What do you observe when caustic soda solution is added to the following solution, first a little and then in excess :

(a) FeCl₃

(b) ZnSO₄

(c) Pb(NO₃)₂

(d) CuSO₄

Write balanced equations for these reactions.

(a) When caustic soda solution is added to FeCl₃ dropwise, a reddish brown ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{yellow}}{{\text{FeCl}$3}} + \underset{\text{colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}3↓} + \underset{\text{colourless}}{3\text{NaCl}}

(b) When caustic soda solution is added to Zinc sulphate dropwise, a white gelatinous ppt is obtained, which dissolves in excess of NaOH:

$\underset{\text{colourless}}{{\text{ZnSO}$4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}2↓} + \underset{\text{ colourless}}{\text{Na}2\text{SO}4}

${\text{Zn(OH)}$2} + \underset{\text{ excess}}{2\text{NaOH}} \longrightarrow \underset{\text{colourless}}{\text{Na}2\text{ZnO}2↓} + 2\text{H}2\text{O}

(c) When caustic soda solution is added to Pb(NO₃)₂ dropwise, a chalky white ppt is obtained, which dissolves in excess of NaOH:

$\underset{\text{colourless}}{{\text{Pb(NO}$3)2}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}2↓} + \underset{\text{ colourless}}{2\text{NaNO}3}

${\text{Pb(OH)}$2} + \underset{\text{excess}}{2\text{NaOH}} \longrightarrow \underset{\text{sodium plumbite - colourless}}{\text{Na}2\text{PbO}2↓} + 2\text{H}2\text{O}

(d) When caustic soda solution is added to CuSO₄ dropwise, a pale blue ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{blue}}{{\text{CuSO}$4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}2↓} + \underset{\text{colourless}}{\text{Na}2\text{SO}4}