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  4. When heated in air, brown copper powder turns black. This black powder would turn brown again when heated with

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Q. When heated in air, brown copper powder turns black. This black powder would turn brown again when heated with

KVPYKVPY 2017Redox Reactions

Solution:

When heated in air, brown copper turns black. This black powder would turn brown again when heated with hydrogen $(H_2)$. This is because here, $H_2$ will act as air reducing agent that reduces $CuO$ into $Cu$, whereas air $(O_2)$ will act as an oxidising agent.
$\underset{\text{Brown}}{Cu} + O_2 \xrightarrow{\Delta} \underset{\text{Black}}{CuO}$
$ \underset{\text{Black}}{CuO} + H_2 \xrightarrow{\Delta} Cu + H_2O$