1) S + O2 = SO2
2)
SO2 + 2NaOH = Na2SO3 + H2O
SO2 + 2Na[+] + 2OH[-] = 2Na[+] + SO3[2-] + H2O
SO2 + 2OH[-] = SO3[2-] + H2O
3)
Na2SO3 + BaCl2 = BaSO3 + 2NaCl
2Na[+] + SO3[2-] + Ba[2+] + 2Cl[-] = BaSO3 + 2Na[+] + 2Cl[-]
Ba[2+] + SO3[2-] = BaSO3
4) BaSo3 = BaO + SO2