Question

A silver article generally turns black when kept in the open for few days. The article when rubbed with toothpaste again starts shining.

a. Why do silver articles turn black when kept in the open for few days? Name the phenomenon involved.

b. Name the black substance formed and give it's chemical formula. ​

Answer 1

a) Silver article turns black when kept in the air because silver article reacts with sulphur compounds such as hydrogen sulphide (H2S) present in air. The phenomenon is called corrosion. For silver particularly, it is called tarnishing of silver.

Answer 2

a. Metals such as silver are attacked by air, moisture, acids, gases, etc. present in air and form undesirable compounds on their surfaces. Silver tarnishes and gets black coating. This process is called corrosion.

b. The black substance formed because silver reacts with H₂S present in air to form silver sulphide(Ag₂S).

Toothpaste generally contains traces of hydrogen peroxide as its one of the constituents. It reacts with black Ag₂S formed on the surface and restore whiteness.

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