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I noticed that when we oxidize magnesium and "ash" it, we are asked to place a few drops of distilled water on the ash and try to determine a faint ammonia smell. We did. My question then is: If the mass of the reactants equals the mass of the products (Conservation of mass law), where did the nitrogen smell come from? I thought we were reacting (burning), magnesium (magnesium and oxygen reactants only) and the product was just magnesium oxide? I balance the equation and I don't see any nitrogen anywhere. I know the atmosphere is 78% nitrogen but how is this affecting the chemical reaction from a chemical equation standpoint?
Answer 1:

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
And 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:

2Mg + 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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