The NDA CDS 1 2026 Examination is a national-level exam conducted by the UPSC to recruit candidates for the Armed Forces—Army, Navy, and Air Force. The written exam includes General Knowledge, English, and Elementary Mathematics, where General Knowledge consists of important topics from Physics, Chemistry, Biology, History, Polity, Economy, Geography, and Current Affairs.
Chemistry MCQs are usually conceptual and application-based, making topics like Redox Reactions and Physical & Chemical Changes essential for scoring high.
Class 3 Topic Overview
1. Introduction
In today’s class, we focus on two fundamental areas of Chemistry that frequently appear in NDA CDS exams—Redox Reactions and Physical & Chemical Changes. These concepts form the foundation for understanding chemical behavior, reaction mechanisms, and real-life applications. Mastering these ideas helps in answering objective questions quickly and accurately.
2. Subtopics to be Covered
A. Redox Reactions
1. Concept of Oxidation and Reduction
- Oxidation: Loss of electrons / gain of oxygen / loss of hydrogen
- Reduction: Gain of electrons / loss of oxygen / gain of hydrogen
- Oxidizing and reducing agents
2. Redox Reaction in Terms of Electron Transfer
- Electron donor and electron acceptor
- Identification of oxidation and reduction half-reactions
3. Oxidation Number Method
- Rules for assigning oxidation numbers
- Calculating oxidation numbers in compounds and ions
- Identifying change in oxidation state
4. Balancing Redox Reactions
- Oxidation number method
- Ion-electron method (acidic and basic medium)
5. Types of Redox Reactions
- Combination reactions
- Decomposition
- Displacement reactions
- Disproportionation reactions
6. Applications of Redox Reactions
- Corrosion
- Respiration
- Photosynthesis
- Metal extraction
- Everyday chemical reactions
B. Physical and Chemical Changes
1. Concept of Physical Change
- No new substance formed
- Reversible in nature
- Changes in state, shape, size
- Examples: melting of ice, boiling of water, breaking glass
2. Concept of Chemical Change
- New substances formed
- Irreversible in most cases
- Involves chemical reactions
- Examples: rusting of iron, burning, cooking food
3. Key Differences Between Physical and Chemical Changes
- Change in composition
- Energy change
- Reversibility
- Observations like colour change, gas formation, precipitate, heat/light
4. Indicators of Chemical Reactions
- Evolution of gas
- Formation of precipitate
- Colour change
- Change in temperature (exothermic, endothermic)
5. Daily Life Examples
- Dissolving sugar vs burning sugar
- Evaporation vs combustion
- Cutting paper vs burning paper
3. Conclusion
In this class, we explored the basics of Redox Reactions and Physical & Chemical Changes, both crucial for understanding chemical processes and for tackling NDA CDS 2026 Chemistry MCQs. A strong grasp of oxidation-reduction principles and differences between types of changes strengthens conceptual clarity and improves accuracy in objective questions. These topics form the backbone of many advanced concepts in chemistry and are highly relevant to real-life applications as well.
