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

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

32 of its 45 questions also appear in 31/3/1 or 31/3/2; 13 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

    Also in 31/3/1 (Q3)

    The most common method of extraction of metals from their oxide ores is :
    1. (a)Reduction with carbon
    2. (b)Reduction with hydrogen
    3. (c)Reduction with aluminium
    4. (d)Electrolytic reduction
    Show answer
    (a) Reduction with carbon
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/3/1 (Q1), 31/3/2 (Q3)

    Consider the following chemical equation : To balance this chemical equation, the values of 'p', 'q', 'r' and 's' must be respectively :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/3/1 (Q4), 31/3/2 (Q1)

    Given below are the structures of some hydrocarbons. Select the two structures which are related to each other from the given options :
    (i) C C C C H H H H H H H H (ii) C C C C H H H H H H H H H H (iii) C C C H H H H H H H C H H H (iv) C C C C Cl H H H H H H H
    1. (a)(i) and (iv)
    2. (b)(ii) and (iv)
    3. (c)(ii) and (iii)
    4. (d)(i) and (iii)
    Show answer
    (c) (ii) and (iii)
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/3/1 (Q5), 31/3/2 (Q2)

    Choose the incorrect statement about the common reaction used in hydrogenation of vegetable oils.
    1. (a)It is an addition reaction.
    2. (b)It takes place in the presence of nickel or palladium catalyst.
    3. (c)The product contains only single bonds between carbon atoms.
    4. (d)It is an addition reaction which occurs in the presence of an acid catalyst.
    Show answer
    (d) It is an addition reaction which occurs in the presence of an acid catalyst.
    Step-by-step solution →
  5. Q5 1 mark

    Also in 31/3/1 (Q2), 31/3/2 (Q5)

    Which of the given option represents a family of salts ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/3/1 (Q9), 31/3/2 (Q8)

    Select a pair of bisexual flowers from the following :
    1. (a)Papaya and mustard
    2. (b)Hibiscus and mustard
    3. (c)Hibiscus and papaya
    4. (d)Hibiscus and watermelon
    Show answer
    (b) Hibiscus and mustard
    Step-by-step solution →
  7. Q7 1 mark
    The growth of the pollen tubes towards ovules is an example of :
    1. (a)Phototropism
    2. (b)Hydrotropism
    3. (c)Geotropism
    4. (d)Chemotropism
    Show answer
    (d) Chemotropism
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/3/1 (Q6), 31/3/2 (Q6)

    Match Column-I with Column-II and select the correct option from the choices provided.
    Column-I and Column-II
    Column-IColumn-II
    a. Site of fertilisation of egg with the sperm(i) Vagina
    b. Site of implantation of embryo(ii) Uterus
    c. Site of entry of sperm into the female reproductive tract(iii) Oviduct
    d. Site through which the waste materials generated by the developing embryo are removed(iv) Placenta
    (v) Cervix
    1. (a)a-(ii), b-(i), c-(v), d-(iv)
    2. (b)a-(iii), b-(i), c-(v), d-(iv)
    3. (c)a-(iv), b-(ii), c-(iii), d-(i)
    4. (d)a-(iii), b-(ii), c-(i), d-(iv)
    Show answer
    (d) a-(iii), b-(ii), c-(i), d-(iv)
    Step-by-step solution →
  9. Q9 1 mark

    Also in 31/3/1 (Q8), 31/3/2 (Q9)

