Examples of PrecipitationExamples of Precipitation• 2NaOH(aq) + Pb(NO3)2(aq) Pb(OH)2(s) + NaNO3(aq)

• A white precipitate of lead hydroxide is formed which dissolved when excess NaOH is added.

• Al(OH)3 and Zn(OH)2 are also soluble in excess NaOH.

After excess NaOH is added…

Examples of PrecipitationExamples of Precipitation• FeCl3(aq) + 3NaOH(aq) Fe(OH)3(s) + 3NaCl(aq)

• A reddish-brown precipitate of iron(III) hydroxide is seen.

Examples of PrecipitationExamples of Precipitation• FeSO4(aq) + 2NaOH(aq) Fe(OH)2(s) + Na2SO4(aq)

• A dirty-green precipitate of iron(II) hydroxide is seen.

Examples of PrecipitationExamples of Precipitation

• 2NaOH(aq) + CuSO4(aq) Cu(OH)2(s) + Na2SO4(aq)

• A pale blue precipitate of copper hydroxide is seen.

Examples of PrecipitationExamples of Precipitation• CuSO4(aq) + 2NH3(aq) + 2H2O(l) Cu(OH)2(s) + (NH4)2SO4(aq)

• A pale blue precipitate of copper hydroxide is seen which dissolved to give a dark blue solution when excess aqueous NH3 is added.

• Zn(OH)2 is also soluble in excess aqueous NH3.

After excess aq. NH3 is added…

Examples of PrecipitationExamples of Precipitation• KI(aq) + Pb(NO3)2(aq) PbI2(s) + KNO3(aq)

• A bright yellow precipitate of lead(II) iodide is seen.

QUIZQUIZ• Which ions cannot be identified using

precipitation reactions?

A: Cl-, I- and SO42-

B: Al3+, Zn2+, Pb2+, Cu2+, Fe2+ and Fe3+

C: K+ and Na+ Answer: Yes, the answer is C, since all salts of K+ and Na+ are soluble in water, hence cannot be precipitated.

• Cl- and I- can be precipitated as AgCl and AgI respectively using AgNO3.

• SO42- can be precipitated as BaSO4 using Ba(NO3)2.

• The cations in B can be precipitated as hydroxides.

QUIZQUIZ• Which ions can form a hydroxide which can

dissolve in excess NaOH?

A: Fe2+, Fe3+ and Cu2+

B: Zn2+, Pb2+ and Al3+

C: Mg2+ and Ca2+

Answer: Yes, the answer is B.

Zn2+, Pb2+ and Al3+ form insoluble hydroxides which react further with OH- to form soluble complexes.

In excess NaOH, Zn(OH)2 forms sodium zincate, Na2Zn(OH)4. Pb(OH)2 forms sodium plumbate, Na2Pb(OH)4. Al(OH)3 forms sodium aluminate, NaAl(OH)4.