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CBSE Class 10 Science 2025 — Paper 31/S/3

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

33 of its 45 questions also appear in 31/S/1, 31/S/2 and other papers; 12 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark
    Select from the following decomposition reactions in which the source of energy for decomposition is heat :
    1. (i)
    2. (ii)
    3. (iii)
    4. (iv)
    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(iii) and (iv)
    4. (d)(i) and (iv)
    Show answer
    (d) (i) and (iv)
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/S/1 (Q5)

    The number of covalent bond(s) present in a nitrogen molecule is/are : (Atomic number of nitrogen is 7)
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/S/1 (Q4)

    Consider the reactions of dil. hydrochloric acid with the following metals : Aluminium, Copper, Iron, Magnesium and Zinc The correct observation is :
    1. (a)Most vigorous reaction with aluminium and no reaction with iron
    2. (b)Most vigorous reaction with magnesium and no reaction with iron
    3. (c)Most vigorous reaction with magnesium and no reaction with copper
    4. (d)Most vigorous reaction with zinc and no reaction with copper
    Show answer
    (c) Most vigorous reaction with magnesium and no reaction with copper
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/S/1 (Q1), 31/S/2 (Q8)

    From the following, select a process in which combination reaction occurs :
    1. (a)Black and white photography
    2. (b)Burning of carbon (coal)
    3. (c)Burning of methane
    4. (d)Refining of copper
    Show answer
    (b) Burning of carbon (coal)
    Step-by-step solution →
  5. Q5 1 mark

    Also in 2024 · 31/1/1 (Q6), 2024 · 31/1/2 (Q2), 2024 · 31/1/3 (Q9)

    Oxides of aluminium and zinc are :
    1. (a)Acidic
    2. (b)Amphoteric
    3. (c)Basic
    4. (d)Neutral
    Show answer
    (b) Amphoteric
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/S/1 (Q8), 31/S/2 (Q1)

    Application of Abscisic acid on plants promotes :
    1. (a)Development of fruits
    2. (b)Wilting of leaves
    3. (c)Elongation of stem
    4. (d)Formation of flowers
    Show answer
    (b) Wilting of leaves
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/S/1 (Q6)

    The valves in the human heart ensure that :
    1. (i)Oxygen-rich blood flows from left atrium to left ventricle only.
    2. (ii)Deoxygenated blood flows from left atrium to left ventricle only.
    3. (iii)Oxygen-rich blood flows from right atrium to right ventricle only.
    4. (iv)Deoxygenated blood flows from right atrium to right ventricle only.
    1. (a)(i) and (iii)
    2. (b)(ii) and (iv)
    3. (c)(i) and (iv)
    4. (d)(ii) and (iii)
    Show answer
    (c) (i) and (iv)
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/S/1 (Q7), 31/S/2 (Q2)

    The part of the hindbrain present immediately above the medulla is :
    1. (a)Cerebellum
    2. (b)Pons
    3. (c)Cerebrum
    4. (d)Spinal cord
    Show answer
    (b) Pons
    Step-by-step solution →
  9. Q9 1 mark

    Also in 31/S/1 (Q10)

    In garden pea, violet colour flower is the dominant trait over white colour flower which is the recessive trait. When parent plant with violet coloured flowers (Vv) is crossed with a parent plant with white flowers, the possible percentage of occurrence of traits in the F1 progeny would be :
    1. (a) violet
    2. (b) violet, white
    3. (c) violet, white
    4. (d) violet, white
    Show answer
    (b) violet, white
    Step-by-step solution →
  10. Q10 1 mark

    Also in 31/S/1 (Q9), 31/S/2 (Q10)

    The end product of meiosis in a reproductive cell in plants is :
    1. (a)Zygote
    2. (b)Pollen grains
    3. (c)Sperms
    4. (d)Egg
    Show answer
    (b) Pollen grains
    Step-by-step solution →
  11. Q11 1 mark

    Also in 31/S/1 (Q15), 31/S/2 (Q12)

    The organism occupying the fourth trophic level in a food chain operating in grassland is :
    1. (a)Green plants
    2. (b)Frog
    3. (c)Grasshopper
    4. (d)Snake
    Show answer
    (d) Snake
    Step-by-step solution →
  12. Q12 1 mark

