CBSE Class 10 Science 2020 · Paper 31/1/3 · Question 28

Carbon cannot reduce the oxides of sodium, magnesium and aluminium because these metals have more affinity for oxygen than carbon: they hold oxygen more strongly than carbon does, so carbon cannot remove the oxygen from their oxides. These metals are placed towards the top of the reactivity series ($\text{K}$, $\text{Na}$, $\text{Ca}$, $\text{Mg}$, $\text{Al}$, above zinc). They are obtained from their ores by electrolytic reduction, that is, by electrolysis of their molten compounds: sodium and magnesium from their molten chlorides, aluminium from its oxide. Example, sodium from sodium chloride: molten $\text{NaCl}$ is taken in an electrolytic cell and an electric current is passed; sodium is deposited at the cathode (negative electrode) and chlorine gas is liberated at the anode (positive electrode). At the cathode: ${\text{Na}}^{+}+e^{-}\to \text{Na}$; at the anode: $2{\text{Cl}}^{-}\to {\text{Cl}}_2+2e^{-}$; overall: $2\text{NaCl}\to 2\text{Na}+{\text{Cl}}_2$. Any other correct example, such as magnesium from molten magnesium chloride, earns the same credit.