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Class 10 Science assertion and reason questions 2026-27: 39 chapter-wise, with the reason explained
This page has 39 assertion and reason questions for Class 10 Science, three for each of the 13 chapters of the NCERT Science textbook (Reprint 2026-27). Each answer says whether the assertion is true, whether the reason is true, and, when both are true, whether the reason really explains the assertion. The page the idea comes from is named under every answer.
These questions are our own, drafted with AI help. Every statement was checked against the textbook page named under it and every answer letter was re-checked by a test script, but no practising teacher has reviewed them yet. None of them is a textbook question. If you find a mistake, please tell us.
What an assertion and reason question is
You get two statements. The Assertion (A) is a claim. The Reason (R) is offered as the cause of that claim. You choose one of four options, the same four the CBSE sample paper prints:
| Option | Means |
|---|---|
| (A) | Both A and R are true, and R is the correct explanation of A. |
| (B) | Both A and R are true, and R is not the correct explanation of A. |
| (C) | A is true but R is false. |
| (D) | A is false but R is true. |
All 39 questions below use these four options, so they are printed once here and not under every question.
Where they sit in the 2026-27 sample paper
CBSE's Science sample paper for 2026-27 has 39 questions in three sections: Section A Biology, Section B Chemistry and Section C Physics (instructions, PDF page 1). Four of them are assertion and reason questions of 1 mark each: Questions 8 and 9 in Biology (PDF pages 2 to 3), Question 24 in Chemistry (page 7) and Question 32 in Physics (pages 11 to 12). That is 4 of the 80 theory marks. The marking scheme gives option D for both Biology items (PDF page 1), which is a useful warning: the answer is not always (A).
The Class X Science curriculum lists assertion-reasoning questions among the question types in the paper (question paper design, PDF page 9).
How to answer in three checks
- Is A true? Read it on its own, as if R were not there. If A is false, the answer can only be (D), or neither, if R is false too; the paper does not use that case.
- Is R true? Read it on its own too. If A is true and R is false, the answer is (C).
- Does R explain A? Only when both are true. Put "because" between them: "A, because R". If that sentence gives the actual cause, the answer is (A). If R is a true fact about the same topic but not the cause, the answer is (B).
The third check is where most marks are lost. A true reason that sits on the same textbook page is not automatically the explanation.
What the 2026-27 paper leaves out
CBSE's curriculum for 2026-27 says some topics are taught but assessed only in school, not in the board paper: periodic classification of elements, evolution, and the electric motor, electromagnetic induction and the electric generator (pages 4 to 6). The colour of the Sun at sunrise and sunset is left out, and information printed in boxes in the textbook is not assessed (page 6). None of the questions below uses those parts. The curriculum's "Note for Teachers" (page 6) also lists heredity and the effects of electric current as not assessed in the year-end exam, while the unit lists on pages 5 and 6 include them, so check with your teacher which reading your school follows before you practise the Chapter 8 and Chapter 12 sets.
Chemistry
Chapter 1: Chemical Reactions and Equations
Q1. Assertion (A): A strip of zinc kept in blue copper sulphate solution gets a reddish-brown coating. Reason (R): Zinc is more reactive than copper, so it displaces copper from copper sulphate.
Answer: (A). A is true: the reddish-brown layer is copper metal set free on the zinc. R is true and explains A: a more reactive metal pushes a less reactive one out of its compound, Zn + CuSO₄ → ZnSO₄ + Cu. (Displacement reactions, page 11.)
Q2. Assertion (A): The burning of natural gas is an exothermic reaction. Reason (R): Respiration is an exothermic process.
Answer: (B). Both are true: methane burns in oxygen to give carbon dioxide and water with the release of heat, and respiration also gives out energy (page 7). But R does not explain A. Respiration being exothermic says nothing about why natural gas burning releases heat; they are two separate examples of the same kind of reaction.
Q3. Assertion (A): White silver chloride kept in sunlight slowly turns grey. Reason (R): In sunlight, silver chloride combines with oxygen from the air to form grey silver oxide.
Answer: (C). A is true. R is false: light decomposes silver chloride into silver and chlorine, 2AgCl → 2Ag + Cl₂, and the grey colour is silver metal. Nothing combines with oxygen here; it is a decomposition reaction caused by light. (Page 9.)
Chapter 2: Acids, Bases and Salts
Q4. Assertion (A): Tooth decay starts when the pH in the mouth falls below 5.5. Reason (R): Tooth enamel, made of calcium hydroxyapatite, is the hardest substance in the body.
