₂SO₄

  First add NaOH in drops and observe, then add NaOH in excess and observe...

  
  

  Observations

  1. A dirty green precipitate is obtained which is insoluble in excess NaOH.

  2. The dirty green precipitate is Fe(OH)₂





• 2. Ferric Chloride and Sodium Hydroxide

  
  

  FeCl₃ (aq) + NaOH (aq) = Fe(OH)₃   + NaCl

  First add NaOH in drops and observe, then add NaOH in excess and observe...

  
  

  Observations

  1. A reddish brown precipitate is obtained which is insoluble in excess NaOH.

  2. The reddish brown precipitate is Fe(OH)₃





• 3. Copper Sulphate and Sodium Hydroxide

  
  

  CuSO₄ (aq) + NaOH (aq) = Cu(OH)₂   + Na₂SO₄

  First add NaOH in drops and observe, then add NaOH in excess and observe...

  
  

  Observations

  1. A pale blue precipitate is obtained which is insoluble in excess NaOH.

  2. The pale blue precipitate is Cu(OH)₂





• 4. Zinc Sulphate and Sodium Hydroxide

  
  

  ZnSO₄ (aq) + NaOH (aq) = Zn(OH)₂   + Na₂SO₄

  First add NaOH in drops and observe, then add NaOH in excess and observe...

  
  

  Observations

  1. When, drops of NaOH is added to ZnSO₄, a gelatinous white precipitate is obtained.

  2. When, NaOH is added in excess to ZnSO₄, the gelatinous white precipitate dissolves in it.

  3. A gelatinous white precipitate is obtained which is "soluble" in excess NaOH.

  4. The gelatinous white precipitate is Zn(OH)₂





• 5. Lead Nitrate and Sodium Hydroxide

  
  

  Pb(NO₃)₂ (aq) + NaOH (aq) = Pb(OH)₂   + NaNO₃

  First add NaOH in drops and observe, then add NaOH in excess and observe...

  
  

  Observations

  1. When, drops of NaOH is added to Pb(NO₃)₂, a chalky white precipitate is obtained.

  2. When, NaOH is added in excess to Pb(NO₃)₂, the chalky white precipitate dissolves in it.

  3. A chalky white precipitate is obtained which is "soluble" in excess NaOH.

  4. The chalky white precipitate is Pb(OH)₂





• 6. Barium Chloride and Sodium Sulphate

  
  

  BaCl₂ (aq) + Na₂SO₄ (aq) = BaSO₄   + NaCl

  
  

  Observations

  1. A white precipitate is obtained which is insoluble in dilute HCl.

  2. The white precipitate is BaSO₄

  Imp :- This reaction is applicable for any sulphate.





• 7. Lead Nitrate and dil. Hydrochloric acid

  
  

  Pb(NO₃)₂ (aq) + dil.HCl = PbCl₂   + HNO₃

  Observations

  3. A white precipitate is obtained which dissloves in hot water and reappears on cooling.

  4. The white precipitate is PbCl₂





• 7. Silver Nitrate and dil. Hydrochloric acid

  
  

  AgNO₃ (aq) + dil.HCl = AgCl   + HNO₃

  Observations

  3. A curdy white precipitate is obtained which is "insoluble" in HNO₃ but "soluble" in excess of NH₄OH

  4. The curdy white precipitate is AgCl