In basic redox chemistry, which of the following is not a characteristic feature of an oxidation reaction according to modern definitions?

Introduction / Context:
Oxidation and reduction reactions are central concepts in chemistry, from corrosion and combustion to metabolism and industrial processes. Over time, the definition of oxidation has been refined, but several equivalent descriptions are still used in textbooks. This question asks you to identify which statement is not a characteristic of oxidation, meaning it actually describes the opposite process, reduction. Recognising these alternative definitions prevents confusion when you encounter redox reactions in different contexts.

Given Data / Assumptions:

• The focus is on oxidation reactions.
• The options mention addition of hydrogen, addition of oxygen, loss of electrons, and addition of an electronegative element.
• We assume modern redox definitions that relate oxidation to both electron transfer and changes in element combinations.
• We also assume basic familiarity with the concept of reduction as the opposite of oxidation.

Concept / Approach:
Historically, oxidation was defined as a process involving addition of oxygen or removal of hydrogen. Later, a more general electron based definition was adopted: oxidation is the loss of electrons, and reduction is the gain of electrons. Another equivalent way of thinking is that oxidation involves addition of an electronegative element (like oxygen, halogens) or removal of an electropositive element (like hydrogen). Reduction, on the other hand, can involve the addition of hydrogen, loss of oxygen, or gain of electrons. Thus, while addition of oxygen, loss of electrons, and addition of an electronegative element all describe oxidation, addition of hydrogen is characteristic of reduction, not oxidation.

Step-by-Step Solution:
Step 1: Recall the electron based definition. Oxidation is loss of electrons; reduction is gain of electrons. Step 2: Recall the oxygen and hydrogen based definitions. Oxidation often involves addition of oxygen or removal of hydrogen, while reduction often involves addition of hydrogen or removal of oxygen. Step 3: Examine each option. Addition of oxygen is consistent with oxidation. Loss of electrons is the core modern definition of oxidation. Addition of an electronegative element like oxygen or halogens is also oxidation. Addition of hydrogen, however, is characteristic of reduction, not oxidation. Step 4: Conclude that “It involves addition of hydrogen” is not a characteristic of oxidation.

Verification / Alternative check:
Consider a simple example: the conversion of ethene (C2H4) to ethane (C2H6) by addition of hydrogen. Here, hydrogen is added, and the carbon atoms become more reduced (they gain electrons in effect); this is a reduction process. In contrast, when iron reacts with oxygen to form rust (iron oxide), oxygen is added and iron is oxidised. Another example is the reaction of hydrogen sulfide with chlorine, where sulfur is oxidised as it loses electrons and the electronegative element chlorine is added. These examples illustrate that addition of hydrogen is linked with reduction, whereas oxidation is linked with oxygen addition, electron loss, or addition of electronegative elements.

Why Other Options Are Wrong (i.e., they truly describe oxidation):
Option B (It involves addition of oxygen): This matches the classical definition of oxidation in many reactions, especially combustion and metal oxidation. Option C (It involves loss of electrons): This is the core modern definition of oxidation in redox chemistry. Option D (It involves addition of an electronegative element): Adding elements like oxygen or chlorine generally corresponds to oxidation, as these species attract electrons.

Common Pitfalls:
A frequent problem is mixing up the older oxygen based definition with the hydrogen based one and forgetting which direction corresponds to oxidation versus reduction. Another issue is focusing only on one definition (for example, only “gain of oxygen”) and not being able to recognise oxidation when it occurs through a different mechanism such as electron loss. To avoid these mistakes, remember the summary: oxidation is loss of electrons, gain of oxygen, or loss of hydrogen; reduction is gain of electrons, loss of oxygen, or gain of hydrogen. In that summary, “addition of hydrogen” clearly belongs on the reduction side, not oxidation.

Final Answer:
The process that is not a characteristic of oxidation is It involves addition of hydrogen, because that describes reduction.