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CBSE Class 10 Science 2025 — Paper 31/4/3

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

33 of its 45 questions also appear in 31/4/1 or 31/4/2; 12 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

    Also in 31/4/1 (Q16), 31/4/2 (Q11)

    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 →
  2. Q2 1 mark

    Also in 31/4/1 (Q15), 31/4/2 (Q12)

    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 →
  3. Q3 1 mark
    An element ‘M’ has 25% of the electrons filled in the third shell as in the second shell. The element ‘M’ is :
    1. (a)Sodium
    2. (b)Magnesium
    3. (c)Aluminium
    4. (d)Calcium
    Show answer
    (b) Magnesium
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/4/1 (Q11), 31/4/2 (Q13)

    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 →
  5. Q5 1 mark

    Also in 31/4/1 (Q12), 31/4/2 (Q14)

    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 →
  6. Q6 1 mark

    Also in 31/4/1 (Q1), 31/4/2 (Q4)

    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 →
  7. Q7 1 mark

    Also in 31/4/1 (Q2), 31/4/2 (Q6)

    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 →
  8. Q8 1 mark

    Also in 31/4/1 (Q4), 31/4/2 (Q7)

    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 →
  9. Q9 1 mark

    Also in 31/4/1 (Q5), 31/4/2 (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 (Q14), 31/4/2 (Q15)

    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 →
  11. Q11 1 mark

    Also in 31/4/1 (Q13), 31/4/2 (Q16)

    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 →
  12. Q12 1 mark

    Also in 31/4/1 (Q6), 31/4/2 (Q10)

    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 →
  13. Q13 1 mark

    Also in 31/4/1 (Q7), 31/4/2 (Q3)

    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 →
  14. Q14 1 mark

    Also in 31/4/1 (Q9), 31/4/2 (Q2)

    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 →
  15. Q15 1 mark

    Also in 31/4/1 (Q8)

    In a bisexual flower the male gametes are present in the :
    1. (a)anther
    2. (b)ovary
    3. (c)stigma
    4. (d)filament
    Show answer
    (a) anther
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/4/1 (Q10), 31/4/2 (Q1)

    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 →
  17. Q17 1 mark

    Also in 31/4/1 (Q19), 31/4/2 (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): 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 →
  18. Q18 1 mark

    Also in 31/4/1 (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 large animals, oxygen can reach different parts of the animal’s body easily.

    Reason (R): Respiratory pigments take up oxygen from the air and carry it to body tissues.

    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 →
  19. Q19 1 mark

    Also in 31/4/1 (Q18), 31/4/2 (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
    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): White light is dispersed by a glass prism into seven colours.

    Reason (R): The red light bends the least while the violet the most when a beam of white light passes through a glass prism.

    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
    Name the compound used in black and white photography. State whether the reaction that occurs is exothermic or endothermic. Give justification for your answer.
    Show answer

    Silver bromide is used in black and white photography; its decomposition by light is endothermic as it absorbs energy.

    The compound is silver bromide, (silver chloride, , behaves in the same way). In light it decomposes: . The reaction is endothermic, because in it the silver bromide decomposes by absorbing the energy of the light that falls on it.
    Step-by-step solution →
  2. Q22(a) 2 marks
    Show the formation of calcium chloride by the transfer of electrons from one element to the other. Atomic Number of calcium and chlorine is 20 and 17 respectively.
    Show answer

    Calcium gives one electron to each of two chlorine atoms, forming and two ions that attract to give .

    Calcium () gives its two outermost electrons, one to each of two chlorine atoms (): and . Both ions then have the octet , and the oppositely charged ions attract to form calcium chloride: .
    Step-by-step solution →
    OR
    Q22(b) 2 marks
    “Aluminium oxide is an amphoteric oxide.” Justify this statement giving chemical equation for the reactions involved.
    Show answer

    Aluminium oxide reacts with both an acid and a base to give a salt and water, so it is amphoteric.

