The first step in this sequence is the combustion of carbon (C) to produce carbon dioxide (CO2).

C + O2 -> CO2

The next step is the reaction of carbon dioxide (CO2) with calcium oxide (CaO) to form calcium carbonate (CaCO3).

CO2 + CaO -> CaCO3

The third step involves the decomposition of calcium carbonate (CaCO3) to produce carbon dioxide (CO2) and calcium oxide (CaO).

CaCO3 -> CO2 + CaO

The final step is the reaction of carbon dioxide (CO2) with sodium oxide (Na2O) to form sodium carbonate (Na2CO3).

CO2 + Na2O -> Na2CO3

Overall, the complete sequence of reactions is:

C + O2 -> CO2
CO2 + CaO -> CaCO3
CaCO3 -> CO2 + CaO
CO2 + Na2O -> Na2CO3