An element X which is in Grey colour on exposure to moist air turns black and a new compound Y is formed. The substance X and Y are

The given X with grey color may be Ag . When ‘X’ reacts with moist air it will convert into black color solid compound, Y. Then the compound “Y must be ${\text{Ag}}_{\text{2}}\text{S}$”.  Therefore, the net chemical equation is

$\begin{array}{l}2\text{\hspace{0.17em}Ag\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}+2{\text{\hspace{0.17em}H}}_{\text{2}}\text{S\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}+{\text{\hspace{0.17em}\hspace{0.17em}O}}_{\text{2}}\to 2{\text{\hspace{0.17em}Ag}}_{\text{2}}\text{S\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}+2{\text{\hspace{0.17em}\hspace{0.17em}H}}_{\text{2}}\text{O}\\ \text{\hspace{0.17em}\hspace{0.17em}Grey\hspace{0.17em}color\hspace{0.17em} \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}Black\hspace{0.17em}color}\end{array}$

Therefore, ${\text{X = Ag \&\hspace{0.17em}Y = Ag}}_{\text{2}}\text{S}$