What is the definition of an oxidising agent?

An oxidising agent is defined as a substance that facilitates the oxidation of another species while undergoing reduction itself. This means that the oxidising agent loses electrons, causing the other substance to gain those electrons (thus undergoing oxidation). In the context of the given choices, this definition aligns perfectly with the correct answer, which describes a substance that oxidizes another while being reduced.

To clarify further, in a redox reaction, the oxidising agent causes the oxidation of another reactant and, in the process, it gains electrons, resulting in a decrease in its oxidation state. The role of the oxidising agent is crucial in redox reactions, as it drives the reaction forward by accepting electrons.

The other choices suggest different characteristics that do not capture the essence of what an oxidising agent does. For instance, a substance that reduces another while being oxidized (the first choice) fails to represent the unique role of the oxidising agent. Similarly, the idea of a substance that cannot be reduced or one that remains unchanged during a reaction does not align with the behavior of an oxidising agent, which must experience a change in oxidation state. Thus, choice C is the most accurate representation of an oxidising agent's function in chemical reactions.