Question

Which one of the following metallic hydroxides does not dissolve in sodium hydroxide solution? Suggest the reason as well.
a) Zn(OH)₂ b)Al(OH)₃ c)Fe(OH)₃ d)Pb(OH)₂

Answer 1

The answer is option(a).

Indeed, ZnZn is lower than NaNa in activity series of metals, so the following reaction won't take place

Zn+2NaOH /⟶Zn(OH)2+2Na.Zn+2NaOH ⧸⟶Zn(OH)X2+2Na.
Remember, however, that ZnZn is amphoteric, so it can reacts with a strong base such as NaOHNaOH as an acid forming sodium zincate
Zn+2H2O+2NaOH⟶Na2Zn(OH)4+H2.Zn+2HX2O+2NaOH⟶NaX2Zn(OH)X4+HX2.

P.S. I forget a lot of good ol' chemistry, and I have just realized that usually we do not call metals themselves amphoteric, but rather use this term for their oxides/hydroxides. For metals we just say that some of them (such as ZnZn) dissolve both in acids and in bases. The point is that ZnZn does not react as an acid in the reaction above since the reaction is not an acid-base one, but rather oxidation-reduction. So, the answer then is that ZnZnreacts with aqueous solution of sodium hydroxide because it can be oxidized in these condition in accordance with the reaction mentioned.