Question

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’. Write a balanced chemical equation when ‘X’ is dissolved in water.
A. ${\text{2 Mg}}\,\,\,{\text{ + }}\,\,{{\text{O}}_2} \to \,2\,{\text{MgO}}$
B. ${\text{3 Mg}}\,\,\,{\text{ + }}\,\,{{\text{N}}_2} \to \,{\text{M}}{{\text{g}}_3}{{\text{N}}_2}$
C. ${\text{MgO}}\,\,\,{\text{ + }}\,\,{{\text{H}}_2}{\text{O}} \to \,{\text{Mg}}{\left( {{\text{OH}}} \right)_2}$
D. ${\text{Mg}}\,\,{\text{ + }}\,\,{{\text{H}}_2}{\text{O}} \to \,{\text{Mg}}{\left( {{\text{OH}}} \right)_2}$

Answer 1

Hint: On burning magnesium in oxygen, oxides are formed. Oxides on dissolving in water, hydroxides form. When alkaline earth metal burns in presence of oxygen, the oxides of alkaline earth metal forms.

Complete answer:
When burnt alkaline earth metals are placed in a nitrogen environment, the nitrides of alkaline earth metal forms.
When oxides of alkaline earth metal dissolve in water, the hydroxides of alkaline earth metal form.
When a magnesium ribbon is burnt in oxygen a white compound magnesium oxide ${\text{MgO}}$ is formed accompanied by the emission of light. So, compound ‘X’ is magnesium oxide.
If the burning magnesium ribbon is placed in a nitrogen atmosphere, the magnesium nitride ${\text{M}}{{\text{g}}_{\text{3}}}{{\text{N}}_{\text{2}}}$ forms. So, compound ‘Y’ is a magnesium nitride.
When the white compounds magnesium oxide ${\text{MgO}}$ is dissolved in water the magnesium hydroxide forms.
The reaction is represented as follows:
${\text{MgO}}\,\,\,{\text{ + }}\,\,{{\text{H}}_2}{\text{O}} \to \,{\text{Mg}}{\left( {{\text{OH}}} \right)_2}$

So, option (C) is correct.

Additional Information: Option (A) represents the reaction when magnesium ribbon is burnt in oxygen that is the formation of ‘X’ not the reaction of dissolution of ‘X’ so, option (A) is incorrect.
Option (B) represents the reaction when magnesium ribbon is burnt in nitrogen that is the formation of ‘Y’ not the reaction of dissolution of ‘X’ so option (B) is incorrect.
Option (D) represents the reaction on the dissolution of magnesium ribbon in water not the reaction of dissolution of ‘X’ so option (D) is incorrect.

Note: The alkaline earth metal forms ${\text{MO}}$ type oxide. Only the barium is an exception which forms peroxide. The alkaline earth metal forms ${{\text{M}}_3}{{\text{N}}_2}$ type nitrides. The alkaline earth metal forms ${\text{M}}{\left( {{\text{OH}}} \right)_2}$ type hydroxides.