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CBSE Class 10 Science 2025 — Paper 31/4/2

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

34 of its 45 questions also appear in 31/4/1 or 31/4/3; 11 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

    Also in 31/4/1 (Q10), 31/4/3 (Q16)

    To get an image of magnification on a screen using a lens of focal length cm, the object distance must be :
    1. (a)Less than cm
    2. (b) cm
    3. (c) cm
    4. (d) cm
    Show answer
    (c) cm
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/4/1 (Q9), 31/4/3 (Q14)

    When a pure-tall pea plant is crossed with a pure-dwarf pea plant, the percentage of tall pea plants in and generation pea plants will be respectively :
    1. (a) ;
    2. (b) ;
    3. (c) ;
    4. (d) ;
    Show answer
    (c) ;
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/4/1 (Q7), 31/4/3 (Q13)

    Plants like rose and banana have lost the capacity to produce :
    1. (a)flowers
    2. (b)buds
    3. (c)seeds
    4. (d)fruits
    Show answer
    (c) seeds
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/4/1 (Q1), 31/4/3 (Q6)

    In which one of the following situations a chemical reaction does not occur?
    1. (a)Milk is left open at room temperature during summer
    2. (b)Grapes get fermented
    3. (c)An iron nail is left exposed to humid atmosphere
    4. (d)Melting of glaciers
    Show answer
    (d) Melting of glaciers
    Step-by-step solution →
  5. Q5 1 mark

    Also in 31/4/1 (Q3)

    The property by virtue of which a solid material can be drawn into thin wires is called :
    1. (a)malleability
    2. (b)ductility
    3. (c)rigidity
    4. (d)resistivity
    Show answer
    (b) ductility
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/4/1 (Q2), 31/4/3 (Q7)

    In order to prepare dry hydrogen chloride gas in humid atmosphere the gas produced is passed through a guard tube (drying tube) which contains :
    1. (a)Calcium chloride
    2. (b)Calcium oxide
    3. (c)Calcium hydroxide
    4. (d)Calcium carbonate
    Show answer
    (a) Calcium chloride
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/4/1 (Q4), 31/4/3 (Q8)

    Select from the following a hydrocarbon having one bond and one bond :
    1. (a)Benzene
    2. (b)Cyclohexane
    3. (c)Butyne
    4. (d)Propyne
    Show answer
    (d) Propyne
    Step-by-step solution →
  8. Q8 1 mark
    Which one of the following has half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells?
    1. (a)Male germ cell
    2. (b)Female germ cell
    3. (c)Zygote
    4. (d)Both, male and female germ cells
    Show answer
    (d) Both, male and female germ cells
    Step-by-step solution →
  9. Q9 1 mark

    Also in 31/4/1 (Q5), 31/4/3 (Q9)

    The essential element taken up from the soil by the plants to synthesize proteins is :
    1. (a)Phosphorus
    2. (b)Nitrogen
    3. (c)Iron
    4. (d)Magnesium
    Show answer
    (b) Nitrogen
    Step-by-step solution →
  10. Q10 1 mark

    Also in 31/4/1 (Q6), 31/4/3 (Q12)

    Select TRUE statements about lymph from the following : A. Lymph vessels carry lymph through the body and finally open into larger arteries. B. Lymph contains some amount of plasma, proteins and blood cells. C. Lymph contains some amount of plasma, proteins and red blood cells. D. Lymph vessels carry lymph through the body and finally open into larger veins. The true statements are :
    1. (a)A and B
    2. (b)B and D
    3. (c)A and C
    4. (d)C and D
    Show answer
    (b) B and D
    Step-by-step solution →
  11. Q11 1 mark

    Also in 31/4/1 (Q16), 31/4/3 (Q1)

    Which one of the following gets biomagnified at different levels in a food chain ?
    1. (a)Carbon monoxide
    2. (b)CFC's
    3. (c)DDT
    4. (d)Manure
    Show answer
    (c) DDT
    Step-by-step solution →
  12. Q12 1 mark

    Also in 31/4/1 (Q15), 31/4/3 (Q2)

    In the food chains given below. Select the most efficient food chain in terms of energy :
    1. (a)Grass → Grasshopper → Frog → Snake
    2. (b)Plants → Deer → Lion
    3. (c)Plants → Man
    4. (d)Phytoplankton → Zooplankton → Small Fish → Big Fish
    Show answer
    (c) Plants → Man
    Step-by-step solution →
  13. Q13 1 mark

