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A white residue is obtained when metalli...

A white residue is obtained when metallic Mg is burnt in air. This residue when heated with water emits an ammoniacal smell. Explain these observations.

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On burning metallic magnesium in air, magnesium oxide (MgO) and magnesium nitride `(Mg_3N_2)` are formed. So, the white residue obtained is a mixture ofMgO & `Mg_3N_2`.
`2Mg + O_2 overset(Delta)to 2MgO , 3Mg + N_2 (Delta) to Mg_3 N_2`
This residue when heated with water, results in the hydrolysis of `Mg_3N_2` which emits an arnmoniacal smell.
`Mg_3 N_2 + 6H_2O overset(Delta) to 3Mg (OH)_2 + 2NH_3uarr`
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