Answer: (B). Both are true (page 27). But R does not explain A: the hardness of enamel is not why decay starts at a pH below 5.5. The cause is that enamel, although very hard and not soluble in water, is corroded by acid. Bacteria in the mouth make that acid from sugar and food bits left after eating.
Q5. Assertion (A): The pH of rain water is always exactly 7. Reason (R): Rain water with a pH below 5.6 is called acid rain.
Answer: (D). A is false and R is true, and R itself shows why A fails: rain can be acidic, and rain water with a pH less than 5.6 is acid rain (page 26). A pH of exactly 7 would be neutral, which rain need not be.
Q6. Assertion (A): Baking powder makes a cake soft and spongy. Reason (R): When baking powder is heated or mixed with water, it gives off carbon dioxide, which makes the dough rise.
Answer: (A). Both are true, and R explains A. Baking powder is baking soda mixed with tartaric acid, a weak edible acid. Heat or water sets off a reaction that releases carbon dioxide, and the gas puffs up the dough (page 31).
Chapter 3: Metals and Non-metals
Q7. Assertion (A): A small piece of gallium can melt when held in the palm of the hand. Reason (R): Gallium has a very low melting point.
Answer: (A). Both are true, and R explains A: most metals have high melting points, but gallium and caesium are exceptions with very low ones, low enough for the warmth of a palm to melt them (page 40).
Q8. Assertion (A): Iodine is a metal, because it is lustrous. Reason (R): Iodine is a non-metal that is lustrous.
Answer: (D). A is false and R is true. Lustre alone does not make an element a metal: iodine is listed among the non-metals and is an exception that shines (pages 39 and 40).
Q9. Assertion (A): A clean silver spoon kept in copper sulphate solution does not get a coating of copper. Reason (R): Silver is more reactive than copper.
Answer: (C). A is true, but R is false. In the activity series copper is above silver, so silver is less reactive than copper (Table 3.2, page 45). A metal displaces another from its salt solution only if it is more reactive (page 45). Silver cannot displace copper, so no copper coating forms.
Chapter 4: Carbon and its Compounds
Q10. Assertion (A): Carbon forms bonds mostly by sharing electrons with other atoms. Reason (R): Carbon has four valence electrons, and gaining or losing four electrons to reach a noble gas configuration would be very difficult.
Answer: (A). Both are true, and R explains A. Gaining four electrons would leave six protons holding ten electrons, and losing four would need a large amount of energy. So carbon shares its valence electrons and forms covalent bonds (pages 59 and 60).
Q11. Assertion (A): Saturated hydrocarbons generally burn with a clean flame. Reason (R): Unsaturated carbon compounds burn with a yellow flame and a lot of black smoke.
Answer: (B). Both are true (page 69). But R is about a different group of compounds, so it cannot explain A. The two statements are a comparison, not a cause and its effect.
Q12. Assertion (A): Soap helps to remove oily dirt from clothes. Reason (R): In a micelle, the ionic end of each soap molecule points into the oil droplet and the carbon chain faces the water.
Answer: (C). A is true. R has the ends the wrong way round: soap's ionic end is drawn to water and its long carbon chain to oil, so in a micelle the carbon chains point in, towards the oil, and the ionic ends face outwards. That is how the oily dirt is pulled into the water and rinsed away (page 75).
Biology
Chapter 5: Life Processes
Q13. Assertion (A): The walls of the alveoli in the lungs carry a dense network of blood vessels. Reason (R): The alveoli give the surface where gases are exchanged between air and blood.
Answer: (A). Both are true, and R explains A. The alveoli are the balloon-like ends of the air passages, and the gas exchange happens there. A close web of blood vessels lets oxygen pass into the blood and carbon dioxide pass out of it (page 90).
Q14. Assertion (A): The ventricles of the human heart have thicker muscular walls than the atria. Reason (R): Valves stop blood from flowing backwards when the atria or ventricles contract.
Answer: (B). Both are true (page 92). But R does not explain A. The walls are thicker because the ventricles have to pump blood out into various organs; the valves are a separate feature of the heart.
Q15. Assertion (A): In a fish, blood passes through the heart twice in each cycle round the body. Reason (R): The heart of a fish has only two chambers.