    Also in 31/4/1 (Q11), 31/4/3 (Q4)

    An optical device ‘X’ is placed obliquely in the path of a narrow parallel beam of light. If the emergent beam gets displaced laterally, the device ‘X’ is :
    1. (a)plane mirror
    2. (b)convex lens
    3. (c)glass slab
    4. (d)glass prism
    Show answer
    (c) glass slab
    Step-by-step solution →
  14. Q14 1 mark

    Also in 31/4/1 (Q12), 31/4/3 (Q5)

    A piece of wire of resistance ‘’ is cut lengthwise into three identical parts. These parts are then connected in parallel. If the equivalent resistance of this combination is , then the value of is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (d)
    Step-by-step solution →
  15. Q15 1 mark

    Also in 31/4/1 (Q14), 31/4/3 (Q10)

    The minimum number of identical bulbs of rating 4V; 6W, that can work safely with desired brightness, when connected in series with a 240 V mains supply is :
    1. (a)20
    2. (b)40
    3. (c)60
    4. (d)80
    Show answer
    (c) 60
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/4/1 (Q13), 31/4/3 (Q11)

    An electric bulb is rated V; W. The resistance of its filament when it glows with a power supply of V is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (a)
    Step-by-step solution →
  17. Q17 1 mark
    This question consists of two statements – Assertion (A) and Reason (R). Answer this question selecting the appropriate option (a), (b), (c) and (d) as given below :

    Assertion (A): All exothermic reactions are accompanied with evolution of heat and light.

    Reason (R): Combination reactions may or may not be exothermic.

    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, and 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 31/4/1 (Q20)

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

    Assertion (A): When ciliary muscles contract, eye lens becomes thin.

    Reason (R): Ciliary muscles control the power of the eye lens.

    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, and 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 →
  19. Q19 1 mark

    Also in 31/4/1 (Q18), 31/4/3 (Q19)

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

    Assertion (A): Concentrated nitric acid is diluted by adding water slowly to acid with constant stirring.

    Reason (R): Concentrated nitric acid is easily soluble in water.

    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, and 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 →
  20. Q20 1 mark

    Also in 31/4/1 (Q19), 31/4/3 (Q17)

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

    Assertion (A): In reptiles, the temperature at which the fertilized eggs are kept decides the sex of the offsprings.

    Reason (R): Sex is not genetically determined in some animals.

    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, and 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 →

Questions 21–26 · 2 marks each

  1. Q21 2 marks
    While burning a magnesium ribbon in air, list two safety measures which should be followed. Also state two observations of this activity.
    Show answer

    Hold the ribbon with tongs and keep the eyes away from the flame; it burns with a dazzling white flame, leaving white ash.

    Safety measures: 1. Hold the magnesium ribbon with a pair of tongs, not in the fingers. 2. Keep the burning ribbon as far as possible from the eyes and wear suitable eyeglasses; do not look straight at the dazzling flame. Observations: 1. The ribbon burns with a dazzling white flame, giving out heat and light. 2. It changes into a white powder (ash), mainly magnesium oxide, which is collected in a watch-glass: .
    Step-by-step solution →
  2. Q22(a) 2 marks

    Also in 31/4/1 (Q25(a)), 31/4/3 (Q24(a))

    Draw a ray diagram to show the refraction of a ray of light passing through an equilateral glass prism. Mark the angle through which the emergent ray bends from the direction of the incident ray and also name it.
    Show answer

    The emergent ray turns towards the base; the angle between incident and emergent rays is the angle of deviation .

    The ray bends towards the normal on entering the glass at the first face and away from the normal on leaving at the second face, and it emerges turned towards the base of the prism. The angle between the direction of the incident ray produced and the emergent ray is the **angle of deviation**, . For the drawn construction, with , and , the angles are , , and .
    Step-by-step solution →
    OR
    Q22(b) 2 marks

    Also in 31/4/1 (Q25(b)), 31/4/3 (Q24(b))

    Name the type of lenses required by the persons for the correction of their defect of vision called presbyopia. Write the structure of the lenses commonly used for the correction of this defect giving reason for such designs.
    Show answer

    A convex lens corrects presbyopia; bi-focal lenses, with a concave upper and convex lower part, correct distant and near vision together.

