Question

# The black color of copper oxide turns blue on treating with ${\text{HCl}}$ because of:(A) Formation of cupric chloride(B) Formation of water(C) Release of hydrogen(D) Release of carbon dioxide

Hint: Copper (II) oxide or cupric oxide is an inorganic compound having the chemical formula ${\text{CuO}}$ . It is a solid with blackish color. It is one of the two stable oxides of copper, the other is cuprous oxide. It is produced by heating copper in air at a temperature range of around $300 - {800^\circ }{\text{C}}$ .
$2{\text{Cu}} + {{\text{O}}_2} \to 2{\text{CuO}}$

${\text{CuO + 2HCl}} \to {\text{CuC}}{{\text{l}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}$
Now, this cupric chloride ${\text{CuC}}{{\text{l}}_{\text{2}}}$ salt is blue in color and hence the whole resulting solution appears to be blue in color.
Note: (i) Cupric chloride can also be prepared commercially by the chlorination of copper. In this process, copper at red heat reacts with chlorine gas to give molten cupric chloride. ${\text{Cu(s) + C}}{{\text{l}}_{\text{2}}}{\text{(g)}} \to {\text{CuC}}{{\text{l}}_{\text{2}}}({\text{l}})$
${\text{CuO + 2N}}{{\text{H}}_{\text{4}}}{\text{Cl}} \to {\text{CuC}}{{\text{l}}_{\text{2}}}{\text{ + 2N}}{{\text{H}}_{\text{3}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}$