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 $NaOH$.
$(i)AlC{l}_3+3NaOH\to \underset{\text{white ppt}}{Al(OH{)}_3}\downarrow +NaCl$
$Al(OH{)}_3+NaOH\to \underset{\stackrel{\text{sodium meta}}{\text{aluminate (soluble)}}}{NaAl{O}_2}+2H_{2}O$
$(ii)Zn(OH{)}_2+2NaOH\to \underset{\text{white ppt}}{Zn(OH{)}_2}\downarrow +N{a}_{2}S{O}_4$
$Zn(OH{)}_2+2NaOH\to \underset{\stackrel{\text{sodium zincate}}{\text{(soluble)}}}{N{a}_{2}Zn{O}_2}+2H_{2}O$