    The plant hormone present in greater concentration in the areas of rapidly dividing cells is :
    1. (a)Auxin
    2. (b)Cytokinins
    3. (c)Gibberellins
    4. (d)Abscisic acid
    Show answer
    (b) Cytokinins
    Step-by-step solution →
  10. Q10 1 mark
    Parasitic mode of nutrition is observed in :
    1. (a)Bryophyllum
    2. (b)Hibiscus
    3. (c)Cuscuta
    4. (d)Helianthus
    Show answer
    (c) Cuscuta
    Step-by-step solution →
  11. Q11 1 mark
    A candle flame is placed in front of the reflecting surface of a convex mirror of focal length . If the distance of the flame from the pole of the mirror is ‘’, its image is formed :
    1. (a)at infinite distance from the mirror
    2. (b)behind the mirror at the principal focus
    3. (c)behind the mirror at a distance
    4. (d)behind the mirror at a distance
    Show answer
    (d) behind the mirror at a distance
    Step-by-step solution →
  12. Q12 1 mark
    Select the correct statement from the following :
    1. (a)The size of the molecules of air is larger than the wavelength of visible light.
    2. (b)The blue light has a wavelength about 1.8 times greater than that of red light.
    3. (c)When sunlight passes through the fine particles in air, they scatter the blue colour of visible light more strongly than red.
    4. (d)The light of red colour is scattered the most by fog or smoke.
    Show answer
    (c) When sunlight passes through the fine particles in air, they scatter the blue colour of visible light more strongly than red.
    Step-by-step solution →
  13. Q13 1 mark
    In domestic electric circuits, the colour of insulation covers of wires in the cables of electric iron/electric toaster is generally :
    1. (a)red for live wire, green for neutral wire and black for earth wire
    2. (b)red for live wire, black for neutral wire and green for earth wire
    3. (c)green for live wire, black for neutral wire and red for earth wire
    4. (d)green for live wire, red for neutral wire and black for earth wire
    Show answer
    (b) red for live wire, black for neutral wire and green for earth wire
    Step-by-step solution →
  14. Q14 1 mark

    Also in 31/3/1 (Q15)

    The strength of magnetic field produced inside a long straight current carrying solenoid does not depend upon :
    1. (a)number of turns in the solenoid
    2. (b)direction of current flowing through the solenoid
    3. (c)material of the core filled inside the solenoid
    4. (d)radius of the coil of the solenoid
    Show answer
    (d) radius of the coil of the solenoid
    Step-by-step solution →
  15. Q15 1 mark

    Also in 31/3/1 (Q14), 31/3/2 (Q12)

    Which one of the following statements is not true about a bar magnet ?
    1. (a)It sets itself in north-south direction when suspended freely.
    2. (b)It has attractive power for iron filings.
    3. (c)It produces magnetic field lines.
    4. (d)The direction of magnetic field lines inside a bar magnet is from its north pole to its south pole.
    Show answer
    (d) The direction of magnetic field lines inside a bar magnet is from its north pole to its south pole.
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/3/1 (Q16), 31/3/2 (Q16)

    Other than the abiotic components, which of the given biotic components are not required to make an aquarium with small herbivorous fishes a self-sustaining system ?
    1. (i)Aquatic plants and aquatic animals
    2. (ii)Terrestrial plants and terrestrial animals
    3. (iii)Decomposers as bacteria and fungi
    4. (iv)Consumers as clown fishes and sea urchins
    1. (a)(i) and (iv)
    2. (b)(ii) and (iii)
    3. (c)(i) and (iii)
    4. (d)(ii) and (iv)
    Show answer
    (d) (ii) and (iv)
    Step-by-step solution →
  17. Q17 1 mark
    Two statements are given — 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): Brass is prepared by first melting copper and then dissolving tin into it in a definite proportion.

    Reason (R): The primary metal of brass is copper.

    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 31/3/1 (Q20), 31/3/2 (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) given below.

    Assertion (A): Use of jute bags for shopping reduces pollution.

    Reason (R): Jute is biodegradable and its bag may be reused as and when needed.

    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 →
  19. Q19 1 mark

    Also in 31/3/1 (Q18), 31/3/2 (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) given below.

    Assertion (A): In our actions of writing or talking, our nervous system communicates with the muscles.

    Reason (R): Cranial nerves and spinal nerves form the peripheral nervous system.

    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 →
  20. Q20 1 mark

    Also in 31/3/1 (Q19), 31/3/2 (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) given below.

    Assertion (A): Magnetic field lines around a bar magnet never intersect each other.

    Reason (R): Magnetic field produced by a bar magnet is a quantity that has both magnitude and direction.

    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(a) 2 marks

    Also in 31/3/1 (Q21(a)), 31/3/2 (Q21(a))

    List the possible sources of energy required in decomposition reactions. Illustrate any one with a suitable example.
    Show answer

    Heat, electricity or light can supply the energy; for example, electricity splits water into hydrogen and oxygen.

    The energy needed for a decomposition reaction can be supplied as heat, as electricity or as light. Example (electricity): when a current is passed through acidified water, it decomposes into hydrogen at the cathode and oxygen at the anode in the volume ratio , .
    Step-by-step solution →
    OR
    Q21(b) 2 marks

    Also in 31/3/1 (Q21(b)), 31/3/2 (Q21(b))

    What is observed when hydrated ferrous sulphate crystals are heated in a dry boiling tube ? Give balanced chemical equation(s) of the reaction(s) that occur(s).
    Show answer

    Green crystals turn white, then reddish brown, giving off gases with the smell of burning sulphur.

