It appears you have thought this through and
have nearly reached the answer on your own! You
are correct in that you are mainly burning Mg to
form magnesium oxide (MgO). However, as you
noted, the atmosphere is 78% nitrogen, so if you
burned the Mg in air, there is plenty of nitrogen
around. As it turns out, nitrogen will react with
Mg if it is hot enough. Mg burns very hot and is
more than sufficient to get the reaction with
nitrogen going to create some magnesium nitride
(Mg3N2). When you add water
to it, you form MgO and ammonia by the reaction:
Mg3N2 + 3
H20 --> 3 MgO + 2 NH3
that is why you could smell ammonia!
Here is an interesting variation of that
experiment that you could try. Mg does not
necessarily need O2 to form MgO. In
fact, if you light Mg and then dip it into
CO2, it will continue to "burn"
according to the following equation:
CO2 --> 2MgO + C
By immersing it
in an atmosphere of CO2, you can
eliminate the reaction with nitrogen so that no
Mg3N2 is formed (or at least
very minimal amounts from the transfer). What you
get instead is a mixture of MgO (white powder) and
black carbon ash.
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