Question

A tarnished copper vessel begins to shine again when rubbed with lemon. Why ?

Answer 1

Copper vessels are tarnished due to the copper oxide layer formed on their surface. Copper oxide is basic in nature.

The green coating on copper vessels is due to the formation of the basic copper carbonate formed by the attack of ${\mathrm{O}}_2,{\mathrm{CO}}_2$, and moisture of the air on the copper vessel.

$\underset{\mathrm{Copper}}{2\mathrm{Cu}\left(\mathrm{s}\right)}+\underset{\mathrm{From}\mathrm{moist}\mathrm{air}}{\underbrace{{\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}\to \underset{\mathrm{Basic}\mathrm{Copper}\mathrm{carbonate}\left(\mathrm{Green}\right)}{\underbrace{{\mathrm{CuCO}}_3.\mathrm{Cu}{\left(\mathrm{OH}\right)}_2}}$

Lemon juice is acidic in nature due to the presence of Citric acid$\left({\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_7\right)$.

1. When it is rubbed on the surface of tarnished copper utensils, it reacts with basic Copper oxide and Copper carbonate.
2. Since this is a neutralization reaction, a salt is formed which can then be removed by rinsing the vessel with water.
3. Thus the colored tarnish is removed and the original color and luster of the copper vessel are retained. The chemical reaction is:$\underset{\left(\mathrm{Copper}\right(\mathrm{II})\mathrm{oxide})}{3\mathrm{CuO}}+\underset{\left(\mathrm{Citric}\mathrm{Acid}\right)}{2{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_7}\to \underset{\left(\mathrm{Copper}\left(\mathrm{II}\right)\mathrm{citrate}\right)\left(\mathrm{Salt}\right)}{{\mathrm{Cu}}_3{\left({\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{O}}_7\right)}_2}+\underset{\left(\mathrm{Water}\right)}{3{\mathrm{H}}_2\mathrm{O}}$