    The green crystals first turn white as the water of crystallisation is driven off, and on stronger heating the white powder turns reddish brown while gases with the smell of burning sulphur are given out. ; .
    Step-by-step solution →
  2. Q22 2 marks
    A chemical compound ‘X’ is used to bleach washed clothes in laundry as well as to make drinking water free from germs. Identify ‘X’. How is this compound represented ? Write the method of its preparation along with the chemical equation for the reaction that occurs.
    Show answer

    X is bleaching powder, , made by passing chlorine over dry slaked lime.

    X is bleaching powder. It is represented as (calcium oxychloride), though its actual composition is quite complex. It is prepared by the action of chlorine gas on dry slaked lime: .
    Step-by-step solution →
  3. Q23 2 marks

    Also in 31/3/1 (Q23)

    State the main function of veins in human circulatory system. Why do they not need thick walls ?
    Show answer

    Veins carry blood from the organs back to the heart; their walls can be thin because the blood is at low pressure.

    Veins collect blood from the various organs of the body and carry it back to the heart (all except the pulmonary vein carry deoxygenated blood). They do not need thick walls because the blood in them is no longer under high pressure — most of the pressure has been used up in forcing the blood through the narrow capillaries — so instead of thick elastic walls they have wide lumens and valves that keep the slow blood flowing only towards the heart.
    Step-by-step solution →
  4. Q24 2 marks
    “Sex of the children is determined by type of sex chromosome which they inherit from their father.” Justify the statement.
    Show answer

    The mother always gives X, so the child's sex depends on whether the father's sperm gives X or Y.

    A woman’s sex chromosomes are XX, so every egg she produces carries an X chromosome. A man’s are XY, so half his sperms carry an X and half carry a Y. If an X-bearing sperm fertilises the egg, the child is XX — a girl; if a Y-bearing sperm fertilises it, the child is XY — a boy. The mother contributes an X in every case, so the sex of the child depends on whether it inherits an X or a Y chromosome from the father, which justifies the statement.
    Step-by-step solution →
  5. Q25(a) 2 marks

    Also in 31/3/1 (Q25(a)), 31/3/2 (Q25(a))

    A student has difficulty in reading his textbooks but can read the blackboard clearly while sitting in the last row. Name the defect of vision the student is suffering from. List two reasons due to which this defect arises. Write the nature of the lenses required to correct this defect.
    Show answer

    Hypermetropia, caused by a long eye-lens focal length or a short eyeball, corrected with a convex lens.

    The defect is hypermetropia (far-sightedness). It arises because (i) the focal length of the eye lens has become too long, so the lens converges the rays too little, or (ii) the eyeball has become too short from front to back, so the retina lies in front of the image. It is corrected with a convex (converging) lens of suitable power, which brings the rays from a near object to a focus on the retina.
    Step-by-step solution →
    OR
    Q25(b) 2 marks

    Also in 31/3/1 (Q25(b)), 31/3/2 (Q25(b))

    Draw a ray diagram to show the path of a ray of light falling obliquely on one of the refracting faces of a triangular glass prism and mark the angle of deviation on it.
    Show answer

    The ray bends towards the normal at the first face and away at the second, turning towards the base; .

    The ray bends towards the normal on entering the prism at , travels straight to on the second face, and bends away from the normal on emerging, so that it is turned towards the base of the prism. The angle of deviation is marked between the produced incident ray and the emergent ray, where . For the diagram drawn (, , ): , , and .
    Step-by-step solution →
  6. Q26 2 marks
    Four resistors, each of resistance , are joined end to end to form a square ABCD as shown. Using appropriate formula, determine the equivalent resistance of the combination between its two ends A and B.
    2.0 Ω A B 2.0 Ω 2.0 Ω D C 2.0 Ω
    Show answer

    The equivalent resistance between A and B is .

    The resistor AB () is in parallel with the series path A–D–C–B (), so and .
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks
    State the change that is observed when a China dish containing copper powder is heated over the flame of a burner. Name the phenomenon responsible for the change and write balanced equation for the chemical reaction that occurs. How is this reaction different from the reaction that occurs when copper wares kept in open air slowly lose their shiny brown surface and gain a coat ? Write chemical name of the coating and state its colour.
    Show answer

    Copper turns black by oxidation to copper(II) oxide; the slow dulling of copper wares is corrosion, giving a green coat of basic copper carbonate.

