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CBSE Class 10 Science 2026 — Paper 31/8/3/SB

80 marks · 3 hours · 39 questions (45 with the OR choices)

28 of its 45 questions also appear in 31/8/1/SB, 31/8/2/SB and other papers; 17 are only in this paper.

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Questions 1–9 · 1 mark each

  1. Q1 1 mark

    Also in 31/8/1/SB (Q5), 31/8/2/SB (Q7)

    A man underwent MRI of brain which shows some defects in the cerebellum of his brain. Which activity of that person will be affected ?
    1. (a)Heart beat
    2. (b)Feeling of hunger and thirst
    3. (c)Reflexes
    4. (d)Walking in straight line
    Show answer
    (d) Walking in straight line
    Step-by-step solution →
  2. Q2 1 mark
    Which of the following statements correctly identify the advantages of vegetative propagation ?
    1. (i)Plants can bear flowers and fruits earlier than those produced by seeds.
    2. (ii)The new plants develop as an outgrowth on the parent plant and can be removed easily.
    3. (iii)Plants that have lost the capacity to produce seeds can be propagated.
    4. (iv)All plants produced are genetically similar to parent plant.
    1. (a)(i), (iii), (iv)
    2. (b)(ii), (iii), (iv)
    3. (c)(i), (ii), (iii)
    4. (d)(ii), (iv) only
    Show answer
    (a) (i), (iii), (iv)
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/8/1/SB (Q1), 31/8/2/SB (Q6)

    Select the correct option that shows the path of flow of only the deoxygenated blood through the human circulatory system.
    1. (a)Body left atrium left ventricle lungs right atrium right ventricle body.
    2. (b)Right atrium right ventricle lungs left atrium left ventricle body
    3. (c)Body left atrium left ventricle lungs
    4. (d)Body right atrium right ventricle lungs
    Show answer
    (d) Body right atrium right ventricle lungs
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/8/1/SB (Q7), 31/8/2/SB (Q1)

    Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow seeds (rrYY) were crossed. All the seeds of progeny have round, yellow seeds (RrYy). When plants are self-pollinated, the progeny will have new combination of characters.
    1. (i)Round, yellow
    2. (ii)Round, green
    3. (iii)Wrinkled, yellow
    4. (iv)Wrinkled, green Choose the option that shows new combination of characters in progeny.
    1. (a)(i) and (ii)
    2. (b)(i) and (iv)
    3. (c)(iii) and (iv)
    4. (d)(i) and (iii)
    Show answer
    (b) (i) and (iv)
    Step-by-step solution →
  5. Q5 1 mark

    Also in 31/8/1/SB (Q2), 31/8/2/SB (Q4)

    Which of the following groups consists entirely of substances that are broken down by an enzyme, lipase ?
    1. (a)Carbohydrates, proteins and vitamins
    2. (b)Fats, oils and ghee
    3. (c)Minerals, starch and vitamins
    4. (d)Proteins, sugars and esters
    Show answer
    (b) Fats, oils and ghee
    Step-by-step solution →
  6. Q6 1 mark
    During sudden activity, cramps are caused in the muscles of legs in sports person. Choose the option that indicates the real cause of cramps.
    1. (a)Lack of oxygen and formation of carbon dioxide.
    2. (b)Presence of oxygen and formation of ethanol.
    3. (c)Lack of carbon dioxide and formation of pyruvate.
    4. (d)Lack of oxygen and formation of lactic acid.
    Show answer
    (d) Lack of oxygen and formation of lactic acid.
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/8/1/SB (Q4), 31/8/2/SB (Q2)

    Examine the given food chains to establish the correct one in an ecosystem :
    1. (a)Seed mouse snake deer
    2. (b)Algae small fish large fish hawk
    3. (c)Grasshopper frog snake elephant
    4. (d)Plants rabbit deer wolf
    Show answer
    (b) Algae small fish large fish hawk
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/8/1/SB (Q9), 31/8/2/SB (Q8)

    Question No. 8 & 9 consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option from (a), (b), (c) and (d) given below :

    Assertion (A): The energy transferred to a lion when it eats a deer is only a small fraction of the deer’s total energy content.

    Reason (R): Energy transfer is efficient at all trophic levels with nearly 90% of energy passed on to the next level.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (c) Assertion (A) is true, but Reason (R) is false.
    Step-by-step solution →
  9. Q9 1 mark
    Question No. 8 & 9 consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option from (a), (b), (c) and (d) given below :

    Assertion (A): Motor neurons carry signals from the spinal cord to the effector organ.

    Reason (R): Sensory neurons carry signals from receptors towards the spinal cord.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    Step-by-step solution →

Questions 10–12 · 2 marks each

  1. Q10 2 marks

    Also in 31/8/1/SB (Q12), 31/8/2/SB (Q11)

    The sensitive plant shows movement of leaves in response to touch. Explain such movement briefly.
    Show answer

    On touch, cells at the leaflet base lose water and shrink, so the leaflets fold; this nastic movement does not depend on growth.

