CBSE Class 12 board exam: the topics that carry the marks
In Class 12 a few units carry most of the marks, and the same ideas return as reasoning questions every year. These are the high-weightage topics, each explained in a one-minute animated lesson, in English and Hindi, with the exam-style answer written out.
Physics
Theory is 70 marks with 30 for practicals. Optics, magnetism and electrostatics are the heaviest units in published breakdowns, and electromagnetic induction and modern physics keep returning as reasoning questions.
- Why can electric field lines never cross? — Class 12Electrostatics: one of the three heaviest Physics units
- Electric Field vs Electric Potential — the vector and the scalar
- Why is electric potential a scalar, not a vector? — Class 12
- EMF vs Potential Difference — why a battery reads less than its own label
- Why can't a transformer work on DC? — Class 12Electromagnetic induction and AC: a classic reasoning question
- Why is AC preferred over DC for long-distance transmission?
- Interference vs Diffraction — same superposition, a different number of sourcesWave optics
- Convex vs Concave Lens/Mirror — does convex always mean converging?Ray optics: the heaviest Physics unit, about 14 marks
- Why can a convex mirror never form a real image? — Class 10 Physics
- Nuclear Fission vs Fusion — why splitting AND joining nuclei both release energy
- Alpha vs Beta vs Gamma Radiation — why paper stops one but lead barely stops another
Chemistry
Theory is 70 marks across ten units. Electrochemistry (9) and Aldehydes, Ketones and Carboxylic Acids (8) are the largest, followed by several 7-mark units: Solutions, Chemical Kinetics, d- and f-Block, Coordination Compounds and Biomolecules.
- Galvanic vs Electrolytic Cell — Class 12Electrochemistry: the largest Chemistry unit, 9 of 70 marks
- Molarity vs Molality vs Normality — does heating a solution change its concentration?Solutions: 7 of 70 marks
- Oxidation vs Reduction — redox without a trace of oxygen
- EMF vs Potential Difference — why a battery reads less than its own label
- Why do ionic compounds conduct only when molten or dissolved?
- Sigma vs Pi Bond — why a double bond isn't two single bonds stackedBonding basics that organic chemistry builds on
- Saturated vs Unsaturated Hydrocarbons — the same word as your kitchen oil
Biology
Theory is 70 marks. Genetics and Evolution is the largest unit at 20, then Reproduction at 16, then Biology and Human Welfare and Biotechnology at 12 each and Ecology at 10.
- Transcription vs Translation — Class 12Molecular basis of inheritance: inside the 20-mark Genetics unit
- DNA vs RNA — the master copy never leaves the building
- Genotype vs Phenotype — can you tell someone's genotype just by looking at their phenotype?
- Dominant vs Recessive Traits — does dominant mean more common, or recessive mean it vanishes?
- Codominance vs Incomplete Dominance — does a 'mixed' phenotype always mean the same thing?
- Why is blood group AB the universal recipient? — Class 12 Biology
- Why does the father decide the baby's sex? — Class 10
- Mitosis vs Meiosis — why your body runs two completely different kinds of divisionReproduction: 16 of 70 marks
- Bacteria vs Virus — will antibiotics actually cure your flu?
- How does vaccination protect us from disease? — Class 9
Mathematics
Theory is 80 marks with 20 for internal assessment. Calculus alone is 35 of the 80, about 44 percent. Vectors and 3D Geometry is 14, Algebra 10, Relations and Functions 8, Probability 8 and Linear Programming 5.
- Definite vs Indefinite Integral — Class 12Calculus: 35 of 80 marks
- Differentiation vs Integration — two questions about the same curve
- Domain vs Range — why one value can quietly break a functionRelations and Functions: 8 of 80 marks
- Dot Product vs Cross Product — Class 12Vectors and 3D Geometry: 14 of 80 marks
- What is conditional probability? P(A|B) explained — Class 12Probability: 8 of 80 marks
- Independent vs Mutually Exclusive Events — the trap question examiners love
- Permutations vs Combinations — the one question that decides which formula to use
- Why is 0! equal to 1? — Class 11
Artificial Intelligence (843)
For students taking the Class 12 AI subject (code 843): units include Python Programming, Data Science Methodology, Making Machines See, Big Data, Understanding Neural Networks and Generative AI.
How to use this page
Start with the unit that carries the most marks in your weakest subject, open a lesson, and answer the quick check at the end. Each lesson ends with the way to write the answer in the exam.
Unit marks are from published summaries of the 2025-26 syllabus and are a guide to what to revise, not a prediction. CBSE revises the syllabus every year.
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Common questions
- How are the CBSE Class 12 Science papers structured?
- Physics, Chemistry and Biology each have a 70-mark theory paper and 30 marks for practicals. Mathematics has an 80-mark theory paper and 20 marks of internal assessment.
- Which units carry the most marks?
- In Maths, Calculus is 35 of 80. In Biology, Genetics and Evolution is 20 of 70 and Reproduction is 16. In Chemistry, Electrochemistry is 9 and Aldehydes, Ketones and Carboxylic Acids is 8. In Physics, published breakdowns put Optics, Magnetism and Electrostatics highest. These are summaries of the 2025-26 syllabus; CBSE revises it every year.
- Will these exact questions come in my board exam?
- No one can promise that. Concepts repeat much more than exact wording. Understanding why an answer is true lets you handle a changed question, which is why each lesson explains the reason, not just the answer.