    On heating, the brown surface of the copper powder becomes coated with a black substance, copper(II) oxide. The phenomenon is oxidation — copper gains oxygen from the air: . The dulling of copper wares is different: it happens slowly, at room temperature and without heating, through the action of the moist carbon dioxide and oxygen of the air on the copper — corrosion — rather than by a quick reaction with oxygen alone on heating. The coating formed on copper wares is basic copper carbonate, , which is green.
    Step-by-step solution →
  2. Q28(a) 3 marks

    Also in 31/3/1 (Q28(a)), 31/3/2 (Q28(a))

    Common salt is an important raw material for various chemicals of daily use. State in brief the method of preparation of
    1. (i)Sodium hydroxide, and
    2. (ii)Sodium hydrogen carbonate from common salt. Write balanced chemical equations of the reactions that occur.
    Show answer

    Sodium hydroxide: electrolysis of brine (chlor-alkali process); sodium hydrogencarbonate: ammonia and carbon dioxide passed into brine; both with balanced equations.

    (i) Sodium hydroxide — chlor-alkali process: electricity is passed through a concentrated solution of sodium chloride (brine). Chlorine is liberated at the anode, hydrogen at the cathode, and sodium hydroxide is formed in the solution: . (ii) Sodium hydrogencarbonate — brine is saturated with ammonia and carbon dioxide is then bubbled through it; the sparingly soluble sodium hydrogencarbonate separates out and is filtered off: .
    Step-by-step solution →
    OR
    Q28(b) 3 marks

    Also in 31/3/1 (Q28(b)), 31/3/2 (Q28(b))

    Design an experimental set-up to demonstrate that “Alcohol and glucose contain hydrogen but are not categorised as acids”. Also give the reason to justify this fact.
    Show answer

    The bulb glows with dilute hydrochloric acid but not with glucose or alcohol, as they give no ions in water.

    Set-up: fix two iron nails about cm apart in a rubber cork, stand the cork on a beaker so that the nails dip into the liquid, and connect the nails through a V battery, a torch bulb and a switch. Fill the beaker first with dilute hydrochloric acid — the bulb glows. Wash the beaker and the nails and repeat with a glucose solution and then with alcohol — the bulb does not glow in either case. Reason: a solution conducts only if it contains free ions, so the dark bulb shows that glucose and alcohol give no hydrogen ions in water. A substance is an acid because it produces ions in aqueous solution, not merely because its molecule contains hydrogen; in glucose and alcohol the hydrogen is held by covalent bonds (largely in —OH groups) that do not ionise in water. Hence they contain hydrogen but are not categorised as acids.
    Step-by-step solution →
  3. Q29 3 marks
    Why is blood circulation in vertebrates known as “double circulation”? Trace its path in the form of a flow chart.
    Show answer

    Blood passes twice through the heart in one cycle: through the right side to the lungs, and through the left side to the body.

    In one complete cycle round the body the blood passes through the heart twice — once through the right side on its way to the lungs (pulmonary circulation) and once through the left side on its way to the body (systemic circulation) — so the circulation is called double circulation. (This is the pattern in the vertebrates other than fishes; in fishes the blood passes through the heart only once per cycle.) Flow chart: body organs → vena cava → right atrium → right ventricle → pulmonary artery → lungs (blood is oxygenated) → pulmonary veins → left atrium → left ventricle → aorta → body organs (oxygen is given up) → vena cava → right atrium.
    Step-by-step solution →
  4. Q30 3 marks

    Also in 31/3/1 (Q30), 31/3/2 (Q30)

    A pure pea plant having round (R), yellow (Y) seeds is crossed with another pure pea plant having wrinkled (r), green (y) seeds. Subsequently progeny is self-pollinated to obtain progeny.
    1. (a)What do the seeds of generation look like ?
    2. (b)Give the possible combinations of traits in seeds of generation. Also give their ratio.
    3. (c)State the reason of obtaining seeds of new combination of traits in generation.
    Show answer

    seeds are all round and yellow; gives four combinations in the ratio because the two traits are inherited independently.

