Concept:The reaction in which the addition of oxygen to a substance or the removal of hydrogen from a substance takes place is called an oxidation reaction.Explanation:The taste of butter changes if left for a longer period is an example of oxidation reaction.Butter is a dairy product that contains 80% fat.Butter oxidises when it reacts with oxygen in the air.It becomes rancid as a result of oxidation, i.e it gives foul smell and taste.A white substance is formed when an aqueous solution of barium chloride is mixed with sodium sulphate solution is not an example of an oxidation reaction.When barium chloride and sodium sulphate combine in an aqueous solution, a white precipitate of barium sulphate is formed that is insoluble in water.These reactions are ionic in nature and involve double displacement.A reddish-brown powder that gets coated on articles made of iron is an example of an oxidation reaction.When new, iron articles are shiny, but after a while, they become coated with a reddish brown powder. This is commonly referred to as iron rusting. Thus, iron rusting is an oxidation reaction.Rust is a reddish-brown powder that forms when a metal is attacked by substances such as moisture, acids, and so on.Wine turns sour when exposed to the air is an example of an oxidation reaction. It becomes sour due to aerobic bacteria that allow the wine's alcohol content to oxidise.