    The folding of the leaves of the sensitive plant on being touched is a movement that is independent of growth, that is, a nastic movement. The stimulus is touch. The plant has no nervous tissue, so the information that a leaf has been touched is passed on from one cell to the next as an electrical and chemical signal until it reaches the cells at the base of the leaflets. Plants have no muscle cells; instead, their cells change shape by changing the amount of water in them. The cells in the swelling at the base of each leaflet lose water, become flaccid and shrink, so the leaflets fold up in pairs and the leaf droops. When these cells take up water again they swell, and the leaflets spread out as before, so the movement is reversible.
    Step-by-step solution →
  2. Q11 2 marks
    Discuss two limitations to the use of electrical impulse for coordination, by animals.
    Show answer

    An electrical impulse reaches only cells connected by nervous tissue, and a cell needs time to reset before it can send another impulse.

    Two limitations of using electrical impulses for coordination are: (1) An electrical impulse travels only along nervous tissue, so it reaches only those cells that are connected by nervous tissue and not each and every cell of the animal's body. (2) Once an electrical impulse has been generated in a cell and transmitted, that cell takes some time to reset its mechanisms before it can generate and transmit a new impulse, so cells cannot continually create and transmit electrical impulses. Because of these two limits most multicellular animals also use chemical communication: hormones released into the blood can reach all cells regardless of nervous connections, and the signal can be sent steadily and persistently.
    Step-by-step solution →
  3. Q12(a) 2 marks
    State the two important roles of male sex hormone-testosterone.
    Show answer

    Testosterone regulates the formation of sperms and brings about the changes seen in boys at puberty.

    Testosterone, the male sex hormone secreted by the testes, has these two important roles: (1) It regulates the formation of sperms. The tubules of the testes produce the male gametes, and testosterone controls and maintains that production, so sperms are formed only while the hormone is being secreted. (2) It brings about the changes in appearance seen in boys at the time of puberty, that is, the secondary sexual characters - growth of hair on the face and body, deepening of the voice, broadening of the shoulders and a more muscular build, and enlargement of the penis and the testes.
    Step-by-step solution →
    OR
    Q12(b) 2 marks
    Give two reasons why secretions of prostate gland and seminal vesicles are of significance in human male reproductive system.
    Show answer

    Their secretions form a fluid that makes transport of the sperms easier and also provides nutrition to the sperms.

    The prostate gland and the seminal vesicles add their secretions along the path of the vas deferens, and they matter for these two reasons: (1) The secretions provide a fluid in which the sperms are suspended, and this makes the transport of the sperms easier. Instead of remaining bare cells, the sperms can swim with their tails and be carried along the vas deferens and out through the urethra. (2) The same fluid provides nutrition to the sperms. A sperm is a small cell with a long tail and very little cytoplasm, so it carries almost no food of its own, and the nutrients in the fluid supply the energy it needs for the journey.
    Step-by-step solution →

Questions 13–14 · 3 marks each

  1. Q13 3 marks

    Also in 31/8/1/SB (Q14), 31/8/2/SB (Q13)

    “Each organism has two sets of all genes, one inherited from each parent i.e. both parents contribute equally to the DNA of the progeny during sexual reproduction”. If such is the case, then how is the stability of DNA of the species ensured ? Discuss.
    Show answer

    Gametes get half the chromosomes (one set), and fertilisation restores the full number (), so the DNA of the species stays stable.

    If both parents really passed on the two sets of genes each of them carries, the progeny would have four sets, the next generation eight, and the amount of DNA would double in every generation. This does not happen, because sexual reproduction includes a halving step. Each body cell has two copies of each chromosome, one from the male parent and the other from the female parent. When germ cells are formed, each germ cell (gamete) takes only one chromosome from each pair, which may be of either maternal or paternal origin, so a gamete carries one set of genes instead of two and half the number of chromosomes; in human beings a body cell has 46 chromosomes (23 pairs) while a sperm or an egg has 23. When two germ cells unite at fertilisation, the zygote once again has two copies of each chromosome, 23 plus 23 giving 46, so the normal number of chromosomes is restored in the progeny. This re-establishment of the number of chromosomes, and so of the amount of DNA, in every generation ensures the stability of the DNA of the species.
    Step-by-step solution →
  2. Q14 3 marks
    (A) If no decomposers in an ecosystem, what will happen to the garbage and dead animal and plant wastes. (B) As a student, what are the four changes in your daily habits you would adopt to reduce generation of non-biodegradable wastes ?
    Show answer

    Without decomposers, dead matter piles up and nutrient cycles break down; use cloth bags, steel bottles, refills and segregation to cut non-biodegradable waste.

