What does oxidation refer to in a chemical reaction?

Oxidation in a chemical reaction is defined primarily as the loss of electrons or the addition of oxygen to a substance. This process is a key component of redox (reduction-oxidation) reactions, where one species loses electrons and another gains them. When a substance undergoes oxidation, it usually increases its oxidation state.

The addition of oxygen often accompanies the loss of electrons because, in many reactions, oxygen can act as an electron acceptor. For instance, when iron rusts, it oxidizes as it reacts with oxygen in the presence of moisture, forming iron oxide.

The other options do not accurately capture the definition of oxidation. The gain of electrons describes reduction rather than oxidation. The exchange of carbon atoms is not relevant to oxidation reactions specifically. Finally, the reduction of hydrogen ions does not encompass the broader concept of oxidation, which is more concerned with electron transfer and the alteration of oxidation states.