Which one of the following is not an example of an oxidation reaction ?

Oxidation reactions involve the addition of oxygen or the loss of electrons. The change in the taste of butter (Option 1) is due to oxidative rancidity, where fats and oils are oxidized when exposed to air [1]. The reddish-brown coating on iron (Option 3) is rust, formed by the oxidation of iron in the presence of moisture and air. The souring of wine (Option 4) occurs when ethanol is oxidized into acetic acid. In contrast, the reaction between barium chloride and sodium sulfate (Option 2) is a double displacement reaction [1]. In this process, the compounds exchange ions to form a white precipitate of barium sulfate and sodium chloride [2]. Since there is no change in the oxidation states of the elements involved, it is classified as a precipitation reaction rather than an oxidation-reduction reaction.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > 1.3.2 Rancidity > p. 13
2. [2] https://en.wikipedia.org/wiki/Rancidification