    Presbyopia is corrected with a **convex** (converging) lens, because the defect is a loss of the power of accommodation and it is near vision that fails. Sometimes such a person also suffers from myopia and then needs correction for distant as well as near vision; for this the lenses commonly used are **bi-focal lenses**. Structure: the upper part of each lens is a concave lens and the lower part is a convex lens. Reason: distant objects are viewed by looking straight ahead, through the upper concave part which corrects distant vision, while reading is done by looking downwards, through the lower convex part which corrects near vision — so one pair of spectacles corrects both defects.
    Step-by-step solution →
  3. Q23 2 marks
    “In human alimentary canal the small intestine is designed to absorb the digested food.” Justify this statement.
    Show answer

    It completes digestion, is a long coiled tube whose villi enlarge the absorbing surface, and blood vessels carry the food to every cell.

    The small intestine is designed to absorb the digested food because: 1. The digestion of carbohydrates, proteins and fats is completed in it, so the food there is in small soluble molecules (glucose, amino acids, fatty acids and glycerol) that can be absorbed. 2. It is very long and coiled, and its inner lining has numerous finger-like projections called villi, which greatly increase the surface area for absorption. 3. The villi are richly supplied with blood vessels, which take the absorbed food to each and every cell of the body.
    Step-by-step solution →
  4. Q24 2 marks
    Pure-tall (TT) pea plants are crossed with pure-dwarf (tt) pea plants. The pea plants obtained in generation are then self pollinated to produce generation.
    1. (i)What do the plants of generation look-like? Justify your answer.
    2. (ii)What is the ratio of pure-tall plants to pure-dwarf plants in generation ?
    Show answer

    (i) All plants are tall (Tt) as tallness is dominant; (ii) pure-tall : pure-dwarf plants in are 1 : 1.

    (i) All the plants of the generation are tall. Each receives T from the pure-tall parent and t from the pure-dwarf parent, so every plant is Tt, and since tallness (T) is dominant over dwarfness (t), the plant looks tall. (ii) Selfing Tt gives TT : Tt : tt = 1 : 2 : 1 in the generation, so the ratio of pure-tall (TT) to pure-dwarf (tt) plants is 1 : 1.
    Step-by-step solution →
  5. Q25(a) 2 marks

    Also in 31/4/1 (Q22(a))

    Show the formation of magnesium chloride by electron transfer. Write the name of the cation and anion present in the compound formed. (Atomic Number of Mg , Cl )
    Show answer

    Magnesium gives two electrons to two chlorine atoms, forming (cation) and (anion) in .

    Magnesium () loses its two outer electrons to give (), and each of two chlorine atoms () gains one to give (), so . The cation present is the magnesium ion, , and the anion is the chloride ion, .
    Step-by-step solution →
    OR
    Q25(b) 2 marks

    Also in 31/4/1 (Q22(b))

    How is zinc extracted from its ore? Name the processes involved in the extraction and write chemical equations for the reactions that occur during these processes.
    Show answer

    Zinc ore is converted to zinc oxide by roasting or calcination, and the oxide is reduced with carbon.

    Zinc is a metal of the middle of the reactivity series, so its ore is first converted into zinc oxide and the oxide is then reduced with carbon. The processes are: (i) **Roasting** of the sulphide ore zinc blende — ; (ii) **Calcination** of the carbonate ore calamine — ; (iii) **Reduction** of the oxide with carbon — . The impure zinc is finally purified by electrolytic refining.
    Step-by-step solution →
  6. Q26 2 marks
    State the role of an electric fuse, used in series with an electrical appliance. Why should in an electric circuit a fuse with defined rating not be replaced by one with a larger rating?
    Show answer

    A fuse in series melts on excess current and breaks the circuit; a larger-rated fuse would not melt at a harmful current.

