'A' Level Chemistry Problem Analysis: Solubility And Heating of Zinc Hydroxide

(i) Is zinc hydroxide soluble in water? How about in excess OH-(aq)?

(ii) If NaOH solution is heated, does it yield Na2O and water? What about Zn(OH)2?

Thought process:

Zn(OH)2 is an insoluble precipitate, however it can form a soluble complex tetrahydroxozincate(II) ion [Zn(OH-)4]2-(aq) in excess OH-(aq). The complex ion is soluble because it is capable of ion-dipole interactions with water, unlike Zn(OH)2(s).

NaOH is stable (owing to the fact the Na+ ino has relatively low charge density), and therefore will not dehydrate to form Na2O. When excess water is eliminated through evaporation, one will end up with a white solid which is NaOH(s).

On the other hand, the Zn2+ ion has a relatively higher charge density, is more unstable and hence Zn(OH)2 will readily shed water when heated to form ZnO(s).

Zinc oxide is yellow when hot, white when cold; the yellow colour arises from interactions in the crystal lattice, and not due to d-d transition (when an electron moves from one energy level to another and both levels are d-orbitals), since Zn2+ has no partially filled d-orbitals and doesn’t qualify as a transition metal.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


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