Most metal oxide formation lines have positive slope (Delta S is negative for M + O2 → MO: gas consumed → disorder decreases). At melting/boiling points, slope changes (phase change alters entropy). Carbon line is special: (a) C + O2 → CO2 has nearly zero slope (1 mol gas → 1 mol gas, Delta S ≈ 0). (b) 2C + O2 → 2CO has negative slope (1 mol gas → 2 mol gas, Delta S positive). At high T, the 2C + O2 → 2CO line goes very low → carbon becomes an excellent reducing agent. Aluminium line (4Al + 3O2 → 2Al2O3) is very low on the diagram → Al can reduce Fe2O3, Cr2O3, Mn3O4 (thermite reaction) but NOT MgO, CaO, Na2O (these lines are even lower).