    (A) Nothing would be broken down. The garbage and the dead bodies of animals and plants would go on accumulating, so the land and the water would be choked with dead matter, which would also become a source of foul smell and of disease. More seriously, the nutrients locked inside that dead matter - nitrogen, phosphorus, carbon and the minerals - would never be released back into the soil, the water and the air. The nutrient cycles would break down, the producers would be starved of the raw materials they need to make food, and the whole ecosystem would in time collapse. (B) Four changes in daily habits: (i) Carry a cloth or jute bag while shopping and refuse plastic carry bags. (ii) Use a steel or glass water bottle and a steel tiffin box instead of disposable plastic bottles, cups, straws and cutlery. (iii) Buy goods in refill or larger packs rather than in many small plastic-wrapped ones, and choose things that can be repaired or refilled, such as a pen with a refill or a rechargeable cell, over things meant to be thrown away. (iv) Separate the waste at home so that plastic, glass, metal and electronic items are handed over for recycling instead of being mixed with the rest.
    Step-by-step solution →

Question 15 · 4 marks

  1. Q15 4 marks

    Also in 31/8/1/SB (Q15), 31/8/2/SB (Q15)

    The given diagram shows the human kidney and its associated structures. Study the Table I and Table II that shows the concentration of certain substances found within the structure P and Q.
    Table I (structure P) and Table II (structure Q) - concentration of components, in per cent
    ComponentsTable I - Structure P, concentration %Table II - Structure Q, concentration %
    Urea0.051.00
    Amino acids0.060.00
    Salts1.721.60
    Glucose0.100.00
    P Q
    1. (A)Where are kidneys located in the human body ? Name the filtration unit of them.[1]
    2. (B)List two factors on which reabsorption of water from the filtrate depends.[1]
    3. (C)Why is there variation in the concentration of given substance in structure P and Q listed in table I and II respectively ?[2]
    4. (C)OR : If a person is suffering from diabetes, concentration of which substance present in structure Q (listed in table II) will be modified and why ?[2]
    Show answer

    Kidneys lie in the abdomen beside the backbone, with the nephron as filtration unit; filtration then selective reabsorption changes blood into urine.

    (A) The kidneys are a pair of bean-shaped organs lying in the abdomen, one on either side of the backbone, towards the back of the abdominal cavity. Their filtration unit is the nephron. (B) The amount of water reabsorbed from the filtrate depends on (i) how much excess water there is in the body, and (ii) how much dissolved waste there is to be excreted. (C) P is the blood vessel carrying blood into the kidney, so Table I is the composition of blood, and Q is the ureter carrying urine away, so Table II is the composition of urine. In the nephron the blood is first filtered at the glomerulus, so water and all the small dissolved substances enter the tubule, and the filtrate is then selectively reabsorbed as it flows along the tubule. All the glucose and all the amino acids are useful and are taken back completely, so they fall from 0.10% and 0.06% in P to 0.00% in Q. Most of the salts are taken back along with most of the water, so their concentration hardly changes, 1.72% to 1.60%. Urea is a nitrogenous waste and is not reabsorbed, while the water around it is, so it is left behind and becomes concentrated, rising from 0.05% in P to 1.00% in Q. OR (C) The concentration of glucose in structure Q will be modified: there will be more glucose in Q. In a healthy person it is 0.00% because all the filtered glucose is reabsorbed by the tubule. A person suffering from diabetes does not have sufficient insulin, so the glucose level of the blood rises; more glucose is then filtered than the tubule is able to reabsorb, the excess is not taken back and passes out in the urine, so glucose now appears in structure Q at a value above 0.00%.
    Step-by-step solution →

Question 16 · 5 marks

  1. Q16(a) 5 marks

    Also in 31/8/1/SB (Q16(a)), 31/8/2/SB (Q16(b))

    In flowering plants, the process of pollination leads to fertilization.
    1. (i)Explain with the help of a well-labelled diagram how the male germ cell from the pollen grain reaches the female germ cell located in the ovule.
    2. (ii)Differentiate between self-pollination and cross-pollination.
    3. (iii)Identify and name the parts ‘R’ and ‘S’ in the given diagram of seed.
    R S
    Show answer

    Pollen tube carries the male germ cell to the egg; self-pollination is within a flower, cross-pollination between plants; R is plumule, S radicle.

