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Oxidation is the process in which an atom or molecule loses one or more electrons in a chemical reaction. During this process, the oxidation state of the atom or molecule increases. Reduction, on the other hand, is the process in which an atom or molecule gains one or more electrons in a chemical reaction, resulting in a decrease in the oxidation state.

Oxidation and reduction are always linked in reactions because they involve the transfer of electrons. When one species (an atom or molecule) loses electrons, it becomes oxidized. At the same time, another species must gain these electrons, undergoing reduction. This concept of coupled reactions is known as redox (oxidation-reduction) reaction. As a result of the electron transfer between the two species, the oxidizing agent is reduced, while the reducing agent is oxidized. This relationship shows that oxidation cannot take place without reduction and vice versa, as electrons cannot be created or destroyed but only transferred from one species to another.

Consider the example of the reaction between metallic zinc (Zn) and copper sulfate (CuSO4): \(Zn(s) + CuSO4(aq) \rightarrow ZnSO4(aq) + Cu(s)\) In this reaction, metallic zinc loses two electrons and becomes the zinc ion (Zn^(2+)). This is an example of oxidation: \(Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-\) At the same time, the copper ion (Cu^(2+)) in the copper sulfate solution gains two electrons and forms metallic copper: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\) This part of the reaction is an example of reduction. The overall reaction, which demonstrates the simultaneous occurrence of the oxidation of Zn and the reduction of Cu^(2+), is: \(Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)\) Thus, we can conclude that oxidation and reduction must always occur together because they are linked by electron transfer. One species loses electrons, and another species gains those electrons, completing the redox reaction.