$A$ 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.

(N/A) Metals such as silver,when exposed to substances in their surroundings like moisture,acids,or gases,undergo a process called corrosion. In the case of silver,it reacts with sulfur compounds in the air,leading to the formation of a black layer. This phenomenon is known as the corrosion of metals.
$(b)$ The black substance formed is silver sulphide. Its chemical formula is $Ag_2S$.