Question

$A_1$ and $A_2$ are two ores of metal $M.A_1$ on calcination gives black precipitate, $C{O}_2$ and water.
Identify $A_1$ and $A_2$.

Answer 1

$A_1$ is basic copper carbonate $(Cu(OH{)}_2.CuC{O}_3)$ while $A_2$ is $C{u}_{2}S.$ The confirmatory reactions are :
(i) $CuC{O}_3\cdot Cu(OH{)}_2\stackrel{\Delta }{\to }\underset{\text{Black}}{\mathrm{CuO}}+C{O}_2\uparrow +H_{2}O$
(ii) $CuC{O}_3\cdot Cu(OH{)}_2\stackrel{HCl}{\to }CuC{l}_2+C{O}_2\uparrow +H_{2}O$
$CuC{l}_2\stackrel{KI}{\to }\underset{D}{C{u}_2{I}_2}\downarrow +KCl+I_2$
$A_2\stackrel{\text{Roasting}}{\to }C{u}_{2}O+S{O}_2\uparrow$
$C{u}_{2}S+C{u}_{2}O\to Cu+S{O}_2\uparrow$
$S{O}_2$ is a reducing gas that gives green colour with acidified $K_{2}C{r}_2{O}_7$ as
$3S{O}_2+K_{2}C{r}_2{O}_7+H_{2}S{O}_4\to K_{2}S{O}_4+\underset{\text{Green}}{C{r}_2(S{O}_4{)}_3}+4H_{2}O$