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The compounds of Zn, Cd and Hg are usual...

The compounds of Zn, Cd and Hg are usually white.
(ii) A dark blue precipitate is formed when NaOH solution is added to `CuSO_(4)`

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In the compounds of zinc metals, `M^(2+)` ions possess the penultimate d-orbitalsdoubly occupied, i.e. `(n-1)d^(10)` configuration. There is no d-d transition. Hence the compounds of zinc metals are colourless
(i) NaOH reacts with `CuSo_(4)` when dark blue precipitate of `Cu(OH)_(2)` is formed. This precipitate of heating forms CuO which is black in colour. Hence, the colour darkens on heating `CuSO_(4) + 2NaOH to Cu(OH)_(2) + Na_(2) SO_(4)`
`Cu(OH)_(2) tounderset(Black)(CuO) + H_(2)O`
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