Answer: (D). A is false and R is true. A fish heart has two chambers. Blood goes to the gills, picks up oxygen and flows straight on to the rest of the body, so it passes through the heart only once in each cycle. In other vertebrates it passes twice, which is called double circulation (page 92).
Chapter 6: Control and Coordination
Q16. Assertion (A): The roots of a plant grow downwards into the soil. Reason (R): Roots respond to the pull of the earth's gravity.
Answer: (A). Both are true, and R explains A. Growth in response to gravity is geotropism: roots grow downwards, towards the pull of the earth, while shoots usually grow upwards and away from it (page 107).
Q17. Assertion (A): Adrenaline reaches its target organs, such as the heart, through the blood. Reason (R): Adrenaline is secreted by the pituitary gland.
Answer: (C). A is true: adrenaline goes straight into the bloodstream, which takes it round the body to organs such as the heart (page 109). R is false: adrenaline comes from the adrenal glands. The pituitary secretes other hormones, such as growth hormone (pages 109 and 110).
Q18. Assertion (A): A shortage of growth hormone in childhood leads to dwarfism. Reason (R): Growth hormone is secreted by the pituitary gland.
Answer: (B). Both are true (page 110). But R only names where the hormone comes from, so it does not explain A. The explanation of A is that growth hormone regulates the growth and development of the body, so too little of it during childhood stunts growth.
Chapter 7: How do Organisms Reproduce?
Q19. Assertion (A): Spores of bread mould (Rhizopus) can survive for a while before they begin to grow. Reason (R): Each spore has a thick wall that protects it until it lands on another moist surface.
Answer: (A). Both are true, and R explains A. The spores form inside the sporangia, the tiny blobs on the mould's stalks, and their thick covering keeps them safe until they meet a moist surface where they can grow into new Rhizopus (page 118).
Q20. Assertion (A): A Hydra cut into small pieces cannot grow back into whole animals. Reason (R): Simple animals such as Hydra and Planaria show regeneration.
Answer: (D). A is false and R is true. In regeneration, each of the many pieces of a simple animal like Hydra or Planaria can grow into a complete organism (page 116).
Q21. Assertion (A): Pollen grains in a flower are made by the stamens. Reason (R): The stamen is the female reproductive part of a flower.
Answer: (C). A is true. R is false: the stamen is the male reproductive part, and its pollen grains are the yellowish powder that sticks to your fingers. The female part is the pistil, in the centre of the flower (page 120).
Chapter 8: Heredity
Q22. Assertion (A): Every egg cell of a woman carries an X chromosome. Reason (R): A woman's two sex chromosomes are a matched pair, and both are X.
Answer: (A). Both are true, and R explains A. A germ cell takes one chromosome from each pair. Since a woman's pair is XX, whichever one an egg receives, it is an X (page 132).
Q23. Assertion (A): Whether a child is a boy or a girl depends on the sex chromosome it inherits from the father. Reason (R): A child that inherits an X chromosome from its father will be a boy.
Answer: (C). A is true: every child gets an X from the mother, so the father's chromosome decides. R is false: an X from the father makes the child XX, a girl; a Y from the father makes the child XY, a boy (page 132).
Q24. Assertion (A): A child gets almost the same amount of genetic material from its mother as from its father. Reason (R): Each germ cell carries only one set of genes, so the child receives one set from each parent.
Answer: (A). Both are true, and R explains A. Body cells carry two sets of genes. A germ cell carries one, so when an egg and a sperm fuse the child gets one set from each parent, and each trait can be influenced by both (pages 129, 131 and 132).
Physics
Chapter 9: Light: Reflection and Refraction
Q25. Assertion (A): Concave mirrors are often used as shaving mirrors. Reason (R): Concave mirrors are used in vehicle headlights to get a powerful parallel beam of light.
Answer: (B). Both are true (page 140). But R does not explain A. A concave mirror is used for shaving because, with the face between its pole and its focus, it gives an enlarged, erect image (Table 9.1, page 138). The headlight use is a different property of the same mirror.
Q26. Assertion (A): A light ray that goes through the optical centre of a thin lens is bent towards the principal axis. Reason (R): A ray parallel to the principal axis passes through the principal focus after refraction through a convex lens.
Answer: (D). A is false: a ray through the optical centre passes on without any deviation (page 151). R is true: it is one of the standard rays used for drawing images formed by a convex lens (page 153).
Q27. Assertion (A): Light travels faster in ice than in rock salt. Reason (R): Ice has a lower refractive index (1.31) than rock salt (1.54).
