When green coloured crystals of a …

The salt $X$ is likely ammonium dichromate, $(NH_4)_2Cr_2O_7$, which is orange, but it turns green when it decomposes. When heated, it undergoes a decomposition reaction to produce chromium(III) oxide ($Cr_2O_3$), nitrogen gas ($N_2$), and water vapor ($H_2O$). The green-colored substance is chromium(III) oxide, and the gas with a pungent smell is nitrogen dioxide ($NO_2$) which is usually formed due to secondary reactions. The decomposition reaction of ammonium dichromate can be represented by the following chemical equation: $$(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O$$ During the heating process, some $NH_3$ may be released, which can further oxidize to $NO_2$ in the presence of heat and oxygen. So, the salt $X$ is ammonium dichromate, $(NH_4)_2Cr_2O_7$, and the gas $Y$ is nitrogen dioxide, $NO_2$. Here is the full balanced chemical equation: $$(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O$$ When considering secondary reactions for the release of $NO_2$: $$4NH_3 + 7O_2 \rightarrow 4NO_2 + 6H_2O$$