Question

Which of the following is not a redox reaction?
A) Burning of candle
B) Rusting of iron
C) Dissolving salt in water
D) Dissolving Zinc in dil ${H_2}S{O_4}$

Answer 1

Redox reactions are those reactions in which Oxidation and Reduction both take place simultaneously. Oxidation is the loss of electrons and reduction is the gain of electrons. Oxidation often leads to increase in Oxidation number and reduction leads to decrease in the oxidation number.

Complete Step By Step Answer:
We are given four reactions. We can find if it is redox or not by knowing if the oxidation number of any species has changed. If there is change in the oxidation states (decrease as well as increase) we can term it as a redox reaction.
A) The first reaction given is the burning of the candle. We all know that Oxygen gas supports combustion and is also responsible for the burning of candles, Burning of paper etc. The reaction that occurs during burning of candle is as follows; When a candle is lit, the heat vaporizes the liquid wax and starts to break the Hydrocarbons into Hydrogen and Carbon. The vapourised molecules react with the Oxygen to create heat, light, water vapour and carbon Dioxide. Here the Oxygen acts as the Oxidant and Hydrocarbon is the reductant. Hence it is a redox reaction.
B) Rusting of iron occurs when Iron is exposed to Oxygen and moisture. The reaction that occurs is:
$4Fe + 3{O_2} + {H_2}O \to 4Fe{(OH)_3}$
In this reaction Iron is Oxidized from zero to +3 state and Oxygen is reduced from 0 to -2 state. Hence this is a redox reaction.
C) Dissolving salt in water doesn’t accompany any chemical reaction as it doesn’t produce any new ion or compound. Dissolving salt in water gives a homogenous solution. Hence this is a Redox Reaction.
D) When Zinc is dissolved in Sulphuric acid the reaction that occurs is:
$Zn + {H_2}S{O_4} \to ZnS{O_4} + {H_2}$
Here the Oxidation state of Zn changes from 0 to +2. It is oxidized and Hydrogen is reduced from +1 to 0. Hence it is a redox reaction.
The correct answer is Option (C).

Note:
If we are asked to find out whether a reaction is redox or not, remember to find out the oxidation states of all the species involved. Both Oxidation and Reduction must take place for the reaction to be redox. Remember that the Oxidation state of Neutral Species is Zero.