Question

a silver article turns black when kept in the open for few days the article when rubbed with toothpaste it regards it's shine
a) why it again starts shining by rubbing with toothpaste ?

Answer 1

When silver articles are kept in the open, then a layer of silver oxide is formed on the articles due to the reaction of silver with the oxygen present in the air. Silver oxide is black in colour. Finally, after sufficient time, it will form silver sulphide after coming in contact with oxygen and Hydrogen sulphide present in the atmospheric air. Silver sulphide is also black in colour.

4Ag + O2 + 2H2S → 2Ag2S + 2H2O

This process is termed as tarnishing of silver or corrosion. As toothpastes consist of several abrasive substances (like, Aluminium hydroxide), this layer of silver sulphide can be removed by rubbing the article with toothpaste.

3Ag2S + 2AI → 6Ag + Al2S3

This reaction leaves the silver metal all shiny.