    Role of the fuse: an electric fuse is a safety device connected in series with the appliance. Its wire, of a material of suitable low melting point, carries the whole current of the appliance; if the current exceeds the specified value because of overloading or a short circuit, the heat produced melts the fuse wire and breaks the circuit. The current stops, and the appliance and the wiring are protected from damage and fire. Why not a larger rating: the rating is chosen just above the normal current of the appliance. A fuse of larger rating would melt only at a much larger current, so an excessive current that is harmful to the appliance and the wiring could flow without the fuse melting. The appliance could be damaged, the wires could overheat and a fire could start, so the fuse would no longer protect the circuit.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks

    Also in 31/4/1 (Q33), 31/4/3 (Q31)

    What are decomposers? Give two examples. State how they maintain a balance in an ecosystem.
    Show answer

    Decomposers (bacteria, fungi) break down dead matter into simple substances, clean the environment and return nutrients to the soil for plants.

    Decomposers are micro-organisms that break down the complex organic matter of dead plants and animals, and their waste products, into simple inorganic substances that return to the soil, water and air. Two examples are bacteria and fungi. They maintain the balance of an ecosystem in two ways: they clean the environment by breaking down dead remains, which would otherwise pile up, and they release the nutrients locked in that dead matter back into the soil, where green plants absorb them and use them again to make food. Since matter must be used over and over in an ecosystem, decomposers are the step that closes the nutrient cycle and keeps the supply of nutrients available to the producers.
    Step-by-step solution →
  2. Q28 3 marks
    Samples of four metals A, B, C and D were added one by one to the following solutions. The results obtained were tabulated as follows : Use the table above to answer the following questions about metals A, B, C and D :
    1. (i)Which is the least reactive metal?
    2. (ii)What would be observed if C is added to a solution of copper sulphate?
    3. (iii)Arrange the metals A, B, C and D in the order of their decreasing reactivity.
    Results of adding each metal to each solution
    Metal / SolutionIron SulphateCopper SulphateZinc SulphateAluminium Sulphate
    A–DisplacementNo reactionNo reaction
    BDisplacementDisplacementDisplacement–
    CDisplacement?–No reaction
    DNo reactionNo reactionNo reactionNo reaction
    Show answer

    D is least reactive; C displaces copper from copper sulphate; the order is .

    (i) D is the least reactive metal: it displaces none of iron, copper, zinc or aluminium from their sulphate solutions. (ii) C displaces copper from the copper sulphate solution: the blue colour of the solution fades and a reddish-brown coating of copper is deposited on C. C displaces iron, so it is more reactive than iron, which itself displaces copper. (iii) B > C > A > D (B is aluminium, C is zinc, A is iron, and D lies below copper).
    Step-by-step solution →
  3. Q29 3 marks
    1. (i)Study the diagram and name the parts marked as A, B, C and D.
    2. (ii)Write the function of A and C.
    A B D C
    Show answer

    A pulmonary artery, B pulmonary vein, C aorta, D vena cava; A carries blood to the lungs and C to the body.

    (i) A – pulmonary artery; B – pulmonary vein; C – aorta; D – vena cava. (ii) A, the pulmonary artery, carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation. C, the aorta, carries oxygenated blood from the left ventricle to all parts of the body.
    Step-by-step solution →
  4. Q30 3 marks

    Also in 31/4/1 (Q32)

    The electrical resistivity of three materials A, B and C at 20°C is given below :
    1. (i)Classify these materials as conductor, alloy and insulator.
    2. (ii)Give one example of each of these materials and state one use of each material in the design of an electrical appliance say an electric stove or an electric iron.
    Electrical resistivity of three materials at 20°C
    MaterialResistivity ( m)
    A
    B
    C
    Show answer

    A is an insulator (ebonite), B an alloy (manganin), C a conductor (copper); the alloy is used for the heating element.

    (i) A is an insulator, B is an alloy and C is a conductor. (ii) C is copper ( m); it is used for the connecting wires and the lead that carry the current to the stove or the iron, because its very low resistivity wastes almost no energy on the way. B is an alloy such as manganin ( m); alloy wire is used for the heating element or coil of the stove or iron, because its high resistivity produces a large amount of heat and it does not oxidise readily at a high temperature. A is an insulator such as ebonite or hard rubber ( m); it is used for the handle of the iron and for the covering of the wires, so that the current cannot pass to the user.
    Step-by-step solution →
  5. Q31 3 marks
    If we want to obtain a real and magnified image of an object by using a concave mirror of focal length 18 cm. Where should the object be placed? Use mirror formula to determine the object distance for an image of magnification by this mirror to justify your answer.
    Show answer

    The object goes between F and C; for it is 27 cm from the mirror.