    Also in 31/S/1 (Q14), 31/S/2 (Q13)

    The strength of the magnetic field inside a current carrying long straight solenoid is :
    1. (a)maximum at its centre.
    2. (b)minimum at its centre.
    3. (c)maximum at the ends and minimum at its centre.
    4. (d)uniform throughout its length.
    Show answer
    (d) uniform throughout its length.
    Step-by-step solution →
  13. Q13 1 mark

    Also in 31/S/1 (Q12), 31/S/2 (Q15)

    An optical device which always produces images of is :
    1. (a)Plane mirror
    2. (b)Convex mirror
    3. (c)Concave mirror
    4. (d)Convex lens
    Show answer
    (a) Plane mirror
    Step-by-step solution →
  14. Q14 1 mark

    Also in 31/S/1 (Q13), 31/S/2 (Q14)

    Which one of the following factors affects the electrical resistivity of a conductor ?
    1. (a)Temperature
    2. (b)Length
    3. (c)Shape
    4. (d)Thickness
    Show answer
    (a) Temperature
    Step-by-step solution →
  15. Q15 1 mark
    Concave mirror finds its application in which of the following cases ?
    1. (a)A solar furnace
    2. (b)A rear-view mirror in vehicles
    3. (c)To view full size erect image of distant tall buildings
    4. (d)To produce divergent beam of light
    Show answer
    (a) A solar furnace
    Step-by-step solution →
  16. Q16 1 mark
    In an ecosystem, 10% of energy available for transfer from one trophic level to the next in a food chain is in the form of :
    1. (a)heat energy
    2. (b)chemical energy
    3. (c)mechanical energy
    4. (d)light energy
    Show answer
    (b) chemical energy
    Step-by-step solution →
  17. Q17 1 mark

    Also in 31/S/1 (Q19), 31/S/2 (Q18)

    Two statements are given below — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): In our homes we receive supply of electric power through a main supply. One of the wires in this supply, usually with red insulation, is called live wire and another wire with green insulation is called neutral wire.

    Reason (R): In our country, the potential difference between the live wire and the neutral wire is 220 volts.

    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
    (d) Assertion (A) is false, but Reason (R) is true.
    Step-by-step solution →
  18. Q18 1 mark

    Also in 31/S/1 (Q17), 31/S/2 (Q19)

    Two statements are given below — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Decomposition of vegetable matter into compost is an exothermic reaction.

    Reason (R): Decomposition reactions need energy to break down the reactants.

    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
    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    Step-by-step solution →
  19. Q19 1 mark

    Also in 31/S/1 (Q20), 31/S/2 (Q17)

    Two statements are given below — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Due to scattering of sunlight, the clear sky predominantly appears blue.

    Reason (R): The wavelength of blue light is nearly times the wavelength of red light.

    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
    (c) Assertion (A) is true, but Reason (R) is false.
    Step-by-step solution →
  20. Q20 1 mark

    Also in 31/S/1 (Q18), 31/S/2 (Q20)

    Two statements are given below — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Transpirational pull helps in the absorption and upward movement of water and dissolved minerals in plants.

    Reason (R): The effect of root pressure in transport of water is more significant at night.

    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
    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    Step-by-step solution →

Questions 21–26 · 2 marks each

  1. Q21 2 marks

    Also in 31/S/1 (Q26), 31/S/2 (Q24)

    Draw a ray diagram to show the path of a ray of light which falls obliquely on one of the faces of an equilateral triangular prism made of glass and mark on it the angle of incidence () and the angle of deviation ().
    Show answer

    The ray bends towards the normal at and away at , giving angle of deviation for .

    The ray diagram is shown in the figure. The ray strikes the face obliquely, bends towards the normal on entering the glass, travels across the prism, and bends away from the normal as it emerges from the face , so it is deviated towards the base. The angle of incidence is marked between the incident ray and the normal at the first face, and the angle of deviation is marked between the incident ray produced forward and the emergent ray. For the case drawn, and with , so .
    Step-by-step solution →
  2. Q22 2 marks

    Also in 31/S/1 (Q21), 31/S/2 (Q22)

    Translate the following statements into balanced chemical equations :
    1. (a)Aluminium reacts with copper chloride to form aluminium chloride and copper.
    2. (b)Zinc reacts with sodium hydroxide to give sodium zincate and hydrogen gas.
    Show answer

    The balanced equations are and .

