Question

What would be the color of the solution when copper oxide and dilute hydrochloric acid are mixed?

Answer 1

1. When black color copper oxide $\left(\mathrm{CuO}\right)$ is reacted with hydrochloric acid (HCl) it produces a bluish-green color salt called cupric chloride or copper ($\mathrm{{\rm I}{\rm I}}$) chloride $\left({\mathrm{CuCl}}_2\right)$ and water.
2. Due to the formation of this salt, the solution appears bluish-green in color.

Chemical reaction involved:

$\underset{(\mathrm{Copper}\mathrm{oxide})}{\mathrm{CuO}\left(\mathrm{s}\right)}+\underset{(\mathrm{Hydrochloric}\mathrm{acid})}{2\mathrm{HCl}\left(\mathrm{aq}\right)}\stackrel{}{\to }\underset{(\mathrm{Bluish}-\mathrm{green}\mathrm{colour})}{\underset{(\mathrm{Cupric}\mathrm{chloride})}{{\mathrm{CuCl}}_2\left(\mathrm{aq}\right)}}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Hence, bluish-green will be the color of the solution when copper oxide and dilute hydrochloric acid are mixed.