    To get a real and magnified image, the object must be placed between the principal focus F and the centre of curvature C of the mirror, that is, between cm and cm from the mirror. Justification: with cm and , gives ; then gives , so cm. The object is cm in front of the mirror, between F ( cm) and C ( cm); the image forms at cm, beyond C, and is real, inverted and twice the size of the object.
    Step-by-step solution →
  6. Q32(a) 3 marks

    Also in 31/4/1 (Q27(a)), 31/4/3 (Q30(a))

    Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical equation :
    Show answer

    Equations are balanced to obey the law of conservation of mass; the balanced equation is .

    A chemical equation is balanced because a reaction only rearranges atoms, so the number of atoms of every element (and hence the total mass) must be the same on both sides. The law behind it is the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction, so the total mass of the products equals the total mass of the reactants. Balanced equation:
    Step-by-step solution →
    OR
    Q32(b) 3 marks

    Also in 31/4/1 (Q27(b)), 31/4/3 (Q30(b))

    Define a precipitation reaction. Give its example and also express the reaction that occurs in the form of a balanced chemical equation.
    Show answer

    A precipitation reaction forms an insoluble solid on mixing solutions, e.g. sodium sulphate and barium chloride give white barium sulphate.

    A precipitation reaction is a reaction in which two solutions are mixed and an insoluble substance, called a precipitate, is formed and separates out as a solid. Example: when sodium sulphate solution is mixed with barium chloride solution, a white precipitate of barium sulphate is formed.
    Step-by-step solution →
  7. Q33 3 marks

    Also in 31/4/1 (Q30), 31/4/3 (Q33)

    State two limitations of electrical impulses in multicellular organisms. Why is chemical communication better than electrical impulses as a means of communication between cells in multicellular organisms?
    Show answer

    Impulses reach only nerve-connected cells and need reset time; chemical signals reach every cell via the blood and can be sustained.

    Two limitations of electrical impulses: (i) They reach only those cells of the body that are connected by nervous tissue, and not each and every cell of the organism. (ii) Once a cell has generated and transmitted an impulse it must take time to reset its mechanisms before it can send another, so impulses cannot be created and transmitted continuously. Chemical communication is better because a chemical compound released by one cell diffuses out and is carried by the blood to all parts of the body, so it can reach every cell and not just the cells with a nerve supply; and the cell making it can go on making it, so the signal is steady and long lasting instead of being limited by a reset time. Chemical communication is slower than an electrical impulse, but it is far more widespread and can be sustained.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks
    1. (i)What are structural isomers ? Draw structural isomers of butane (). Give reason why propane has no structural isomers ?
    2. (ii)What happens when butane is burnt in air ? Write chemical equation for the reaction. Differentiate between the flames obtained when butane and butyne both are burnt in air in the similar conditions.
    Show answer

    Butane has two structural isomers, n-butane and iso-butane; propane has none; butane burns with a blue flame, butyne with a sooty yellow one.

    (i) Structural isomers are compounds that have the same molecular formula but different structures. Butane, , has two: n-butane, , a straight chain, and iso-butane (2-methylpropane), , a branched chain; their full structures are drawn in the figure. Propane, , has no structural isomers because its three carbon atoms can be joined in only one way, as a chain of three; a branched chain needs at least four carbon atoms. (ii) Butane burns in air to form carbon dioxide and water, giving out a large amount of heat and light: . Butane, a saturated hydrocarbon, burns with a clean blue flame, while butyne, an unsaturated hydrocarbon, burns with a yellow, sooty flame giving black smoke.
    Step-by-step solution →
    OR
    Q34(b) 5 marks
    1. (i)Give reason why carbon can neither form cations nor anions but forms covalent compounds.
    2. (ii)What is meant by functional group in carbon compounds ? Write in tabular form the structural formula and the functional group present in the following compounds : (a) Ethanol (b) Ethanoic acid
    Show answer

    Carbon shares electrons because gaining or losing four is too difficult; ethanol has the group and ethanoic acid the group.