    An amphoteric oxide reacts with both acids and bases to give a salt and water. Aluminium oxide does both: with hydrochloric acid it behaves as a base, , and with sodium hydroxide it behaves as an acid, (sodium aluminate). So aluminium oxide is an amphoteric oxide.
    Step-by-step solution →
  3. Q23 2 marks
    Name the tissues which form the vascular bundle. State their function in plants.
    Show answer

    Xylem and phloem form the vascular bundle: xylem conducts water and minerals, and phloem conducts food.

    The vascular bundle is formed of the conducting tissues xylem and phloem. Xylem conducts water and the minerals dissolved in it from the roots to the leaves and the other parts of the plant. Phloem conducts the food made in the leaves (the soluble products of photosynthesis), along with amino acids and other substances, to the other parts of the plant, both upwards and downwards; this is called translocation.
    Step-by-step solution →
  4. Q24(a) 2 marks

    Also in 31/4/1 (Q25(a)), 31/4/2 (Q22(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
    Q24(b) 2 marks

    Also in 31/4/1 (Q25(b)), 31/4/2 (Q22(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 →
  5. Q25 2 marks

    Also in 31/4/1 (Q24)

    Explain with the help of a flow chart that in human beings father is responsible for the sex (male or female) of the child.
    Show answer

    Every egg carries X but sperms carry X or Y, so the father's sperm decides the child's sex.

    A human mother is and a father is . In gamete formation each gamete receives one chromosome of each pair, so every egg carries , while half the sperms carry and half carry . At fertilisation, an X-carrying sperm gives a zygote , which develops into a girl, and a Y-carrying sperm gives , which develops into a boy. Since the mother can contribute only an X, the sex of the child depends entirely on which kind of sperm fertilises the egg, so the father is responsible for the sex of the child.
    Step-by-step solution →
  6. Q26 2 marks
    Draw the pattern of magnetic field lines due to a current carrying straight conductor. Mark on it the direction of current in the conductor and the direction of the magnetic field developed. Name the rule that helps us to determine the direction of magnetic field lines in this case.
    Show answer

    Concentric circles round the wire, anticlockwise as seen from above for an upward current; the rule is the right-hand thumb rule.

    The magnetic field lines are concentric circles centred on the conductor, in planes perpendicular to it, drawn farther apart away from the wire. With the current marked upwards along the conductor, the field lines run anticlockwise when seen from above (see the figure). The direction is given by the right-hand thumb rule (Maxwell’s corkscrew rule): hold the conductor in the right hand with the thumb along the current, and the curled fingers show the direction of the field lines.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks

    Also in 31/4/1 (Q31)

    If we want to obtain a virtual 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 produced by this mirror to justify your answer.
    Show answer

    Place the object between the pole and the focus; for , cm, so it is 9 cm from the mirror and cm.

    The object must be placed between the pole and the principal focus of the mirror, that is at a distance less than the focal length of cm. Taking cm and : gives , and substituting in gives , so and cm. The object must therefore be placed cm from the mirror, and since cm is less than the focal length cm this position lies between the pole and the focus, which justifies the answer. The image is then formed at cm, that is cm behind the mirror, and is virtual, erect and twice the size of the object.
    Step-by-step solution →
  2. Q28 3 marks
    Name and describe the most widely used method for refining impure metals ?
    Show answer

    Electrolytic refining: the impure metal is the anode and pure metal the cathode in a salt solution; pure metal deposits on the cathode.