    (a) ; (b)
    Step-by-step solution →
  3. Q23(a) 2 marks

    Also in 31/S/1 (Q22(a)), 31/S/2 (Q21(a))

    1. (i)What is an alloy ?
    2. (ii)Write the composition of solder and the property which makes it suitable for welding electric wires.
    Show answer

    An alloy is a homogeneous mixture of metals; solder is lead and tin, and its low melting point suits welding wires.

    (i) An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. (ii) Solder is an alloy of lead and tin taken in roughly equal amounts, about Pb and Sn; its very low melting point — lower than that of either lead or tin — is the property that makes it suitable for welding electric wires.
    Step-by-step solution →
    OR
    Q23(b) 2 marks

    Also in 31/S/1 (Q22(b)), 31/S/2 (Q21(b))

    During electrolytic refining of silver :
    1. (i)What are the electrodes – cathode and anode made up of ?
    2. (ii)What is anode mud ? Why is it called so ?
    Show answer

    Anode is impure silver and cathode is pure silver; anode mud is the insoluble noble-metal impurity that settles below the anode.

    (i) The anode is a thick block of impure silver and the cathode is a thin strip of pure silver, dipped in a solution of a silver salt such as acidified silver nitrate. (ii) Anode mud is the insoluble impurities — noble metals such as gold and platinum — which do not dissolve in the electrolyte and settle down at the bottom, just below the anode; it is called anode mud because it collects beneath the anode as a mud-like deposit.
    Step-by-step solution →
  4. Q24 2 marks
    Explain how circulation of blood takes place in fishes.
    Show answer

    Blood passes once through the fish's two-chambered heart, going to the gills and then the body: single circulation.

    A fish has a two-chambered heart (one atrium and one ventricle). The heart receives deoxygenated blood from the body and pumps it to the gills, where it takes up oxygen from the water and becomes oxygenated. The oxygenated blood flows directly to the rest of the body and, after giving up its oxygen, returns deoxygenated to the heart. Blood passes through the heart only once in each round, so this is called single circulation.
    Step-by-step solution →
  5. Q25 2 marks
    An object is placed cm in front of a concave mirror of focal length cm. Use mirror formula to determine the position of the image formed in this case.
    Show answer

    cm: the image is formed 36 cm in front of the mirror, beyond , real, inverted and enlarged ().

    cm: the image is formed cm in front of the mirror, on the same side as the object, beyond the centre of curvature. It is real, inverted and enlarged, with .
    Step-by-step solution →
  6. Q26(a) 2 marks

    Also in 31/S/1 (Q24(a)), 31/S/2 (Q26(a))

    Explain how tendrils help a pea plant climb up other plants or fences.
    Show answer

    The part of a tendril touching a support grows more slowly, so the tendril coils round it and anchors the weak stem.

    Tendrils are thin thread-like growths of the stem that are sensitive to touch. When a tendril meets a support, the part of the tendril in contact with the object grows more slowly than the part away from it. This unequal growth makes the tendril bend towards the support, circle around it and cling to it. The coils anchor the weak stem, so the pea plant is held up and can climb another plant or a fence.
    Step-by-step solution →
    OR
    Q26(b) 2 marks

    Also in 31/S/1 (Q24(b)), 31/S/2 (Q26(b))

    List two major limitations of the use of electrical impulses as a means of communication between the cells in most multicellular animals.
    Show answer

    Impulses reach only cells joined by nervous tissue, and a cell needs time to reset before it can send another impulse.

    (i) Electrical impulses reach only those cells that are connected by nervous tissue, and not each and every cell of the animal body. (ii) After a cell has generated and transmitted an impulse it needs time to reset its mechanisms before it can send another, so cells cannot continually create and transmit electrical impulses. These limitations are why most multicellular animals also use chemical communication.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks
    Using flow chart, show the steps involved in the method of extraction of a metal of medium reactivity from its ore.
    Show answer

    The ore is concentrated, converted to the oxide by roasting (sulphide) or calcination (carbonate), reduced with carbon, then refined.

