$A$ magnesium ribbon is burnt in oxygen to give a white compound $X$ accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen,it continues to burn and forms a compound $Y.$
$(a)$ Write the chemical formulae of $X$ and $Y.$
$(b)$ Write a balanced chemical equation,when $X$ is dissolved in water.

(A) When magnesium ribbon burns in oxygen,it forms magnesium oxide $(MgO)$:
$2Mg(s) + O_2(g) \longrightarrow 2MgO(s)$ (Compound $X$)
When magnesium ribbon continues to burn in nitrogen,it forms magnesium nitride $(Mg_3N_2)$:
$3Mg(s) + N_2(g) \longrightarrow Mg_3N_2(s)$ (Compound $Y$)
$(a)$ The chemical formula of $X$ is $MgO$ and the chemical formula of $Y$ is $Mg_3N_2$.
$(b)$ When magnesium oxide $(X)$ is dissolved in water,it forms magnesium hydroxide:
$MgO(s) + H_2O(l) \longrightarrow Mg(OH)_2(aq)$