Sodium burning in pure oxygen, image 1 of 2. This is an exothermic reaction (it releases heat), as seen by the bright flame here. Sodium (Na) is a very reactive metal that must be stored under oil as it reacts with oxygen (O2) in the atmosphere. The equation for this reaction is: 2Na + O2 ---> 2NaO For an image of the combustion jar full of white fumes of sodium oxide (NaO) after this reaction, see image A510/212.