Answer: (A). Both are true, and R explains A. The refractive index of a medium is the speed of light in air divided by its speed in the medium, so the lower the refractive index, the faster light travels in it. The values are from Table 9.3 (page 149).
Chapter 10: The Human Eye and the Colourful World
Q28. Assertion (A): A person with myopia cannot see nearby objects clearly. Reason (R): Myopia is also called near-sightedness.
Answer: (D). A is false: a myopic person sees close things clearly; it is far-off objects that look blurred. That is exactly why it is called near-sightedness, so R is true (page 163).
Q29. Assertion (A): The clear sky looks blue on a sunny day. Reason (R): Air molecules and fine particles in the atmosphere scatter red light more strongly than blue light.
Answer: (C). A is true. R is the wrong way round: particles smaller than the wavelength of visible light scatter the shorter, blue wavelengths far more than the longer red ones. The sky looks blue because this scattered blue light is what comes to our eyes (pages 169 and 170).
Q30. Assertion (A): The ciliary muscles can change the focal length of the eye lens. Reason (R): Light enters the eye through the cornea, a thin transparent membrane at the front of the eyeball.
Answer: (B). Both are true (pages 161 and 162). But R does not explain A. The ciliary muscles change the focal length by changing the curvature of the lens: when they contract, the lens grows thicker and its focal length decreases. The cornea is a different part of the eye.
Chapter 11: Electricity
Q31. Assertion (A): When resistors are joined in series, the total resistance is greater than the largest of them. Reason (R): In a series combination, the total resistance is the sum of the separate resistances.
Answer: (A). Both are true, and R explains A. R(series) = R₁ + R₂ + R₃, and adding positive resistances to the largest one always gives more than it (page 184).
Q32. Assertion (A): A heater coil of nichrome wire is rewound with a nichrome wire three times as long and of the same thickness, so the resistivity of the coil becomes three times as large. Reason (R): The resistance of a conductor is directly proportional to its length.
Answer: (D). A is false: resistivity is a characteristic property of the material and does not depend on the wire's length. It is the resistance that becomes three times as large, not the resistivity. R is true: R = ρl/A, so resistance is proportional to length (page 178).
Q33. Assertion (A): A fuse protects electric circuits and appliances from an unduly high current. Reason (R): A fuse wire is made of a material with a very high melting point, so that it never melts.
Answer: (C). A is true. R is false: the fuse works because it melts. Its wire has a suitable melting point; when a current above the rated value flows, the wire heats up, melts and breaks the circuit (page 190).
Chapter 12: Magnetic Effects of Electric Current
Q34. Assertion (A): Magnetic field lines are closed curves. Reason (R): Outside a magnet they run from the north pole to the south pole, and inside the magnet from the south pole to the north pole.
Answer: (A). Both are true, and R explains A: the lines leave the north pole, enter the south pole and run back through the magnet, so each line closes on itself (page 197).
Q35. Assertion (A): A circular coil of 50 turns gives a stronger magnetic field at its centre than a single turn of the same wire carrying the same current. Reason (R): The current goes round in the same direction in every turn, so the fields of all the turns add up.
Answer: (A). Both are true, and R explains A. Since each turn adds its own field in the same direction, a coil of n turns gives n times the field of one turn: 50 times here (page 201).
Q36. Assertion (A): If the current in a straight wire is increased, the magnetic field it produces at a given point becomes stronger. Reason (R): The magnetic field lines around a straight current-carrying wire are concentric circles centred on the wire.
Answer: (B). Both are true: a compass needle near the wire deflects more when the current goes up, so the field at that point is stronger (page 199), and the field lines form concentric circles (page 199). But R only describes the shape of the field. It says nothing about how the strength depends on the current, so it does not explain A.
Environment
Chapter 13: Our Environment
Q37. Assertion (A): A plastic carry bag buried in the soil can stay there for a very long time. Reason (R): Plastics are not broken down by bacteria or other saprophytes.
Answer: (A). Both are true, and R explains A. Substances that bacteria and saprophytes cannot break down are non-biodegradable, and in ordinary conditions they stay in the environment for years (page 214).
Q38. Assertion (A): Ozone in the upper atmosphere is harmful to life on earth. Reason (R): The ozone layer protects the earth's surface from the Sun's ultraviolet (UV) rays.
Answer: (D). A is false: ozone is a deadly poison, but high up in the atmosphere it does essential work. R is true, and it is the reason the upper-atmosphere ozone layer protects life rather than harming it (page 212).