    (i) Pollination brings the pollen grain to the stigma of the carpel, while the female germ cell lies inside an ovule in the ovary. The pollen grain that lands on a suitable stigma germinates and puts out a pollen tube, which grows through the tissue of the stigma and downward through the style until it reaches the ovary and enters the ovule. The male germ cell travels down this pollen tube and is released inside the ovule, where it fuses with the female germ cell, that is, with the egg. This fusion is fertilisation and it forms the zygote, which divides to form the embryo; the ovule then becomes the seed and the ovary becomes the fruit. The diagram should show the stigma with a germinating pollen grain, the pollen tube growing down the style with the male germ cell in it, the ovary, the ovule and the female germ cell inside it, all labelled. (ii) Self-pollination: pollen grains are transferred from the anther to the stigma of the same flower; cross-pollination: pollen grains are transferred from the anther of one flower to the stigma of a different flower, usually a flower on another plant of the same species. Self-pollination usually needs no outside agent; cross-pollination needs an agent such as wind, water, insects or birds. The progeny of self-pollination show little variation; cross-pollination between different plants brings together genes from two individuals, so its progeny show more variation. (iii) R is the plumule, the part of the embryo above the attachment of the cotyledons, which grows into the shoot; S is the radicle, the part below it, which grows into the root.
    Step-by-step solution →
    OR
    Q16(b) 5 marks

    Also in 31/8/1/SB (Q16(b)), 31/8/2/SB (Q16(a))

    1. (i)Name the special tissue inside the uterus that provides nutrition to the developing embryo. Describe its structure. State its two functions besides nutrition.
    2. (ii)Write any two sexually transmitted diseases in humans and their causative microbial agents.
    Show answer

    The placenta nourishes the embryo, exchanges gases and removes wastes; gonorrhoea (bacterium) and AIDS (virus) are two sexually transmitted diseases.

    (i) The tissue is the placenta. It is a disc-shaped tissue embedded in the wall of the uterus; villi grow out from the embryo's side of the disc into blood spaces (sinuses) filled with the mother's blood, which gives a very large surface for exchange, and the embryo is joined to the disc by the umbilical cord. Two functions besides nutrition: it passes oxygen from the mother's blood to the embryo and carries carbon dioxide back; and it removes the embryo's nitrogenous wastes, chiefly urea, into the mother's blood. (The placenta also secretes hormones that help to maintain the pregnancy.) (ii) Gonorrhoea is caused by the bacterium Neisseria gonorrhoeae, and AIDS is caused by a virus, the Human Immunodeficiency Virus (HIV). Syphilis (bacterium Treponema pallidum) and warts (human papilloma virus) are equally acceptable pairs.
    Step-by-step solution →

Questions 17–24 · 1 mark each

  1. Q17 1 mark
    Which of the following is the name of compound ?
    1. (a)Methane
    2. (b)Methanal
    3. (c)Methanol
    4. (d)Methanoic acid
    Show answer
    (b) Methanal
    Step-by-step solution →
  2. Q18 1 mark
    Which of the following compound is formed when electric current is passed through brine solution ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (a)
    Step-by-step solution →
  3. Q19 1 mark

    Also in 31/5/1 (Q21), 31/5/2 (Q22), 31/5/3 (Q18)

    Which of the following will not undergo addition reaction ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  4. Q20 1 mark
    In the compound , which of the following functional group is present ?
    1. (a)Carboxylic acid
    2. (b)Ester
    3. (c)Ketone
    4. (d)Aldehyde
    Show answer
    (b) Ester
    Step-by-step solution →
  5. Q21 1 mark

    Also in 31/8/1/SB (Q23), 31/8/2/SB (Q21)

    Which of the following acid-base indicators will be used by a visually challenged student to detect the presence of acid in a given solution ?
    1. (a)Blue litmus
    2. (b)Clove oil
    3. (c)Red cabbage extract
    4. (d)Methyl orange
    Show answer
    (b) Clove oil
    Step-by-step solution →
  6. Q22 1 mark

    Also in 31/8/1/SB (Q20), 31/8/2/SB (Q23)

    During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through a guard tube containing calcium chloride. The role of taken in guard tube is to –
    1. (a)Absorb the evolved gas
    2. (b)Moisten the gas
    3. (c)Absorb moisture from the gas
    4. (d)Absorb chloride ions from the evolved gas
    Show answer
    (c) Absorb moisture from the gas
    Step-by-step solution →
  7. Q23 1 mark
    Which of the following solution will not conduct electricity ?
    1. (a)Hydrochloric acid
    2. (b)Sodium hydroxide
    3. (c)Sulphuric acid
    4. (d)Glucose
    Show answer
    (d) Glucose
    Step-by-step solution →
  8. Q24 1 mark

    Also in 31/8/1/SB (Q24), 31/8/2/SB (Q24)

    For question number 24, two statements are given-one labelled as Assertion (A) and the other labelled as Reason (R). Answer this question selecting the appropriate option from (a), (b), (c) and (d) given below :

    Assertion (A): Sodium acetate is a basic salt.

    Reason (R): It is because it is a salt of weak acid and strong base.

    1. (a)Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
    Step-by-step solution →

Question 25 · 2 marks

  1. Q25 2 marks
    Write balanced chemical equations for the reactions taking place when :
    1. (i)Aluminium metal reacts with steam.
    2. (ii)Magnesium reacts with dilute .
    Show answer

    Aluminium with steam gives and hydrogen; magnesium with dilute HCl gives and hydrogen.

