Answer the following question:

The gas used as reducing agent in reducing copper oxide to copper.

• Ammonia $\left({\mathrm{NH}}_3\right)$gas is used as a reducing agent in reducing copper oxide $\left(\mathrm{CuO}\right)$ to copper $\left(\mathrm{Cu}\right)$.
• The reaction between copper oxide and ammonia gas: $$\begin{gathered} 2{\mathrm{NH}}_3\left(\mathrm{g}\right)+3\mathrm{CuO}\left(\mathrm{s}\right)\to {\mathrm{N}}_2\left(\mathrm{g}\right)+3\mathrm{Cu}\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \left(\mathrm{ammonia}\right)(\mathrm{copper}(\mathrm{nitrogen})(\mathrm{copper})(\mathrm{water}) \\ \mathrm{oxide}) \end{gathered}$$

Here in this reaction, we can see that ammonia gas is reacting with copper oxide and producing nitrogen gas, copper and water.

Hence from all this, we can conclude that the gas that is used as a reducing agent in reducing copper oxide to copper is $\left({\mathrm{NH}}_3\right)$ ammonia gas.