A Level Chemistry OCR Practice Exam

Which of the following defines oxidation in a chemical reaction?

1. Gain of electrons

2. Loss of electrons or an increase in oxidation number

3. Formation of a covalent bond

4. Decrease in temperature

The correct answer is: Loss of electrons or an increase in oxidation number

Oxidation in a chemical reaction is accurately defined by the loss of electrons or an increase in oxidation number. When a substance undergoes oxidation, it typically involves the transfer of electrons to another species, resulting in an increase in its oxidation state. For example, when iron (Fe) is oxidized to form iron ions (Fe²⁺ or Fe³⁺), it loses electrons, which leads to a higher oxidation state. This process is essential in redox (reduction-oxidation) reactions, where one species is oxidized while another is reduced. The other concepts presented in the choices do not align with the definition of oxidation. The gain of electrons corresponds to reduction, which is the opposite process. The formation of a covalent bond involves sharing electrons rather than transferring them, which is unrelated to oxidation or reduction. A decrease in temperature has no direct connection to oxidation; it pertains to thermodynamics and kinetics of reactions rather than the electron transfer processes involved in oxidation.