    (i) Carbon (atomic number 6, electrons 2, 4) has four valence electrons. To form a anion it would have to gain four electrons, and it would be difficult for a nucleus of six protons to hold on to ten electrons. To form a cation it would have to lose four electrons, which needs a very large amount of energy and would leave six protons holding only two electrons. So carbon shares its four valence electrons with other atoms, forming covalent bonds, and its compounds are covalent. (ii) A functional group is an atom or a group of atoms that replaces hydrogen in a hydrocarbon and gives the compound its characteristic chemical properties, whatever the length of the carbon chain. The full structural formulae, with every bond drawn, are shown in the figure.
    Step-by-step solution →
  2. Q35(a) 5 marks

    Also in 31/4/1 (Q36(a)), 31/4/3 (Q34(a))

    (i) Draw the pattern of the magnetic field lines for the two parallel straight conductors carrying current of same magnitude 'I' in opposite directions as shown. Show the direction of magnetic field at a point O which is equidistant from the two conductors. (Consider that the conductors are inserted normal to the plane of a rectangular cardboard.) (ii) In our houses we receive A.C. electric power of 220 V. In electric iron or electric heater cables having three wires with insulation of three different colours – red, black and green are used to draw current from the mains.
    1. (a)What are these three different wires called? Name them colourwise.
    2. (b)What is the potential difference between the red wire and the black wire?
    3. (c)What is the role of the wire with green insulation in case of accidental leakage of electric current to the metallic body of an electrical appliance?
    I I O
    Show answer

    (i) Circular field lines round each wire; at O both fields point out of the paper and add; (ii) live, neutral, earth; 220 V.

    (i) Round each conductor the magnetic field lines are concentric circles lying in planes perpendicular to it, crowded near the conductor and farther apart away from it. Because the currents are opposite, the two sets of circles run in opposite senses, so between the conductors both fields point the same way and add, while outside the pair they oppose. By the right-hand thumb rule, the left conductor (current downwards) gives at O, which lies to its right, a field directed out of the plane of the paper, and the right conductor (current upwards) gives at O, which lies to its left, a field also directed out of the plane of the paper. The magnetic field at O is therefore perpendicular to the plane of the paper and directed **outwards**, towards the reader, and since O is equidistant from the two equal currents its magnitude is twice the field of either conductor alone. Put in a way that does not depend on how the figure is viewed: the field at O is perpendicular both to the currents and to the line joining the two conductors, so on a rectangular cardboard held normal to the conductors it lies in that card, at right angles to the line joining them. (ii)(a) Red is the live wire, black is the neutral wire and green is the earth wire. (b) The potential difference between the red (live) wire and the black (neutral) wire is V, because the neutral wire is at the potential of the earth. (c) The green earth wire is connected to a metal plate buried deep in the earth and so provides a low-resistance path to the ground for the metallic body of the appliance. If current leaks on to that body the earth wire carries it away to the earth, keeping the body at the earth's potential so that a person touching the appliance does not get a severe shock, and the large current that flows blows the fuse.
    Step-by-step solution →
    OR
    Q35(b) 5 marks

    Also in 31/4/1 (Q36(b)), 31/4/3 (Q34(b))

    (i) By using the given experimental set-up. How can it be shown that :
    1. (a)a force is exerted on the current-carrying conductor AB when it is placed in a magnetic field.
    2. (b)the direction of force can be reversed in two ways. (ii) When will the magnitude of the force be highest? (iii) State Fleming's left hand rule.
    S N A B K − +
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    A current-carrying rod in a magnetic field is displaced; reversing current or field reverses the force; it is greatest when they are perpendicular.

    (i)(a) Complete the circuit and insert the key K. Current flows through the suspended rod AB, which lies between the poles of the horseshoe magnet, and the rod is at once displaced sideways (towards the left in the given figure); on removing the key it returns to rest. Since the freely hung rod moves only while current flows, a force must act on a current-carrying conductor placed in a magnetic field. (i)(b) The direction of that force is reversed in two ways: first, by reversing the direction of the current through AB, that is by interchanging the terminals of the battery while leaving the magnet as it is; second, by reversing the direction of the magnetic field, that is by interchanging the poles of the horseshoe magnet while leaving the current as it is. In each case the rod is displaced the opposite way. (Doing both together leaves the direction unchanged.) (ii) The magnitude of the force is highest when the direction of the current is perpendicular to the direction of the magnetic field, since is greatest at . (iii) Fleming's left hand rule: stretch the thumb, forefinger and centre finger of the **left** hand so that they are mutually perpendicular; if the forefinger points along the magnetic field and the centre finger along the current, then the thumb points along the direction of the force, that is of the motion, of the conductor.
    Step-by-step solution →
  3. Q36(a) 5 marks

    Also in 31/4/1 (Q35(a))

    1. (i)What is regeneration? Give one example of an organism that shows this process and one organism that does not. Why does regeneration not occur in the latter?
    2. (ii)Water in a pond appears dark green and contains filamentous structures. Name these structures and the method by which they reproduce. Explain the process.
    Show answer

    Regeneration occurs in Planaria but not in humans, whose cells are too specialised; the green pond filaments are Spirogyra, which reproduce by fragmentation.