    (a) All the seeds are round and yellow; their genotype is , and round and yellow are the dominant traits. (b) The seeds show four combinations — round yellow, round green, wrinkled yellow and wrinkled green — in the ratio . (c) Seed shape and seed colour are controlled by two different pairs of genes which are inherited independently of each other, so the plant forms four kinds of gamete, , , and , in equal numbers. The recombinant gametes and combine at random during fertilisation and give seeds with trait combinations that neither parent had.
    Step-by-step solution →
  5. Q31 3 marks
    A person is suffering from an eye defect in which the far point of the eye is much nearer than infinity. Name the defect of vision the person is suffering from. List two main causes of this defect. Write the type of the corrective lens and draw a ray diagram to show the function of the corrective lens.
    Show answer

    Myopia, caused by excessive lens curvature or an elongated eyeball, is corrected by a concave lens.

    The person has myopia (near-sightedness): distant objects cannot be seen clearly because their image is formed in front of the retina. Two main causes: (i) excessive curvature of the eye lens; (ii) elongation of the eyeball. It is corrected with a concave (diverging) lens of suitable power. Ray diagram: parallel rays from a distant object pass through the concave lens and diverge as if they came from the far point F of the eye; the eye lens then focuses them on the retina, so the image is formed on the retina.
    Step-by-step solution →
  6. Q32 3 marks
    1. (a)Name and state the rule which determines the force on a current carrying conductor placed in a uniform magnetic field.
    2. (b)Consider the following three diagrams in which the entry of a positive charge in a magnetic field is shown. Identify giving reason the case in which the force experienced by the charge is (i) maximum, and (ii) minimum.
    I. Magnetic field II. Magnetic field III. Magnetic field +Q +Q +Q
    Show answer

    Fleming's left-hand rule; force is maximum in diagram III (charge at right angles to field) and zero in diagram I (along field).

    (a) Fleming’s left-hand rule: stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular; if the forefinger points along the magnetic field and the middle finger along the current, the thumb points in the direction of the force on the conductor. (b) (i) The force is maximum in diagram III, because there the charge moves at right angles to the magnetic field. (ii) The force is minimum — zero — in diagram I, because there the charge moves along the magnetic field lines. In II the charge moves at an angle between and to the field, so the force on it lies between these two values.
    Step-by-step solution →
  7. Q33 3 marks

    Also in 31/3/1 (Q33), 31/3/2 (Q33)

    1. (a)“In a food chain energy flow is unidirectional.” Give two reasons for the given statement.
    2. (b)If 10,000 J energy is available at the producer level, how much energy will be available to the secondary consumers ? Give reason to justify your answer.
    Show answer

    Energy enters only as sunlight and about 90% is lost as heat at each level, so flow is one-way; secondary consumers get 100 J.

    (a) (i) Energy enters the food chain in one place only — as sunlight captured by the green plants — and then passes from producers to herbivores to carnivores, and never back, because no consumer can return energy to the level below it. (ii) At every trophic level about of the energy received is used up in life processes and given off to the surroundings as heat; heat lost in this way cannot be recaptured by any organism, so the flow cannot reverse. (b) J. Producers hold J; the primary consumers get about of it, which is J; the secondary consumers get about of that, which is J. The reason is the ten per cent law — only about a tenth of the energy at one trophic level passes on to the next, and the secondary consumers are two transfers away from the producers.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks

    Also in 31/3/1 (Q34(a)), 31/3/2 (Q35(a))

    1. (i)Consider the following metals : K, Ca, Al, Cu, Ag, Fe Select from the above metals, a metal which I. does not react with oxygen even at high temperature. II. reacts with oxygen at ordinary temperature and forms a protective oxide layer which prevents the metal from further oxidation. III. catches fire when kept in the open. IV. does not burn in oxygen but the hot metal is coated with a black coloured oxide layer.
    2. (ii)What are amphoteric oxides ? With the help of balanced chemical equations show that aluminium oxide is an amphoteric oxide.
    3. (iii)What are alkalis ? Give one example.
    Show answer

    Ag, Al, K and Cu match I to IV; amphoteric oxides react with both acids and bases, as does; alkalis are water-soluble bases.