    Flow chart: Ore of a metal of medium reactivity → Concentration of the ore (gangue removed) → Sulphide ore → Roasting (heated strongly in excess air), or Carbonate ore → Calcination (heated strongly in limited air) → Metal oxide → Reduction to metal (heated with a reducing agent such as carbon) → Metal → Refining (purification, usually electrolytic) → Pure metal. With zinc as the example: ; ; .
    Step-by-step solution →
  2. Q28 3 marks

    Also in 31/S/1 (Q27), 31/S/2 (Q30)

    1. (a)Write the name and chemical formula of a sodium compound which is sometimes added for faster cooking. How is it produced from sodium chloride as one of the raw materials ? Give chemical equation for the reaction involved.
    2. (b)The compound mentioned in (a) above is also an ingredient of antacids. Why ?
    Show answer

    Sodium hydrogencarbonate, , is made from brine, ammonia and carbon dioxide; being a mild base, it neutralises excess stomach acid.

    (a) The compound is sodium hydrogencarbonate, or baking soda, formula . It is made from brine by saturating the sodium chloride solution with ammonia and then passing carbon dioxide through it: ; the sparingly soluble sodium hydrogencarbonate separates out. (b) It is an ingredient of antacids because it is a mild, non-corrosive base and therefore neutralises the excess hydrochloric acid in the stomach, raising the pH and relieving the acidity: .
    Step-by-step solution →
  3. Q29(a) 3 marks

    Also in 31/S/1 (Q30(a)), 31/S/2 (Q33(a))

    Explain the process of translocation of food and other substances in the phloem of plants.
    Show answer

    Sucrose is loaded into the phloem using ATP, water follows by osmosis, and the resulting pressure moves food from source to sink.

    Translocation is the transport of the soluble products of photosynthesis, mainly sucrose, together with amino acids and other substances, through the phloem. It is carried out by the sieve tubes with the help of the companion cells beside them, and unlike transport in the xylem it is an active process that uses energy from ATP. At the source, usually a leaf, sucrose is loaded into the phloem tissue using energy from ATP. This raises the solute concentration inside the sieve tube, so water moves into it by osmosis from the neighbouring xylem. The entry of water builds up a high hydrostatic pressure at the source, and this pressure moves the material in the phloem to the tissues where the pressure is lower, the sinks. At the sink the sucrose is unloaded and used or stored, and the water returns to the xylem. Because the driving force is created within the phloem itself, translocation takes place in both directions, and the plant can send material wherever it is needed; in spring, sugar stored in the root or stem is carried up to the growing buds.
    Step-by-step solution →
    OR
    Q29(b) 3 marks

    Also in 31/S/1 (Q30(b)), 31/S/2 (Q33(b))

    Differentiate between xylem and phloem on the basis of the following :
    1. (i)Direction of transport of the substances
    2. (ii)Major driving forces involved in the transport
    3. (iii)Nature of the substance(s) transported
    Show answer

    Xylem moves water and minerals upwards passively; phloem moves sucrose and other food both ways using ATP energy.

    (i) Direction of transport: in xylem the transport is unidirectional, water and minerals moving only upwards from the roots to the rest of the plant; in phloem it is bidirectional, the material moving from a source to a sink and so travelling both upwards and downwards. (ii) Major driving forces: in xylem the transport is passive and physical, driven mainly by the transpiration pull created by evaporation of water from the leaves, helped by root pressure, and no ATP is used; in phloem the transport is active, energy from ATP being used to load sucrose into the sieve tube, after which water enters by osmosis and the resulting high hydrostatic pressure drives the solution towards the sink. (iii) Nature of the substances transported: xylem carries water and dissolved inorganic minerals absorbed from the soil, while phloem carries the soluble products of photosynthesis, mainly sucrose, together with amino acids and other organic substances.
    Step-by-step solution →
  4. Q30 3 marks
    Explain how urine is produced in kidneys.
    Show answer

    Urine forms in nephrons by filtration of blood in the glomerulus, then selective reabsorption of useful substances and water in the tubule.

