NDA & CDS 1 2026 Exam Chemistry –MCQ Class 5

Introduction

Redox reactions (Reduction–Oxidation reactions) are one of the most important concepts in chemistry and frequently appear in competitive exams like NDA & CDS Examination. A redox reaction involves the transfer of electrons between chemical species. In every redox process, oxidation and reduction occur simultaneously.

• Oxidation is the loss of electrons or increase in oxidation number.
• Reduction is the gain of electrons or decrease in oxidation number.

For example:$$Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$$Zn+Cu2+→Zn2++Cu

Here:

• Zinc loses electrons → Oxidation
• Copper ion gains electrons → Reduction

Thus, the substance that loses electrons is called the reducing agent, while the substance that gains electrons is called the oxidizing agent.

Redox reactions are fundamental to many natural and industrial processes such as corrosion, respiration, combustion, electrochemical cells, and metallurgy.

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Key Concepts in Redox Reactions

1. Oxidation and Reduction

Originally defined in terms of oxygen and hydrogen:

• Oxidation → Addition of oxygen / removal of hydrogen
• Reduction → Removal of oxygen / addition of hydrogen

Modern definition focuses on electron transfer.

Example:$$2Mg + O_2 \rightarrow 2MgO$$2Mg+O2​→2MgO

• Magnesium is oxidized
• Oxygen is reduced

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2. Oxidation Number Concept

The oxidation number represents the apparent charge of an atom in a compound.

Basic rules:

• Free elements → oxidation number 0
• Oxygen usually –2
• Hydrogen usually +1
• Alkali metals +1, alkaline earth metals +2

Example:$$H_2SO_4$$H2​SO4​

Oxidation numbers:

• H = +1
• O = −2
• S = +6

Changes in oxidation number help identify which species undergo oxidation or reduction.

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3. Oxidizing and Reducing Agents

• Oxidizing agent → causes oxidation and itself gets reduced.
• Reducing agent → causes reduction and itself gets oxidized.

Example:$$Fe^{2+} + Cl_2 \rightarrow Fe^{3+} + 2Cl^-$$Fe2++Cl2​→Fe3++2Cl−

• $Cl_2$Cl2​ → Oxidizing agent
• $Fe^{2+}$Fe2+ → Reducing agent

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4. Types of Redox Reactions

1. Combination reactions
2. Decomposition reactions
3. Displacement reactions
4. Disproportionation reactions

Example of disproportionation:$$2H_2O_2 \rightarrow 2H_2O + O_2$$2H2​O2​→2H2​O+O2​

Here, hydrogen peroxide undergoes both oxidation and reduction.

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Importance of Redox Reactions

Redox processes are essential in:

• Electrochemistry and batteries
• Corrosion and rusting
• Photosynthesis and respiration
• Metallurgy (extraction of metals)
• Industrial chemical reactions

They also play a crucial role in energy production and biological systems.

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Relevance for NDA/CDS 1 2026 Exam

For the NDA & CDS Examination, redox reactions are commonly tested through objective-type MCQs. Important areas to focus on include:

• Identification of oxidation and reduction
• Oxidation number calculations
• Recognizing oxidizing and reducing agents
• Balancing redox equations
• Types of redox reactions

Questions are usually conceptual and calculation-based, requiring a clear understanding of electron transfer and oxidation number changes.

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Conclusion

Redox reactions form the backbone of many chemical and biological processes. Understanding the concepts of oxidation, reduction, oxidation numbers, and oxidizing/reducing agents allows students to analyze a wide variety of chemical reactions effectively. For aspirants preparing for the NDA/CDS 1 2026 examination, mastering redox reactions is essential, as these concepts frequently appear in MCQs and help build a strong foundation for advanced topics such as electrochemistry and chemical energetics.