    (i) I — Silver ; II — Aluminium ; III — Potassium ; IV — Copper . (ii) Amphoteric oxides are metal oxides that react with both acids and bases to produce a salt and water. Aluminium oxide does both: with an acid, , and with a base, ; hence it is amphoteric. (iii) Alkalis are bases that are soluble in water and give hydroxide ions in solution. Example: sodium hydroxide, (potassium hydroxide and calcium hydroxide are others).
    Step-by-step solution →
    OR
    Q34(b) 5 marks

    Also in 31/3/1 (Q34(b)), 31/3/2 (Q35(b))

    1. (i)With the help of balanced chemical equations state the process of extracting (I) mercury from its ore called cinnabar, and (II) copper from its sulphide ore.
    2. (ii)Silver and copper articles slowly lose their shiny surfaces when exposed to air. Name the compounds formed on (I) silver articles, and (II) copper articles in the form of coating.
    Show answer

    Mercury comes from roasting then heating cinnabar, copper from self-reduction of with ; coatings are black silver sulphide and green basic copper carbonate.

    (i)(I) Mercury from cinnabar — two heatings. Cinnabar is first roasted in air, which converts the sulphide into mercury(II) oxide: . On heating further, the oxide itself is reduced to the metal: . (I I) Copper from copper glance — the ore is partly roasted in air, giving copper(I) oxide: . The oxide is then heated with the copper(I) sulphide left over, which reduces it to the metal: . (ii)(I) On silver articles: silver sulphide, — a black coating formed with the hydrogen sulphide in the air. (I I) On copper articles: basic copper carbonate, — a green coating formed with moist carbon dioxide in the air.
    Step-by-step solution →
  2. Q35(a) 5 marks

    Also in 31/3/1 (Q36(a)), 31/3/2 (Q34(a))

    1. (i)“In refraction of light through a rectangular glass slab, the emergent ray is always parallel to the direction of the incident ray.” Why ? Explain with the help of a ray diagram. What happens when a ray of light falls normally on one of the faces of a rectangular glass prism ? Draw diagram.
    2. (ii)An object is placed at a distance of 30 cm from the optical centre of a concave lens of focal length 20 cm. Use Lens formula to determine the position of the image formed in this case.
    Show answer

    Parallel faces give , so the emergent ray is parallel but shifted; a normal ray goes straight; cm.

    (i) Let the ray meet the first face at and be refracted at ; then . The two faces of a rectangular slab are parallel, so their normals are parallel and the ray inside the glass makes equal angles with them: the angle of incidence at the second face is . At the second face the light goes from glass to air, so , giving , that is . The emergent ray therefore makes the same angle with its normal as the incident ray does with its own, and the two normals are parallel, so the emergent ray is parallel to the incident ray. It is only shifted sideways, by the lateral displacement. When a ray falls **normally** on a face, , so and : the ray is not bent at all, it passes straight through the glass and emerges along the same line, undeviated and with no lateral displacement. (ii) Here cm and cm. From , , so cm. The image is formed cm from the optical centre on the same side as the object; with it is virtual, erect and diminished.
    Step-by-step solution →
    OR
    Q35(b) 5 marks

    Also in 31/3/1 (Q36(b)), 31/3/2 (Q34(b))

    1. (i)A student wishes to study the image formation by a concave mirror using candle flame as object. State the type of the image formed by the mirror and mention the change in the image formed, if any, that he observes when the candle flame is gradually moved away from the pole of the mirror. Draw a ray diagram to show the image formation when the object distance is nearly equal to the radius of curvature of the mirror.
    2. (ii)A convex mirror used for rear-view on an automobile has a focal length of m. If a bus is located at m from this mirror, use mirror formula to find the position of the image of the bus as seen in the mirror.
    Show answer

    Real, inverted image that shrinks towards F as the flame moves away; the bus's image is 2.0 m behind the mirror.

    (i) The image formed by a concave mirror depends on where the flame is. With the flame between the pole and the focus the image is virtual, erect and enlarged, and lies behind the mirror. As the flame is gradually moved away from the pole, the image changes: at it is formed at infinity, highly enlarged, real and inverted; between and it is real, inverted and enlarged and lies beyond ; at it is real, inverted and of the same size as the flame and lies at ; and beyond it is real, inverted and diminished and lies between and , shrinking towards as the flame goes further away. So the image turns from virtual and erect to real and inverted as the flame passes , and the real image then travels in from infinity towards , getting steadily smaller. The ray diagram for an object distance nearly equal to the radius of curvature shows the object at : a ray parallel to the axis reflects through , a ray through reflects parallel to the axis, and the two meet at to give a real, inverted image of the same size. (ii) Here m (convex) and m. From , , so m. The image is formed m behind the mirror; with it is virtual, erect and one-third the size of the bus.
    Step-by-step solution →
  3. Q36(a) 5 marks