    Urine is made in the nephrons, the filtration units of the kidney. Filtration: blood brought by the renal artery is filtered in the glomerulus inside Bowman's capsule, and water, glucose, amino acids, salts and urea pass into the capsule as the initial filtrate. Selective reabsorption: as the filtrate flows along the tubule, glucose, amino acids, salts and a major amount of water are taken back into the surrounding capillaries, the amount of water depending on the body's needs. The fluid left, mainly water with urea, is urine; it drains through the collecting duct into the ureter and is stored in the urinary bladder.
    Step-by-step solution →
  5. Q31 3 marks
    1. (a)Write mathematical expression for Joule’s law of heating.
    2. (b)Calculate the amount of heat generated while transferring coulomb of charge in two hours through a potential difference of V.
    Show answer

    Joule's law is , and the heat generated is J.

    (a) Joule's law of heating: , the heat produced in a resistor of resistance that carries a current for a time . (b) h s, so A and J (equally, J).
    Step-by-step solution →
  6. Q32 3 marks

    Also in 31/S/1 (Q33), 31/S/2 (Q27)

    1. (a)Define the term biological magnification.
    2. (b)In the given aquatic food chain, which trophic level will have highest effect of biological magnification and why ? Phytoplankton Zooplankton Fishes Human
    Show answer

    Biological magnification is the progressive rise of harmful chemicals along a food chain; it is highest in human beings, the last level.

    (a) Biological magnification is the progressive increase in the concentration of harmful non-biodegradable chemicals, such as pesticides, at each successive trophic level of a food chain. (b) In the given chain the highest effect of biological magnification will be at the last trophic level, that is in Human beings, the fourth trophic level. The reason is that these chemicals cannot be broken down or excreted, so they are stored in the body of every organism that takes them in. At each step an organism eats a large number of organisms of the level below, so almost all the chemical stored in them is collected into one body, while only about ten per cent of the biomass is passed on. The concentration therefore multiplies at every step and is greatest in the organism at the end of the chain.
    Step-by-step solution →
  7. Q33 3 marks

    Also in 31/S/1 (Q32)

    As shown in the figure a small aluminum rod AB is suspended horizontally between the poles of a strong horseshoe magnet. This rod is also connected with a battery and a key. Study the arrangement shown. (a) State Fleming's left-hand rule. (b) Apply Fleming's left-hand rule to determine :
    1. (i)What is observed when a current is passed from B to A in the rod ?
    2. (ii)What is observed when a current is passed from A to B in the rod ?
    3. (iii)What is observed when the rod AB is aligned parallel to the magnetic field and current is passed through it from B to A ? Justify your answer in this case.
    S N A B − + K
    Aluminium rod AB suspended between the poles of a horseshoe magnet and joined to a battery and a key.
    Show answer

    Rod moves left for current B to A, right for A to B; parallel to the field there is no force.

    (a) Fleming's left-hand rule: stretch the thumb, the forefinger and the middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force, and hence of the motion, of the conductor. (b) In the figure the S pole is above the rod and the N pole below it, so the field in the gap is vertically upward. (i) With the current from B to A, the rod is displaced towards the left, in the direction of the arrow marked at A. (ii) With the current from A to B, the current is reversed while the field is unchanged, so the force reverses and the rod is displaced towards the right. (iii) With the rod aligned parallel to the magnetic field, no displacement is observed. The force on a current-carrying conductor is , where is the angle between the conductor and the field; it is maximum when the conductor is perpendicular to the field and zero when the conductor is parallel to it, since .
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks
    1. (i)Differentiate between self-pollination and cross-pollination.
    2. (ii)Explain the events taking place in a flower after a pollen lands on a suitable stigma, till fruit-formation.
    Show answer

    Self-pollination stays within one flower, cross-pollination goes between flowers; after fertilisation the ovule becomes the seed and the ovary the fruit.

