Question

What happens when aluminium and zinc salts react with an excess of $ NaOH $ ?

• A. White precipitate is formed
• B. White precipitate of both $ Zn $ and $ Al $ first formed redissolve in excess of $ NaOH $
• C. White precipitate of $ Al $ redissolves but that of $ Zn $ does not
• D. White precipitate of $ Zn $ redissolves and that of $ Al $ does not

Answer 1

Answer: (B)

When $NaOH$ reacts with salts of $Al$ and $Zn$, it forms white precipitate which dissolve in excess of $NaOH$.
$(i)\, AlCl_3 + 3NaOH \rightarrow \underset{\text{white ppt}}{Al(OH)_3} \downarrow+ NaCl$
$Al(OH)_3 + NaOH \rightarrow \underset{\overset{\text{sodium meta}}{\text{aluminate (soluble)}}}{NaAlO_2} + 2H_2O$
$(ii)\, Zn(OH)_2 + 2NaOH \rightarrow \underset{\text{white ppt}}{Zn(OH)_2}\downarrow + Na_2SO_4$
$Zn(OH)_2 + 2NaOH \rightarrow \underset{\overset{\text{sodium zincate}}{\text{(soluble)}}}{Na_2ZnO_2} + 2H_2O$