    (i) . Aluminium takes the oxygen from steam to form aluminium oxide and hydrogen gas is set free. (ii) . Magnesium displaces hydrogen from the dilute acid, giving magnesium chloride and hydrogen gas.
    Step-by-step solution →

Questions 26–27 · 3 marks each

  1. Q26(a) 3 marks

    Also in 31/8/1/SB (Q26(a)), 31/8/2/SB (Q27(a))

    1. (i)State any two reasons for carbon forming a large number of compounds. Why does carbon form compounds mainly by covalent bonds ?
    2. (ii)Identify the compound which contains aldehyde as its functional group : , , ,
    Show answer

    Catenation and tetravalency let carbon form many compounds; carbon shares its four valence electrons, forming covalent bonds; has the aldehyde group.

    (i) Two reasons: (1) Catenation — carbon atoms link to one another by strong covalent bonds and build long straight chains, branched chains and closed rings; (2) Tetravalency — carbon has four valence electrons, so one carbon atom can bond with four other atoms of carbon or of other monovalent elements, and because the carbon atom is small these bonds are strong and stable. Carbon forms compounds mainly by covalent bonds because it has four electrons in its outermost shell: it can neither gain four electrons (a nucleus of six protons cannot hold ten electrons) nor lose four electrons (that would need a very large amount of energy), so it attains a noble-gas arrangement by **sharing** its four valence electrons, and a shared pair of electrons is a covalent bond. (ii) (propanal) — it carries the aldehyde group .
    Step-by-step solution →
    OR
    Q26(b) 3 marks

    Also in 31/8/1/SB (Q26(b)), 31/8/2/SB (Q27(b))

    An organic compound ‘X’ is a constituent of wine. ‘X’ on reacting with acidified forms another compound ‘Y’ with molecular formula . ‘Y’ produces brisk effervescence with sodium carbonate.
    1. (i)Identify ‘X’ and ‘Y’.
    2. (ii)Name the gas evolved when ‘Y’ reacts with sodium carbonate.
    3. (iii)Write the chemical equations involved for the chemical reactions taking place.
    Show answer

    X is ethanol and Y is ethanoic acid; the gas evolved is carbon dioxide.

    (i) ‘X’ is ethanol, — the alcohol present in wine; ‘Y’ is ethanoic acid (acetic acid), , whose molecular formula is . (ii) The gas evolved is carbon dioxide, . (iii) and
    Step-by-step solution →
  2. Q27 3 marks

    Also in 31/8/1/SB (Q27), 31/8/2/SB (Q26)

    (A) Name the acid present in ant sting and give its chemical formula. Give the common method to get relief from the pain and irritation caused by ant sting. (B) Two solutions ‘X’ and ‘Y’ have pH values and respectively. Which of these will turn blue litmus to red and which will turn phenolphthalein from colourless to pink ?
    Show answer

    Ant-sting acid is methanoic acid, relieved by baking soda; X (pH 3.0) turns blue litmus red; Y (pH 9.5) turns phenolphthalein pink.

    (A) The acid present in an ant sting is methanoic acid (formic acid), . Relief is obtained by rubbing the stung area with a mild base such as moist baking soda (sodium hydrogencarbonate, ), which neutralises the acid; calamine solution, containing zinc carbonate, may be used in the same way. (B) ‘X’, of pH , is acidic and will turn blue litmus red. ‘Y’, of pH , is basic and will turn phenolphthalein from colourless to pink.
    Step-by-step solution →

Question 28 · 4 marks

  1. Q28 4 marks

    Also in 31/8/1/SB (Q28), 31/8/2/SB (Q28)

    The Statue of Liberty is one of the seven wonders. It is present on Liberty island in New York harbour in U.S.A. It is made up of a metal. Over a period of time, the metal’s colour has changed and statue gives greenish look. Answer the following questions based on the above information :
    1. (A)What is corrosion ?[1]
    2. (B)Why do silver articles turn black after some time ?[1]
    3. (C)Copper reacts with moist gas ‘A’ present in the air and slowly loses its shiny surface. It turns green. Identify ‘A’ and the substance of green coating formed on the surface of copper.[2]
    4. (C)OR : What is an alloy ? How is an alloy made ?[2]
    Show answer

    Corrosion eats away a metal's surface; silver turns black due to silver sulphide; gas A is carbon dioxide, the green coating basic copper carbonate.

