Question

Question 38

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.

Answer 1

(a) Metals such as silver when attacked by moisture, acids, gases etc, present in their surroundings, undergo gradual deterioration and this phenomenon is called corrosion.

(b) The black substance is the coating of silver sulphide ${\text{Ag}}_2\text{S}$ formed when silver $\text{Ag}$ reacts with ${\text{H}}_2\text{S}$ present in air. The reaction occurs as follows:

$\text{Ag}+{\text{O}}_2+{\text{H}}_2\text{S}\to {\text{Ag}}_2\text{S}+{\text{H}}_2\text{O}$