    (i) Regeneration is the ability of a fully differentiated simple organism to grow a complete new individual from a piece of its body: if it is cut or broken into pieces, many of those pieces develop into separate whole organisms. It is carried out by specialised cells, which multiply and then change into the various cell types and tissues. Planaria (or Hydra) shows regeneration; a human being does not. Regeneration does not occur in a human being because the human body is highly differentiated: its cells are specialised for single functions and organised into interdependent tissues, organs and organ systems, so they can no longer divide freely and turn into every other cell type, and a fragment therefore cannot rebuild the missing organs or the whole body plan. (ii) The green filamentous structures are Spirogyra, a filamentous green alga. They reproduce by fragmentation, a form of asexual reproduction. On maturing, the filament simply breaks up into two or more fragments; each fragment then divides and grows into a new complete Spirogyra filament. Since the body is only a chain of similar cells with no division of labour, every piece can grow into a whole organism, and because a single parent is involved with no fusion of gametes the new filaments are identical to the parent.
    Step-by-step solution →
    OR
    Q36(b) 5 marks

    Also in 31/4/1 (Q35(b))

    (i) Name the part performing following functions in human male reproductive system :
    1. (a)Carries sperm
    2. (b)Production of male gametes
    3. (c)Whose secretion makes the transport of sperms easier
    4. (d)Provide suitable temperature for sperm formation (ii) Write any two characteristics of sperms. (iii) What are surgical contraceptive methods? Give the side effect caused by this procedure.
    Show answer

    The parts are vas deferens, testes, seminal vesicles with prostate and scrotum; sperms are motile and haploid; surgical methods block gametes.

    (i) (a) Vas deferens (the sperm duct) carries the sperm; (b) the testes produce the male gametes, the sperms; (c) the seminal vesicles and the prostate gland, whose secretions make the transport of sperms easier and also nourish them; (d) the scrotum, which holds the testes outside the abdominal cavity and so keeps them at a temperature lower than the normal body temperature, as sperm formation requires. (ii) A sperm is a very small motile cell made up mainly of genetic material, with a head carrying the nucleus, a middle piece and a long tail that helps it swim towards the female germ cell; and it is haploid, carrying only half the number of chromosomes (23 in a human) so that the full number is restored at fertilisation. (iii) Surgical contraceptive methods block the passage of the gametes by an operation so that fertilisation cannot occur: in the male the vas deferens is cut and tied (vasectomy) and in the female the fallopian tubes are cut and tied (tubectomy). Side effect: if the operation is not performed properly under sterile conditions it can cause infection and other complications at the site of surgery.
    Step-by-step solution →

Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/4/1 (Q39), 31/4/3 (Q37)

    The students in a class took a thick sheet of cardboard and made a small hole in its centre. Sunlight was allowed to fall on this small hole and they obtained a narrow beam of white light. A glass prism was taken and this white light was allowed to fall on one of its faces. The prism was turned slowly until the light that comes out of the opposite face of the prism appeared on the nearby screen. They studied this beautiful band of light and concluded that it is a spectrum of white light.
    1. (i)Give any one more instance in which this type of spectrum is observed.[1]
    2. (ii)What happens to white light in the above case?[1]
    3. (iii)(A)List two conditions necessary to observe a rainbow.[2]
    4. (iii)(B)OR : Draw a ray diagram to show the formation of a rainbow. Mark on it, points (a), (b) and (c) as given below : (a) Where dispersion of light occurs. (b) Where light gets reflected internally. (c) Where final refraction occurs.[2]
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    A rainbow is another spectrum; white light disperses into VIBGYOR in a prism; a rainbow needs water droplets and the Sun behind the observer.

