Explain the following:
(i). Copper hydroxide is soluble in ammonium hydroxide but not in sodium hydroxide.
(ii). A dark blue precipitate is formed when sodium hydroxide solution is added to copper sulphate solution. The precipitate darkens on heating.

Explanation

(i). `Cu(OH)_2` dissolves by forming a complex.
`Cu(OH)_2+4NH_4OHto[Cu(NH_3)_4](OH)_2+4H_2O`
`Cu(OH)_2` is insoluble in NaOH as no such complex is formed.
(ii). `NaOH` reacts with `CuSO_4` when dark blue ppt. Of `Cu(OH)_2` is formed. This ppt. On heating forms CuO which is black in colour. Hence the colour darkens on heating.
`CuSO_4+2NaOHtounderset((Blue))(Cu(OH)_2)+Na_2SO_4`
`Cu(OH)_2overset(Delta)tounderset((Black))(CuO)+H_2O`