    (i) Self-pollination is the transfer of pollen from the anther to the stigma of the same flower; it can take place without an outside agent, and the offspring vary less. Cross-pollination is the transfer of pollen from one flower to the stigma of another, by agents such as wind, water or animals; between flowers of different plants it combines germ-cells from two parents and gives more variation. (ii) The pollen grain germinates on the stigma and a pollen tube grows out of it, travels through the style and reaches the ovule in the ovary. The male germ-cell carried by the tube fuses with the female germ-cell in the ovule (fertilisation) to form a zygote. The zygote divides to form an embryo, the ovule develops a tough coat and becomes the seed, and the ovary grows and ripens into the fruit; the petals, sepals, stamens, style and stigma may shrivel and fall off.
    Step-by-step solution →
    OR
    Q34(b) 5 marks
    1. (i)What is the site of fertilisation of egg in a female reproductive tract ?
    2. (ii)Fertilised egg on implantation in uterus lining continues to grow and develop to become a foetus. Explain how it gets nourishment and removes metabolic wastes during this period.
    Show answer

    Fertilisation occurs in the oviduct; the embryo gets glucose and oxygen and loses wastes through the placenta.

    (i) Fertilisation takes place in the oviduct (fallopian tube): the egg released by the ovary passes into the oviduct, and the sperms, travelling up through the cervix and the uterus, meet it there. (ii) After implantation the embryo is nourished through the placenta, a disc-like tissue embedded in the uterine wall. It has villi on the embryo's side and blood spaces of the mother around the villi, giving a large surface area. Glucose and oxygen pass from the mother's blood into the embryo's blood through the placenta, which nourishes it, and the wastes the embryo produces, such as carbon dioxide, pass the other way into the mother's blood, to be removed by her body.
    Step-by-step solution →
  2. Q35(a) 5 marks

    Also in 31/S/1 (Q35(a))

    1. (i)Define the term power of accommodation of human eye. What happens to the image distance in the eyes when the distance of an object is increased gradually from about m to m from our eyes ? Name and explain the role of the part of human eye responsible for it in this case.
    2. (ii)A person is unable to see distinctly the objects placed beyond m from his eyes. Name the defect of vision the person is suffering from. List two possible causes of this defect and write the type of lenses used for the correction of this defect.
    Show answer

    Image distance stays constant as the ciliary muscles change the lens focal length; the person has myopia, corrected by a concave lens.

    (i) The power of accommodation of the human eye is the ability of the eye lens to adjust its focal length, by changing its curvature, so that objects at different distances are focused clearly on the retina. As the object is moved gradually from about m to about m, the image distance in the eye does not change: it remains constant, equal to the fixed distance between the eye lens and the retina, about cm, because the retina cannot move. The part responsible is the ciliary muscle. As the object recedes, the ciliary muscles relax, the eye lens becomes thinner, its curvature decreases and its focal length increases, so the image of the more distant object is still formed on the retina. (ii) The person has a far point of m instead of infinity, so he is suffering from myopia, that is short sightedness. Two possible causes are excessive curvature of the eye lens and elongation of the eyeball. The defect is corrected by using a concave, that is a diverging, lens of suitable power; here a lens of focal length m, of power D, would be required.
    Step-by-step solution →
    OR
    Q35(b) 5 marks

    Also in 31/S/1 (Q35(b))

    1. (i)Name and explain the phenomenon of light responsible to make the path of a beam of light visible when it enters a smoke/dust-filled room through a small hole. Also state the dependence of the colour of the light we receive on the size of the particles of the medium through which the light passes.
    2. (ii)A person suffering from presbyopia needs spectacles having bifocal lenses. If the power of two lenses used in his spectacles is D and D, which one of the two lenses is for the correction of his distant vision and what is its focal length ?
    Show answer

    Tyndall effect makes the beam visible, with fine particles scattering blue; the concave D lens is for distant vision, with m.

    (i) The phenomenon is the Tyndall effect, the scattering of light by the tiny particles of smoke and dust suspended in the air. As the beam passes through the room these particles scatter the light in all directions, and some of the scattered light reaches our eyes from the side, so the whole path of the beam becomes visible. The colour of the light we receive depends on the size of the scattering particles: very fine particles scatter mainly blue light, that is light of shorter wavelengths, while particles of larger size scatter light of longer wavelengths, and if the particles are large enough the scattered light may even appear white. (ii) In a bifocal lens the upper concave portion corrects distant vision and the lower convex portion is used for reading. Since , a negative power means a concave, diverging lens, so the lens for the correction of distant vision is the one of power D. Its focal length is m, that is cm.
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  3. Q36(a) 5 marks
    1. (i)Give the relation between hydrogen ion concentration of an aqueous solution and its pH.
    2. (ii)Concentrated acids should not be diluted by adding water to acid. Why ?
    3. (iii)Why do same molar concentrations of hydrochloric acid and acetic acid not produce same amounts of hydrogen ions ?
    Show answer

    Higher hydrogen-ion concentration means lower pH; diluting acid is exothermic, so acid goes into water; hydrochloric acid ionises fully, acetic acid partly.

