If a substance gains oxygen during a reaction, it is being oxidized. If it loses oxygen, it is being reduced. Oxidation and Reduction always take place together, so that if one substance is oxidized, another is reduced. Using this idea, say which substance is oxidised and which substance is reduced in each reaction.

i) Mg + O₂→ 2MgO

(s) (g) (s)

ii) ZnO + C →Zn + CO

iii) Fe₂ O₃+ 3CO →2Fe + 3CO₂

iv) Cr₂O₃ + 2Al → + 2Cr + Al₂O₃

(i) Mg + O₂→ 2MgO

As oxidation is the process of addition of oxygen and in the above reaction which is a combination reaction oxygen is being added to magnesium to form magnesium oxide. Hence, Magneium is oxidised in the above reaction.

(ii) ZnO + C →Zn + CO

In the above reaction which could be treated as a single displacement reaction oxygen is being removed from the compound Zinc Oxide i.e. Zinc is losing the oxygen atom and Carbon is gaining the oxygen atom. Hence, Zinc is oxidized and Carbon is reduced in the above reaction.

(iii) Fe₂ O₃+ 3CO →2Fe + 3CO₂

In the above reaction, Iron is losing the oxygen atom and Carbon is gaining the oxygen atom. As oxidation processes in which a substance gains oxygen during a reaction and reduction is a process in which a substance losses oxygen during a reaction. Hence, in the above reaction Iron is reduced and Carbon is oxidised.

(iv) Cr₂O₃ + 2Al → + 2Cr + Al₂O₃

In the above reaction Chromium is losing oxygen and Aluminium is gaining the oxygen atoms. Hence, Chromium is being reduced and Aluminium is oxidised.