A piece of magnesium ribbon was heated to redness in an atmosphere of nitrogen and then cooled with water. The gas evolved is
 

Magnesium metal reacts with nitrogen in air to form magnesium nitride which on hydrolysis gives ammonia gas.

$3\mathrm{Mg}+{\mathrm{N}}_2\stackrel{\mathrm{burning}}{\to }{\mathrm{Mg}}_3{\mathrm{N}}_2$

${\mathrm{Mg}}_3{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{Mg}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_3$