    (i) The pH of a solution decreases as its hydrogen-ion concentration increases: the higher the concentration of hydrogen ions, the lower the pH (below acidic, neutral, above basic); in symbols, . (ii) Dissolving an acid in water is highly exothermic. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns, and the glass container may break due to excessive local heating; so the acid is added slowly to water, with constant stirring. (iii) Hydrochloric acid is a strong acid and ionises completely in water, whereas acetic acid is a weak acid and ionises only partially, so the same molar concentration of acetic acid produces far fewer hydrogen ions.
    Step-by-step solution →
    OR
    Q36(b) 5 marks
    Give reason :
    1. (i)Crystals of washing soda change to white powder on exposure to air.
    2. (ii)Plaster of Paris should be stored in moisture-proof containers.
    3. (iii)Baking soda can be used on bee sting area to get relief.
    4. (iv)Distilled water does not conduct electricity.
    5. (v)Farmers treat the soil of their fields with quick lime.
    Show answer

    Washing soda loses water of crystallisation, Plaster of Paris sets with moisture, distilled water has no ions, and bases neutralise acids.

    (i) Washing soda crystals, , lose most of their water of crystallisation on exposure to air and change to the white powder . (ii) Plaster of Paris takes up moisture and sets into a hard mass of gypsum, , so it must be kept dry until it is used. (iii) A bee sting leaves an acid, which causes pain and irritation; baking soda is a mild base that neutralises it. (iv) Distilled water contains practically no ions, and ions are needed to carry a current through water. (v) Quick lime is a base; it neutralises the excess acidity of the soil and makes it suitable for crops.
    Step-by-step solution →

Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/S/1 (Q38), 31/S/2 (Q39)

    The process of determining the sex of a newborn is purely genetic. But still many people in some societies believe that only the woman (mother) is responsible for determination of the sex of the child and blame her. In fact, the sex of the child is genetically determined. But there are some animals where sex is not genetically determined.
    1. (a)Define sex chromosomes.[1]
    2. (b)Mention one example of animals where sex is not genetically determined. How is sex determined in these animals ? Explain.[1]
    3. (c)(i)Does the genetic combination of the mother play a significant role in determining the sex of newborn ? Explain.[2]
    4. (c)(ii)OR : How is equal genetic contribution of male and female parents ensured in progeny ?[2]
    Show answer

    Sex chromosomes (XX, XY) decide sex; turtle sex depends on egg temperature; the father's sperm decides a child's sex; each parent gives 23 chromosomes.

    (a) Sex chromosomes are the pair of chromosomes that determines the sex of an individual, as opposed to the autosomes, which are alike in both sexes. In human beings they are the twenty-third of the 23 pairs: a female has two X chromosomes (XX) and a male has one X and one Y chromosome (XY). (b) Turtles are one such example (crocodiles and some lizards behave in the same way); snails are another. In turtles the sex is not fixed by the chromosomes but by the environment: the temperature at which the fertilised eggs are incubated during the embryo's development decides whether the young that hatch are males or females. In a snail, an individual can itself change sex. (c)(i) No. The mother is XX, so every egg she produces necessarily carries an X chromosome and she has no other kind of egg to give. The father is XY, so half his sperms carry X and half carry Y. An X-bearing sperm gives an XX zygote, a girl, and a Y-bearing sperm gives an XY zygote, a boy, in equal proportion. The sex of the newborn is therefore decided by the father's sperm, and the mother's genetic combination plays no part in it. (c)(ii) Each parent contributes exactly one chromosome of every pair. A human body cell has 23 pairs, that is 46 chromosomes; during the formation of the germ cells this number is halved, so the egg carries 23 chromosomes and the sperm carries 23; at fertilisation they fuse and the zygote regains 23 + 23 = 46. Since one chromosome of every pair comes from the mother and the other from the father, the progeny inherits one copy of each gene from each parent, which is what makes the genetic contribution of the two parents equal.
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  2. Q38 4 marks