    (A) Corrosion is the process in which the surface of a metal is slowly eaten away by the action of air, moisture, acids or other chemicals around it, the metal being converted into a surface compound such as an oxide, a carbonate or a sulphide. (B) Silver articles turn black because silver reacts with the sulphur present in the air, as hydrogen sulphide, to form a black coating of silver sulphide, : . (C) ‘A’ is carbon dioxide, , and the green coating formed on copper is basic copper carbonate, : . (C) OR : An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. It is made by first melting the main metal, then dissolving the other metals or non-metals in it in a fixed proportion, and then cooling the mixture to room temperature.
    Step-by-step solution →

Question 29 · 5 marks

  1. Q29(a) 5 marks

    Also in 31/8/1/SB (Q29(a)), 31/8/2/SB (Q29(a))

    Write chemical equations for the following when
    1. (a)Silver bromide is exposed to sunlight.
    2. (b)A piece of lead metal is dropped into copper (II) chloride solution.
    3. (c)Hydrogen gas is passed over heated copper oxide.
    4. (d)Methane gas is burnt in air.
    5. (e)Quick lime is mixed with water.
    Show answer

    Silver bromide decomposes in sunlight, lead displaces copper, hydrogen reduces copper oxide, methane burns, and quick lime combines with water.

    (a) — photolytic decomposition. (b) — displacement. (c) — reduction of copper oxide. (d) — combustion. (e) — combination.
    Step-by-step solution →
    OR
    Q29(b) 5 marks

    Also in 31/8/1/SB (Q29(b)), 31/8/2/SB (Q29(b))

    1. (a)Study the experimental setup shown in the diagram and write chemical equation for the same. Name and define the type of reaction. Mention the colour of the products formed.
    2. (b)What type of reaction takes place when aqueous solution of lead (II) nitrate and potassium iodide are mixed together ? Write chemical equation also.
    A pair of tongs Boiling tube Lead nitrate Burner
    Show answer

    Heated lead nitrate decomposes into yellow lead oxide, brown nitrogen dioxide and oxygen; lead nitrate and potassium iodide solutions give a double displacement precipitate.

    (a) . It is a thermal decomposition reaction - a reaction in which a single compound breaks down on heating into two or more simpler substances. Colours of the products: lead oxide is a yellow solid, nitrogen dioxide is a reddish brown gas and oxygen is colourless. (b) A double displacement reaction, and since an insoluble solid separates out it is a precipitation reaction: , the precipitate being yellow lead iodide.
    Step-by-step solution →

Questions 30–32 · 1 mark each

  1. Q30 1 mark
    Choose the correct option :
    1. (a)Concave mirror and concave lens always form virtual image.
    2. (b)Image formed by a plane mirror and a convex mirror has magnification .
    3. (c)Concave lens and convex mirror always forms virtual image.
    4. (d)Image formed by a plane mirror and a concave lens has magnification .
    Show answer
    (c) Concave lens and convex mirror always forms virtual image.
    Step-by-step solution →
  2. Q31 1 mark

    Also in 31/8/1/SB (Q31), 31/8/2/SB (Q30)

    In the given ray diagram the values of the angle of incidence and the angle of reflection are :
    N 40°
    1. (a),
    2. (b),
    3. (c),
    4. (d),
    Show answer
    (d) ,
    Step-by-step solution →
  3. Q32 1 mark

    Also in 31/8/1/SB (Q32), 31/8/2/SB (Q32)

    For question number 32, two statements are given – one labelled as Assertion (A) and the other labelled as Reason (R). Answer this question selecting the appropriate option from (a), (b), (c) and (d) given below :

    Assertion (A): The bending of a straight wire in the form of current carrying circular coil will change the pattern of magnetic field around it.

    Reason (R): Strength of magnetic field depends upon number of turns of coil and current flowing through the coil.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
    Step-by-step solution →

Questions 33–34 · 2 marks each

  1. Q33 2 marks
    In the given electric circuit, what will be the reading in instrument P and Q ?
    P Q 12 Ω 12 V
    Show answer

    P is a voltmeter reading 12 V and Q is an ammeter reading 1 A.

    P is connected across the 12 resistor, so it is a voltmeter, and it reads the full potential difference of the battery, V. Q is connected in series in the loop, so it is an ammeter, and it reads the current A.
    Step-by-step solution →
  2. Q34(a) 2 marks

    Also in 31/8/1/SB (Q33(a)), 31/8/2/SB (Q34(a))

    What is meant by scattering of light ? How do our observations differ when white light (sun light) is scattered from
    1. (i)very fine particles of the medium and
    2. (ii)very large particles of the medium ?
    Show answer

    Scattering sends light off in different directions; very fine particles give blue scattered light, very large particles give white.

    Scattering of light is the phenomenon in which a part of the light falling on the particles of a medium - molecules, dust, smoke or water droplets - is redirected (sent out again) in different directions, away from its original straight path. (i) Very fine particles: they scatter mainly the shorter wavelengths, so the scattered light appears **blue**. This is why the clear sky looks blue. (ii) Very large particles: they scatter all wavelengths almost equally, so the scattered light appears **white**. This is why clouds, fog and mist look white.
    Step-by-step solution →
    OR
    Q34(b) 2 marks

    Also in 31/3/1 (Q34(b)), 31/3/2 (Q33(b)), 31/3/3 (Q34(b)), 31/8/1/SB (Q33(b)), 31/8/2/SB (Q34(b))

    With the help of a ray diagram, explain any phenomenon of formation of spectrum in nature.
    Show answer

    A rainbow forms when raindrops disperse sunlight by refraction, internal reflection and refraction, giving a spectrum with red outside and violet inside.

