5.16 The Reaction Between Magnesium and CO2

Chemical Concepts Demonstrated: Active metal chemistry, the limited usefulness of CO2 fire extinguishers


  • A slab of dry ice is cut in half and a small "crater" is melted into one of the pieces.
  • An ignited magnesium strip is placed in the "crater", and the other half of the dry ice is placed on top.


    The magnesium strip burns brightly in the air, but continues to burn in the carbon dioxide environment.

Explanations (including important chemical equations):

    This reaction is a combustion and oxidation.  What's unusual, however, is that magnesium is reactive enough to be combusted and oxidized in a reaction with carbon dioxide:

    2 Mg + CO2 -> 2 MgO + C

    Under normal combustion/oxidation circumstances, oxygen is the reactant.

    Carbon dioxide fire extinguishers work by smothering a fire in carbon dioxide.  This is only an effective means of extinguishing a fire if carbon dioxide itself cannot be used as a fuel source.