Why do silver articles turn black when kept open for a few days?
$(a)$ Name the phenomenon involved
$(b)$ Name the black substance formed and give its chemical formula

Hint – In this question use the concept that the traces of hydrogen sulfide which is present in air reacts with silver to form silver sulfide that is $A{g_2}S$, it is black in color. Use this concept to get the answer of this problem statement.

Complete answer:

As we know there is some is some amount of traces of hydrogen sulfide (H2S) is present in the air so silver react with this compound and forms silver sulfide (Ag2S) which is black in nature and one of the most insoluble salts of silver and this is what causes the tarnish (i.e. a result of exposure to air and moisture).
The chemical reaction involved in this process is shown below,
$ \Rightarrow 2Ag\left( {{\text{silver}}} \right) + {H_2}S \to A{g_2}S + {H_2}$
$\left( i \right)$ So the phenomenon is called corrosion.
For silver particularly, it is called tarnishing of silver.
$\left( {ii} \right)$ And the substance which is formed is silver sulfide (Ag2S) which is black in nature.
So this is the required answer.

Note – A very interesting aspect of ${H_2}S$, is that the effect of it to high exposure greater than ( 100 ppm) can be dangerous and life-threatening. Effects include shock, convulsions, inability to breathe, rapid unconsciousness, coma or even to death.