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Option 1: The taste of butter changing over time is an example of oxidation. When unsaturated fats in butter are exposed to oxygen, they undergo oxidation, leading to a change in taste.
Option 2: This is the correct answer. Mixing barium chloride with sodium sulphate solution results in a precipitation reaction, not an oxidation reaction. No oxygen is involved in this reaction.
Option 3: The reddish-brown powder coating on iron articles is rust, which occurs due to the oxidation of iron in the presence of oxygen and moisture.
Option 4: Wine getting sour over time is also an example of oxidation. The acetic acid-producing bacteria oxidize the ethanol in wine to acetic acid, resulting in a sour taste.
Therefore, option 2 is not an example of an oxidation reaction, while the other options involve oxidation.