Which balanced equation represents an oxidation-reduction reaction?

(1) BaCl2 + Na2SO4==>BaSO4 + 2NaCl

(2) C+H2O = = >CO + H2

(3) CaCO3==>CaO + CO2

(4) Mg (OH) 2 + 2HNO3==>Mg (NO3) 2 + 2H2O

Correct Answer: option 2 i. e. (2) C+H2O = = >CO + H2

Reason:

Reaction is said to be oxidation-reduction (redox) type, when one species undergoes oxidation, while other species undergoes reduction. In present case, during the course of reaction, carbon is oxidized from '0' to '+2'. On other hand, hydrogen is reduced from '+1' to '0'.

The answer is (2). The oxidation-reduction reaction means that there is electrons transfer in the reaction. So there is valence change of at least two elements evolving in the reaction.