Question

How will you distinguish between the two black samples, CuO and MnO₂ with a chemical test?

Answer 1

To distinguish between copper oxide (CuO) and manganese oxide (MnO_​2), first they need to be heated strongly with conc. HCl.
MnO_​2 will evolve a greenish-yellow coloured gas with a pungent odour. It will turn a filter paper, dipped in starch iodide solution, blue black. This gas is chlorine, and its filtrate is brown in colour due to formation of manganese chloride (MnCl₂)_.

_{$$\begin{gathered} {\mathrm{MnO}}_2+4\mathrm{HCl}\stackrel{\mathrm{\Delta }}{\to }{\mathrm{MnCl}}_2+{\mathrm{Cl}}_2+2{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Brown}\mathrm{Greensih}\mathrm{yellow} \end{gathered}$$}

_{On the other hand,} CuO will not evolve any gas on heating. Its filtrate is green in colour due to _{formation of copper chloride (}CuCl₂)​.

$$\begin{gathered} \mathrm{CuO}+2\mathrm{HCl}\stackrel{\mathrm{\Delta }}{\to }{\mathrm{CuCl}}_2+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Green} \end{gathered}$$