    The rainbow is a natural spectrum formed after rain by the dispersion of sunlight by tiny water droplets in the atmosphere. Each droplet acts like a small prism: sunlight is refracted and dispersed as it enters the drop, reflected internally at the back surface, and refracted again as it comes out (see the ray diagram). The colours emerge at slightly different angles - red at about , violet at about to the direction opposite the Sun - so an observer standing with the Sun behind sees a band of colours in the sky, red on the outside and violet on the inside, always on the side opposite the Sun.
    Step-by-step solution →

Questions 35–37 · 3 marks each

  1. Q35 3 marks
    (A) What is meant by presbyopia ? Write the causes of presbyopia also. (B) Draw the ray diagram for correction of presbyopia by using an appropriate lens.
    Show answer

    Presbyopia is loss of accommodation with age, caused by weak ciliary muscles and a hardened lens; a convex lens corrects it.

    (A) Presbyopia is the defect of vision in which the power of accommodation of the eye decreases with ageing: the near point moves away from the eye, so a person cannot see nearby objects clearly and comfortably and finds reading difficult without spectacles. Its causes are the gradual weakening of the ciliary muscles, which can no longer make the eye lens thick enough for a near object, and the gradual hardening of the crystalline eye lens, which loses its flexibility. A person may suffer from both myopia and hypermetropia, and such people often require bi-focal lenses. (B) It is corrected with a convex (converging) lens of suitable power, used as reading glasses. In the ray diagram the book is placed at the normal near point, cm; the convex lens forms a virtual, erect and enlarged image of it at the receded near point of the eye, and the eye then focuses that image sharply on the retina. For cm the lens needed has , that is cm and D.
    Step-by-step solution →
  2. Q36 3 marks

    Also in 31/8/1/SB (Q37), 31/8/2/SB (Q35)

    (A) Write any two precautions to be taken to avoid electrical accidents. (B) State how these precautions prevent possible damage to the electric circuit.
    Show answer

    Fit a fuse in the live wire and earth metal-bodied appliances; the fuse breaks the circuit, earthing gives a low-resistance path.

    (A) Two precautions: (i) Fit an electric fuse (or an MCB) of the proper rating in series in the **live** wire of every circuit and of every high-power appliance. (ii) Earth every metal-bodied appliance - connect its metal casing through the green earth wire and the third pin of a three-pin plug to a metal plate buried deep in the ground. (B) How they prevent damage to the circuit: The fuse is a short piece of wire of high resistivity and low melting point. If the current rises above the rated value because of overloading or a short circuit, the heat produced in it, , melts the fuse wire. The circuit is broken at once and the current stops, so the household wiring, the switches and the appliances are never heated to the point where their insulation melts, they are damaged or a fire starts. Earthing provides the metal body of the appliance with a path of very low resistance to the ground. If the live wire touches the casing, the body is held at the potential of the earth (about V), so nobody gets a shock; and because that path has a very low resistance the leakage current is large, so it melts the fuse and disconnects the faulty appliance before its damaged wiring can overheat. (Other acceptable precautions, with the same kind of explanation: do not overload one socket through a multi-plug; use only ISI-marked, properly insulated wires of adequate thickness; switch off the mains before repairs and never touch switches with wet hands.)
    Step-by-step solution →
  3. Q37 3 marks

    Also in 31/8/1/SB (Q35), 31/8/2/SB (Q36)

    (A) A vertical wire is carrying a current in the upward direction. It is placed in a magnetic field, pointing towards the East direction. Find out the direction of force on the wire. (B) Out of the two current carrying circular coils, coil ‘A’ has 10 turns while coil B has 15 turns. Which of the two coils will produce a stronger magnetic field at its centre if same current is flowing through both the coils ? Explain with reason. (C) In the given figure, AB and CD are two magnetic field lines intersecting each other at point O. If a compass is placed at point O, what will happen ? State your conclusions.
    O A B C D
    Show answer

    Force is towards North; coil B (15 turns) gives the stronger field; the compass cannot point two ways, so field lines never intersect.

