A white solid (X) on heating forms a solid (Y) and a gas (Z). Another solid (B) forms ( X ) by reacting with ( Z ).( Y ) can be converted into ( X )…

A white solid (X) on heating forms a solid (Y) and a gas (Z). Another solid (B) forms $\displaystyle (\mathrm{X})$ by reacting with $\displaystyle (\mathrm{Z}) .(\mathrm{Y})$ can be converted into $\displaystyle (\mathrm{X})$ and $\displaystyle (\mathrm{B})$. What is (Y)?