Question

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.(b) Name the black substance formed and give its chemical formula.

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Solution

(a). Corrosion:It is basically defined as the destruction or the degradation of the metal or the alloy and just because of the chemical or electrochemical reaction within the medium and the environment.Therefore, this phenomenon is actually known as the Corrosion.(b). Formation of black substance:Silver sulfide is actually formed when the ${\mathrm{H}}_{2}\mathrm{S}$ gas present in air slowly reacts with the Silver which is present in the silver article makes it look black in color.So, the layer of Silver sulfide which is deposited on the metal's surface is black in color.Reaction would be: $\underset{\mathrm{Silver}}{2\mathrm{Ag}\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{sulfide}}{{\mathrm{H}}_{2}\mathrm{S}\left(\mathrm{g}\right)}\to \underset{\mathrm{Silver}\mathrm{sulfide}}{{\mathrm{Ag}}_{2}\mathrm{S}\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}}{{\mathrm{H}}_{2}\left(\mathrm{g}\right)}$Therefore, the black substance formed is Silver sulfide and it's chemical formula is ${\mathrm{Ag}}_{2}\mathrm{S}$.

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