    Also in 31/S/1 (Q39), 31/S/2 (Q37)

    In a domestic circuit, six LED bulbs and a ceiling fan are arranged as per the circuit shown in the figure. The source voltage is 220 volts and the power rating of each device is marked in the circuit diagram. Two fuses and are also provided in the circuit. Based on the circuit diagram, answer the following questions :
    K1 K2 F2 − + + − 220 V F1 40 W, 220 V L 20 W, 220 V 10 W, 44 V 10 W, 44 V 10 W, 44 V 10 W, 44 V 10 W, 44 V A B C D E
    1. (a)State what happens when (i) only key is closed, (ii) and key is also closed.[1]
    2. (b)Find the current drawn by the bulb L.[1]
    3. (c)(i)Calculate the resistance of the bulb A. Find the total resistance offered by the circuit to the flow of current when only key is closed.[2]
    4. (c)(ii)OR : What would happen to the glow of the bulbs in the circuit when and both are closed and the fuse melts suddenly ? Give reason to justify your answer.[2]
    Show answer

    With only the fan and L work; adds A to E; A, , total .

    (a)(i) With only closed, only the branch holding fuse , the ceiling fan and bulb L is complete, so the fan runs and bulb L glows, both at their rated 220 V and therefore normally; bulbs A to E do not glow, their branch being open at . The current drawn is A. (ii) When is closed as well, the series string A, B, C, D, E is completed and all five bulbs glow, each receiving V, exactly its rated voltage; the fan and bulb L are unaffected, because the branches are in parallel and each still has the whole 220 V across it, and the total current rises to A. (b) A. (c)(i) . With only closed the only conducting path is the series string of the five bulbs, so the total resistance offered by the circuit is . (c)(ii) The ceiling fan stops and bulb L goes out, because is in series with that branch alone and melting it breaks the branch. The five bulbs A, B, C, D and E continue to glow with exactly the same brightness as before, because they lie in a separate parallel branch that is still connected to the supply through and : the potential difference across that branch is still 220 V, so the current in it is still A and each bulb still has 44 V across it and consumes 10 W.
    Step-by-step solution →
  3. Q39 4 marks

    Also in 31/S/1 (Q37), 31/S/2 (Q38)

    All compounds of carbon which contain only carbon and hydrogen are called hydrocarbons. Among these, saturated hydrocarbons are called 'alkanes', unsaturated compounds with one or more double bonds are called 'alkenes' and compounds containing one or more triple bonds are called 'alkynes'.
    1. (a)Identify the following compound and write its name. [1]
    2. (b)How do saturated and unsaturated carbon compounds differ in terms of the flame produced by them on burning ?[1]
    3. (c)(i)Which type of hydrocarbons undergo addition reactions ? Show with an example.[2]
    4. (c)(ii)OR : What are structural isomers ? Draw two structural isomers of butane ().[2]
    Show answer

    It is propyne, an alkyne; saturated compounds burn with a clean blue flame, unsaturated with a sooty yellow flame; alkenes and alkynes undergo addition.

    (a) The compound contains a carbon-carbon triple bond, so it is an alkyne; with three carbon atoms (molecular formula ) its name is propyne. (b) A saturated carbon compound burns with a clean blue, non-sooty flame, whereas an unsaturated carbon compound burns with a yellow flame and a great deal of black smoke, because its higher percentage of carbon does not burn completely in a limited supply of air. (c)(i) Unsaturated hydrocarbons, that is the alkenes and the alkynes, undergo addition reactions, because their double or triple bond can open. Example: ethene adds hydrogen over a nickel catalyst to give ethane, . (c)(ii) Structural isomers are compounds having the same molecular formula but different structural formulae, and hence different properties. Butane, , has two structural isomers: n-butane, (a straight chain of four carbon atoms), and iso-butane or 2-methylpropane, (a chain of three carbon atoms with a methyl group on the middle one).
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