1. Take a magnesium ribbon and weigh it
2. now burn the magnesium ribbon
3. now collect the ash and weigh it
4. you finds that weight of the ribbon before buring =/= weight of the ribbon after burning
5. you find that the weight of the ribbon i.e., ash is high
6. this is due to the formation of oxide since the burning process only held in the presence of oxygen.
7. hence magnesium combines with oxygen and form magnesium oxide
equation : 2Mg + O2 ----------> 2MgO