Question

Formation of light blue precipitate in a solution treated with $NaOH$ indicates presence of:

• A. $C{a}^{2+}$
• B. $N{H}^{4+}$
• C. $C{u}^{2+}$
• D. Both $A$ and $C$

Answer 1

Answer: (C)

$C{u}^{2+}$

A solution of a copper salt (copper sulphate) was mixed with an equal amount of another solution, in which a physical change is noted, usually a colour change due to a precipitation of the copper insoluble compound or formation of a complex since copper is a transitional metal. It forms a blue precipitate with ammonium hydroxide, sodium hydroxide and sodium carbonate, in which the former results in a deep blue solution of excess ammonia, and the latter two, turns black (copper oxide) on heating. These tests are already unique to copper, but further confirmatory test includes primarily the white lilac ppt (yellow solution) with thiocyanate, and then the reddish brown ppt with Ferro(II) cyanide, the green precipitate with the sulphate, soluble in excess to a pink/colourless solution, and also the brown-black precipitate with the sulphide.