    Also in 31/3/1 (Q35(a)), 31/3/2 (Q36(a))

    1. (i)Identify the parts ‘X’ and ‘Y’ in the figure given below :
    2. (ii)Name the yellowish coloured structures produced by the part labelled as ‘Y’.
    3. (iii)Write the name of the process by which these are transferred to the part labelled as ‘X’.
    4. (iv)Explain the process of seed formation in a flowering plant.
    ‘X’ ‘Y’ Petal Sepal
    Show answer

    X is the stigma, Y the anther; pollen from Y reaches X by pollination, and fertilisation follows, turning the ovule into the seed.

    (i) ‘X’ is the stigma (the top of the pistil) and ‘Y’ is an anther (the swollen tip of a stamen). (ii) The yellowish structures produced by the anther are pollen grains. (iii) They are carried to the stigma by pollination. (iv) Seed formation: after pollination the pollen grain on the stigma germinates and grows a pollen tube down through the style until it reaches an ovule in the ovary. The male germ cell carried in the tube fuses with the egg inside the ovule — this is fertilisation — and a zygote is formed. The zygote divides repeatedly to form the embryo. The ovule then develops a tough coat around the embryo and becomes the seed, while the ovary grows and ripens into the fruit; the petals, sepals, stamens, style and stigma usually wither and fall off.
    Step-by-step solution →
    OR
    Q36(b) 5 marks

    Also in 31/3/1 (Q35(b)), 31/3/2 (Q36(b))

    1. (i)Name the type of asexual mode of reproduction shown in the given figure.
    2. (ii)Identify the unicellular organism in the diagram.
    3. (iii)List any two advantages of asexual reproduction over sexual reproduction.
    4. (iv)Name and explain any one mode of asexual reproduction observed in Hydra.
    (a) (b) (c) (d) (e) (f)
    Show answer

    Leishmania divides by binary fission; asexual reproduction needs one parent and gives identical offspring quickly; Hydra reproduces by budding.

    (i) Binary fission — more precisely longitudinal binary fission, which takes place in a definite orientation with respect to the whip-like flagellum of the cell. (ii) The organism is Leishmania, the protozoan that causes kala-azar. (iii) Two advantages of asexual over sexual reproduction: (1) only one parent is needed — no gametes, no mate and no fertilisation — so it is much faster and produces a large number of offspring in a short time; (2) the offspring are genetically identical to the parent, so a successful set of characters is passed on unchanged, and no energy is spent on gametes, on attracting a mate or on reproductive structures. (iv) Budding. Because of repeated cell division at one particular site on the body wall of Hydra, an outgrowth called a bud develops. The bud grows, and a mouth surrounded by tentacles appears at its free end; when it is fully developed it detaches from the parent body and lives as an independent new individual identical to the parent.
    Step-by-step solution →

Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/3/1 (Q39), 31/3/2 (Q39)

    As shown in the diagram, an electric circuit consisting of an ammeter, a voltmeter, 4 cells of V each, a plug key with a gap XY was set up. Voltmeter and ammeter readings were recorded in the observation table for four arrangements as given below: Arrangement No. 1 - only resistor in gap XY Arrangement No. 2 - only resistor in gap XY Arrangement No. 3 - Resistors and in series in gap XY Arrangement No. 4 - Resistors and in parallel in gap XY Based on the observations, four - graphs A, B, C and D as shown in figure were drawn. Study these graphs.
    A + − X Y V + − + − K
    A B C D O V (volts) I (amperes)
    1. (a)Which one of the graphs represents the series combination of and ?[1]
    2. (b)Which one of these graphs represents the parallel combination of and ?[1]
    3. (c)(i)Show an arrangement of three resistors, each of resistance , so that the combination has a resistance of . Give justification for your answer.[2]
    4. (c)(ii)OR : A battery of V is connected with a series combination of five resistors of , , , and . How much current would flow through the resistor? Justify your answer.[2]
    Show answer

    Graph A is series and D is parallel; two in parallel plus one in series give ; or the current is 4 A.