Q39. Assertion (A): In a food chain of grass, insects, frogs, snakes and hawks, a pesticide sprayed on the grass reaches its highest concentration in the hawks. Reason (R): Such chemicals are not degradable, so they build up progressively at each trophic level.
Answer: (A). Both are true, and R explains A. This build-up along a food chain is called biological magnification, and the top level of the chain collects the most (page 212). The food chain in A is our own example.
Answer key
| Chapter | Questions | Answers |
|---|---|---|
| 1 Chemical Reactions and Equations | Q1 to Q3 | A, B, C |
| 2 Acids, Bases and Salts | Q4 to Q6 | B, D, A |
| 3 Metals and Non-metals | Q7 to Q9 | A, D, C |
| 4 Carbon and its Compounds | Q10 to Q12 | A, B, C |
| 5 Life Processes | Q13 to Q15 | A, B, D |
| 6 Control and Coordination | Q16 to Q18 | A, C, B |
| 7 How do Organisms Reproduce? | Q19 to Q21 | A, D, C |
| 8 Heredity | Q22 to Q24 | A, C, A |
| 9 Light: Reflection and Refraction | Q25 to Q27 | B, D, A |
| 10 The Human Eye and the Colourful World | Q28 to Q30 | D, C, B |
| 11 Electricity | Q31 to Q33 | A, D, C |
| 12 Magnetic Effects of Electric Current | Q34 to Q36 | A, A, B |
| 13 Our Environment | Q37 to Q39 | A, D, A |
Across the 39 questions, 15 answers are (A), 8 are (B), 8 are (C) and 8 are (D). If you got most of the (B) questions wrong, go back to the third check: a true reason from the same page is not always the explanation.
Three traps to watch for
- The swapped reason. R is nearly right but turned around: the ends of a soap molecule (Q12), which colour is scattered more (Q29), or which chromosome makes a boy (Q23). Read every word of R before you call it true.
- The neighbouring fact. R is true and comes from the same page, but it explains something else (Q2, Q11, Q14, Q25). Use the "because" test.
- The assertion that sounds familiar. A uses the right words in the wrong place, such as "near-sightedness" for a person who cannot see near objects (Q28). Check A on its own first.
Practise more
- All 13 chapters with their questions, notes and pages in one place: Class 10 Science question answer.
- A 10-mark test for each chapter: Class 10 Science chapter-wise test 2026-27.
- Longer sets by marks: Class 10 Science important questions 2026-27, competency-based questions and past-paper questions chapter-wise.
- How the board paper is laid out: our Class 10 Science sample paper guide and syllabus guide.
- Free practice, marked instantly in your browser: practice by chapter, and printable worksheets with answer keys: Class 10 worksheets.
Where CMDLearn fits
CMDLearn is building an AI tutor for Classes 1 to 10 that explains a step and checks the child's working, but leaves the writing to the child. It is still in development, and no child uses it yet. If you would like to hear when there is something to try, you can join the waitlist.
Sources
All opened and read on 5 November 2026. CBSE page numbers are PDF pages; NCERT page numbers are the printed page numbers of the textbook.
- CBSE, Science (086) Sample Question Paper, Class X, 2026-27: general instructions (PDF page 1), the assertion and reason instructions and Questions 8 and 9 (pages 2 and 3), Question 24 (page 7), Question 32 (pages 11 and 12): cbseacademic.nic.in/web_material/SQP/ClassX_2026_27/Science-SQP.pdf
- CBSE, Science (086) Marking Scheme, Class X, 2026-27: answers to Questions 8 and 9 (PDF page 1): cbseacademic.nic.in/web_material/SQP/ClassX_2026_27/Science-MS.pdf
- CBSE, Science (Subject Code 086), Class X (2026-27) curriculum: pages 4 to 6 (units, topics assessed only formatively, sunrise and sunset excluded, Note for Teachers, boxed text not assessed), page 9 (question paper design and typology of questions): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf
- NCERT, Science, the textbook for Class X (Reprint 2026-27), Chapters 1 to 13, the pages named under each answer above: ncert.nic.in/textbook/pdf/jesc101.pdf (and so on to jesc113.pdf)
The question format is from CBSE's sample paper, and the science in each answer is from the textbook pages named. The assertions, reasons and worked answers are our own. This guide is general information; follow your teacher, the textbook and CBSE's own notices.
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