    (i) A rainbow in the sky after a rain shower is the same kind of spectrum; so is the band of colours seen on looking at the sky through the spray of a waterfall or a fountain with the Sun behind you. (ii) The white light is dispersed - it splits into its seven component colours, violet, indigo, blue, green, yellow, orange and red (VIBGYOR), forming a band called the spectrum, because the glass has a different refractive index for each colour and so deviates violet the most and red the least (). (iii)(A) Two conditions: (1) there must be water droplets suspended in the air in front of the observer, as during or just after a rain shower; and (2) the Sun must be shining behind the observer, so that the observer faces the droplets with the Sun at their back - a rainbow is always formed in the direction opposite to the Sun. (iii)(B) In the ray diagram of a raindrop: (a) is on the surface where the sunlight enters, where it is refracted and dispersed; (b) is on the far inside surface, where the dispersed light is reflected internally; and (c) is on the lower surface, where the light is refracted again as it leaves the drop and the separated colours travel on to the observer.
    Step-by-step solution →
  2. Q38 4 marks

    Also in 31/4/1 (Q37), 31/4/3 (Q38)

    Common salt is a very important chemical compound for our daily life. It's chemical name is sodium chloride and it is used as a raw material in the manufacture of caustic soda, washing soda, baking soda etc. It is also used in the preservation of pickles, butter, meat etc.
    1. (i)Name the acid and the base from which common salt can be obtained.[1]
    2. (ii)State the nature (acidic/basic/neutral) of sodium chloride. Give reason for the justification for your answer.[1]
    3. (iii)(A)What happens when electric current is passed through an aqueous solution of sodium chloride (called brine)? Name the products obtained along with the corresponding places in the electrolytic cell where each of these products is obtained.[2]
    4. (iii)(B)OR : How is washing soda obtained from sodium chloride? Give chemical equation of the reactions involved in the process.[2]
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    Common salt is neutral and comes from and ; brine electrolysis gives chlorine, hydrogen and sodium hydroxide; the ammonia-soda route gives washing soda.

    (i) Acid: hydrochloric acid ; base: sodium hydroxide , which neutralise as . (ii) Neutral, pH , because sodium chloride is the salt of a strong acid () and a strong base (), so neither nor reacts with water. (iii)(A) The brine decomposes - this is the chlor-alkali process: . Chlorine gas is given off at the anode, hydrogen gas at the cathode, and sodium hydroxide solution is formed near the cathode. (iii)(B) By the ammonia-soda route: ; then ; then , which is washing soda.
    Step-by-step solution →
  3. Q39 4 marks

    Also in 31/4/1 (Q38), 31/4/3 (Q39)

    In life there are certain changes in the environment called ‘stimuli’ to which we respond appropriately. Touching a flame suddenly is a dangerous situation for us. One way is to think consciously about the possibility of burning and then moving the hand. But our body has been designed in such a way that we save ourself from such situations immediately.
    1. (i)Name the action by which we protect ourself in the situation mentioned above and define it.[1]
    2. (ii)Write the role of (a) motor and (b) relay neuron.[1]
    3. (iii)(A)What are the two types of nervous system in human body? Name the components of each of them.[2]
    4. (iii)(B)OR : Which part of the human brain is responsible for : (a) thinking (b) picking up a pencil (c) controlling blood pressure (d) controlling hunger[2]
    Show answer

    Reflex action is an involuntary response via the spinal cord; motor neurons carry it to the muscle; the nervous system has CNS and PNS.

    (i) Reflex action - a sudden, rapid, automatic and involuntary response of the body to a stimulus, carried out through a reflex arc in the spinal cord without the conscious part of the brain being involved. (ii)(a) The motor neuron carries the impulse away from the spinal cord to the effector - the arm muscle - which contracts and pulls the hand back. (b) The relay neuron lies inside the spinal cord and links the sensory neuron to the motor neuron, completing the reflex arc. (iii)(A) The two types are the central nervous system, whose components are the brain and the spinal cord, and the peripheral nervous system, whose components are the cranial nerves arising from the brain and the spinal nerves arising from the spinal cord. (iii)(B) (a) thinking - the cerebrum of the fore-brain; (b) picking up a pencil - the cerebellum of the hind-brain; (c) controlling blood pressure - the medulla of the hind-brain; (d) controlling hunger - the hunger centre of the fore-brain, the hypothalamus.
    Step-by-step solution →

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