Question

If copper is kept open in the air, it slowly loses its shining brown surface and gains a green coating. It is due to the formation of

• A. ${\mathrm{CuSO}}_4$
• B. ${\mathrm{CuCO}}_3$
• C. $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$
• D. $\mathrm{CuO}$

Answer 1

The correct option is B

${\mathrm{CuCO}}_3$

Explanation for correct option:-

Option (A) ${\mathrm{CuCO}}_3$

• We know that the metals are reactive in nature.
• As a result when copper is left exposed to air it reacts with the various gases that are present in the air, like Carbon dioxide and water vapor.
• Due to this reaction, Copper carbonate is formed which results in the dull appearance of the shiny copper metals.
• This forms a green colour coating of Copper carbonate.
• The balanced reaction is:

$\underset{\mathrm{Copper}}{2\mathrm{Cu}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}\mathrm{vapour}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}\to \underset{\mathrm{Copper}\mathrm{carbonate}}{{\mathrm{CuCO}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Copper}\mathrm{hydroxide}}{\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}$

• Hence it is the correct option

Explanation for incorrect option:-

Option (B) ${\mathrm{CuSO}}_4$

• Since the Copper metal only reacts with Carbon dioxide and the moisture, it forms Copper carbonate.
• Since Sulphur is not always present in the air so it does not react to Copper and thus Copper sulphate is not formed.
• Hence it is the incorrect option.

Option (C) $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$

• Nitrogen is not always present in the air so it does not react to Copper and thus Copper nitrate is not formed.
• Since the Copper metal only reacts with Carbon dioxide and the moisture when it is left exposed to air, it forms Copper carbonate.
• Hence it is the incorrect option.

Option (D) $\mathrm{CuO}$

• Copper first reacts with the oxygen present in the surroundings to form Copper oxide.
• This Copper oxide formed then reacts with the carbon dioxide and thus forms Copper carbonate.
• The green colour coating is of Copper carbonate, not Copper oxide
• The complete reaction is:

$$\begin{gathered} \underset{\mathrm{Copper}}{4\mathrm{Cu}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Copper}\left(\mathrm{I}\right)\mathrm{oxide}}{{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)} \\ \underset{\mathrm{Copper}\left(\mathrm{I}\right)\mathrm{oxide}}{{\mathrm{Cu}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Copper}\mathrm{oxide}}{\mathrm{CuO}\left(\mathrm{s}\right)} \\ \underset{\mathrm{Copper}\mathrm{oxide}}{\mathrm{CuO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}\to \underset{\mathrm{Copper}\mathrm{carbonate}}{{\mathrm{CuCO}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Copper}\mathrm{hydroxide}}{\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)} \end{gathered}$$

• Hence it is the incorrect option.

Therefore, If copper is kept open in the air, it slowly loses its shining brown surface and gains a green coating. It is due to the formation of option (A) ${\mathrm{CuCO}}_3$