    (a) Graph A represents the series combination of and . On these axes the slope of a line is , and the series resistance is the largest of the four, so it gives the least steep line. (b) Graph D represents the parallel combination, whose resistance is the smallest of the four and so gives the steepest line. (c)(i) Connect two of the resistors in parallel and the third in series with that pair. The parallel pair gives , so , and the whole combination is . (c)(ii) The five resistors are in series, so the total resistance is and the current drawn is A. The same current flows through every resistor of a series circuit, so the current through the resistor is A.
    Step-by-step solution →
  2. Q38 4 marks

    Also in 31/3/1 (Q37), 31/3/2 (Q37)

    ‘A’ and ‘B’ are two salts used for washing purposes. Salt ‘A’ is used for bathing also. Four test tubes I, II, III and IV as mentioned below are taken. The test tubes are shaken one by one almost identically for the same time and the lengths of foam formed in each test tube is noted.
    Contents of the four test tubes
    Test tubeContents
    IRain water + solution of salt ‘A’
    IIRain water + solution of salt ‘B’
    IIITubewell water + solution of salt ‘A’
    IVTubewell water + solution of salt ‘B’
    1. (a)In which one of the four test tubes is the foam formed the minimum?[1]
    2. (b)Differentiate between salt A and salt B.[1]
    3. (c)(i)What are esters? What happens when an ester reacts with an alkali (say sodium hydroxide)? Give chemical equation for the reaction.[2]
    4. (c)(ii)OR : What is the cause of hardness of water? Sometimes it is observed that while bathing foam is formed with difficulty and an insoluble substance is formed. Name this substance and write the cause of its formation.[2]
    Show answer

    Test tube III gives least foam; A is a soap and B a detergent; an ester with alkali gives a sodium salt and alcohol.

    (a) Test tube III (tubewell water with salt A) shows the minimum foam, because the calcium and magnesium ions of hard tubewell water throw the soap out as insoluble scum. (b) Salt A is a soap - the sodium salt of a long-chain carboxylic (fatty) acid; it is mild enough for bathing and is biodegradable, but it lathers poorly in hard water and forms scum. Salt B is a detergent - the sodium salt of a long-chain benzene sulphonic acid or alkyl hydrogen sulphate; it lathers well even in hard water because its calcium and magnesium salts are soluble. (c)(i) Esters are the sweet-smelling compounds formed when a carboxylic acid reacts with an alcohol in the presence of a little concentrated sulphuric acid. With an alkali an ester is hydrolysed (saponification) to the sodium salt of the acid and the alcohol: . (c)(ii) Hardness of water is caused by dissolved calcium and magnesium salts (hydrogencarbonates, chlorides and sulphates). The insoluble substance formed while bathing is scum - the calcium or magnesium salt of the fatty acid of the soap - formed because and ions displace the sodium of the soap: .
    Step-by-step solution →
  3. Q39 4 marks

    Also in 31/3/1 (Q38), 31/3/2 (Q38)

    A person while climbing up a rocky hill comes into a panic state and fear. His body starts reacting in a “flight-or-flight” condition to adjust to the dangerous and stressful situation. Based on the above facts, answer the questions that follow.
    1. (a)(i)Name the hormone secreted in the blood of the person in this situation.[2]
    2. (a)(ii)OR : Name the source gland of the hormone secreted in this condition.[2]
    3. (b)State any two responses in the body of the person as a result of the secretion of this hormone.[1]
    4. (c)How does the action of the chemical signal in terms of hormones differ from the electrical impulses via nerve cells?[1]
    Show answer

    Adrenaline, from the adrenal medulla; it speeds the heartbeat and breathing; hormones act slower but longer and wider than nerve impulses.

    (a)(i) The hormone secreted into the blood is adrenaline (epinephrine). (a)(ii) Its source gland is the adrenal gland - specifically the adrenal medulla, one gland sitting on top of each kidney. (Only one of these two branches is to be attempted.) (b) Any two of: the heart beats faster, so more oxygenated blood reaches the muscles; the breathing rate increases because the diaphragm and rib muscles contract faster; the small arteries of the skin and digestive system are constricted, diverting blood to the skeletal muscles; the pupils dilate. (c) A hormone is a chemical released into the blood, so it travels more slowly but is carried to every part of the body and acts on all cells having receptors for it, whether or not a nerve reaches them, and its effect is steady and persistent. An electrical impulse travels only along nerve fibres, so it reaches only the cells connected by nervous tissue; it is very fast but short-lived, and the nerve cell must reset itself before it can transmit another impulse.
    Step-by-step solution →

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