    (A) The force on the wire is horizontal and directed towards the **north**. By Fleming's left-hand rule, with the forefinger along the field (East) and the middle finger along the current (vertically up), the thumb points North. (B) Coil B, with turns, produces the stronger field. At the centre of a circular coil , so with the same current and the same radius the field is proportional to the number of turns - each turn adds its own field in the same direction - and , that is times as strong. (C) The figure shows the field at O in two directions at once, so a compass needle placed at O would have to point in two directions (along OB and along OD) at the same time. That is not possible, since a needle can settle in only one direction - the situation shown is unreal. Conclusions: the magnetic field at any point has a single definite direction, and therefore two magnetic field lines never intersect each other.
    Step-by-step solution →

Question 38 · 4 marks

  1. Q38 4 marks

    Also in 31/8/1/SB (Q38), 31/8/2/SB (Q38)

    Four cells, a thin wire AB, a voltmeter, an ammeter, a plug key and some thick connecting wires are taken. The circuit is set up as shown below. The key is inserted into the plug and readings of ammeter (I) and voltmeter (V) are noted. Then the plug key is removed. Now two cells are connected in the circuit in series. Plugging in the key the new values of ‘V’ and ‘I’ are noted. The experiment is repeated by connecting three cells in series and then four cells. In each case ‘V’ and ‘I’ are noted.
    V + − A − + + − A B
    1. (A)Why was the plug key removed after each measurement of ‘V’ and ‘I’ and inserted again for the next step ?
    2. (B)What change takes place in the reading of the voltmeter when a new cell is added in series in the circuit ? Give reason.
    3. (C)In which case will the readings of voltmeter and ammeter be maximum in the above experiment ? What conclusion can be drawn from the straight line graph between ‘V’ and ‘I’ ?
    4. (C)OR : Suppose this experiment is carried out using only one cell but wires of same length & thickness but made of different materials like nichrome wire, fuse wire, copper wire and iron wire which are replaced one by one. Out of V and I which will change and why ?
    Show answer

    Key removed so wire AB stays cool; each added cell raises the voltmeter reading; four cells give maximum readings; the line shows Ohm's law.

    (A) So that current flows through the circuit only while a reading is being taken. A current left flowing through the thin wire AB would heat it (); the resistance of a metal rises with temperature, so would drift and the V-I graph would not come out straight, since Ohm's law holds only at constant temperature. Removing the key lets AB cool to room temperature between readings and also saves the cells. (B) The voltmeter reading increases. Cells joined in series have their emfs added, so each extra cell applies a larger potential difference across AB; as the wire is unchanged its resistance is unchanged, so the current increases in the same proportion and stays constant. (C) Both readings are maximum when all four cells are connected in series, since that gives the greatest total emf. The straight line through the origin shows that , i.e. is constant for the conductor at constant temperature - this is Ohm's law - and that constant, the slope of the line, is the resistance of the wire AB. (C) OR: will change and will remain the same. The same single cell applies the same potential difference in every trial, while the wires have the same length and the same area of cross-section , so by their resistances differ only through the resistivity of the material; hence is different for each wire - largest for copper, which has the lowest resistivity of the four, and smallest for nichrome, which has the highest. This shows that resistance depends on the nature of the material.
    Step-by-step solution →

Question 39 · 5 marks

  1. Q39(a) 5 marks
    1. (i)Which mirror will always form a full length image of a distant object ? Support your answer by a ray diagram for it.
    2. (ii)A convex lens forms an image with magnification . If the object is kept at cm from the lens, find out the focal length of convex lens and nature of image formed.
    Show answer

    (i) A convex mirror; (ii) cm, and the image is virtual, erect and twice the size of the object.

    (i) A convex mirror. For every position of the object it forms a virtual, erect and diminished image between its pole and its focus, and its outward bulge gives it a much wider field of view than a plane or a concave mirror of the same size, so the whole of a distant object is always taken in - which is exactly why a convex mirror is used as a rear-view mirror. The ray diagram shows a parallel ray reflected as though it came from and a ray to the pole reflected at an equal angle on the other side of the axis, their dashed extensions meeting at the small erect image between and . (ii) Focal length cm. Since and cm, the image distance is cm, and . The image is virtual, erect and magnified - twice the size of the object - and it is formed cm from the lens on the same side as the object.
    Step-by-step solution →
    OR
    Q39(b) 5 marks
    1. (i)What is meant by refraction of light ? Write the laws of refraction.
    2. (ii)If two lenses of focal length cm and cm are kept in contact, find out the net power of the lens combination. Will the combination behave as converging lens or a diverging lens ?
    Show answer

    The combination has net power D, so it behaves as a diverging lens of focal length cm.

    (i) Refraction of light is the change in the direction of a ray of light when it passes obliquely from one transparent medium into another; it occurs because the speed of light is different in the two media, so the ray bends towards the normal on entering a denser medium and away from the normal on entering a rarer one. The laws of refraction are: (1) the incident ray, the refracted ray and the normal to the surface at the point of incidence all lie in the same plane; (2) Snell's law - for light of a given colour and a given pair of media, , a constant, which is the refractive index of the second medium with respect to the first. (ii) D and D, so the net power is D. The power is negative, so the combination behaves as a diverging (concave) lens, of equivalent focal length cm.
    Step-by-step solution →

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