    The method is electrolytic refining. The impure metal is made the anode, connected to the positive terminal, and a thin strip of the pure metal is made the cathode, connected to the negative terminal; both dip into a solution of a salt of the metal, the electrolyte (for copper, acidified copper sulphate solution). When a current is passed, the metal of the anode dissolves into the electrolyte as ions () and an equivalent amount of pure metal is deposited from the electrolyte on the cathode (). Soluble impurities go into the solution, and insoluble impurities settle at the bottom of the anode as anode mud.
    Step-by-step solution →
  3. Q29 3 marks
    What is the first step of cellular respiration? In which part of the cell does it occur ? Write the equation for the process of breakdown of glucose in a human cell :
    1. (i)in the presence of oxygen
    2. (ii)due to lack of oxygen
    Show answer

    The first step is glycolysis in the cytoplasm; glucose then gives carbon dioxide and water with oxygen, or lactic acid when oxygen is lacking.

    The first step of cellular respiration is glycolysis: glucose, a six-carbon molecule, is broken down into pyruvate, a three-carbon molecule, with the release of some energy. It occurs in the cytoplasm. (i) In the presence of oxygen, pyruvate is broken down in the mitochondria: ; overall, . (ii) Due to lack of oxygen, in our muscle cells: ; overall, .
    Step-by-step solution →
  4. Q30(a) 3 marks

    Also in 31/4/1 (Q27(a)), 31/4/2 (Q32(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
    Q30(b) 3 marks

    Also in 31/4/1 (Q27(b)), 31/4/2 (Q32(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 →
  5. Q31 3 marks

    Also in 31/4/1 (Q33), 31/4/2 (Q27)

    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 →
  6. Q32 3 marks
    Define the term “potential difference” between two points in an electric circuit carrying current. Name and define its S.I. unit. Also express it in terms of S.I. unit of work and charge.
    Show answer

    Potential difference is the work done per unit charge, ; its SI unit is the volt, and .

    Potential difference between two points in an electric circuit carrying current is the work done to move a unit charge from one point to the other: . Its SI unit is the volt (V): one volt is the potential difference between two points in a current-carrying conductor when joule of work is done to move a charge of coulomb from one point to the other. In terms of the SI units of work and charge, .
    Step-by-step solution →
  7. Q33 3 marks

    Also in 31/4/1 (Q30), 31/4/2 (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

    Also in 31/4/1 (Q36(a)), 31/4/2 (Q35(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
    Q34(b) 5 marks

    Also in 31/4/1 (Q36(b)), 31/4/2 (Q35(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 − +
    Show answer

    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 →
  2. Q35(a) 5 marks
    Name the parts marked as ‘A’ and ‘B’ in the given diagram. Write in detail the changes that take place in a flower when the product of ‘B’ reaches ‘A’ till a fruit is formed.
    A B
    Show answer

    A is the stigma, B the anther; after pollination and fertilisation the ovule becomes a seed and the ovary a fruit.

    A is the stigma and B is the anther. The product of the anther is pollen. (1) Pollination: pollen grains are carried from the anther to the stigma — within the same flower in self-pollination, from another flower in cross-pollination, by agents such as wind, water or animals. (2) On the stigma the pollen grain germinates: a pollen tube grows out of it and travels through the style to reach the ovule in the ovary, carrying the male germ-cell. (3) Fertilisation: in the ovule the male germ-cell fuses with the female germ-cell to form a zygote. (4) The zygote divides several times to form an embryo within the ovule. (5) The ovule develops a tough coat and is gradually converted into a seed. (6) The ovary grows rapidly and ripens to form the fruit, while the petals, sepals, stamens, style and stigma may shrivel and fall off.
    Step-by-step solution →
    OR
    Q35(b) 5 marks
    In human female reproductive system state the changes that take place once fertilisation has taken place. Write the role of placenta in this process. What happens when the egg is not fertilised ?
    Show answer

    After fertilisation the embryo implants in the uterus and the placenta nourishes it; if the egg is unfertilised, the lining is shed as menstruation.

    Once fertilisation has taken place in the oviduct, the zygote divides to form an embryo, which is implanted in the thick, blood-rich lining of the uterus and goes on developing there; the lining is kept, so there is no menstruation during pregnancy, and after about nine months the child is born by rhythmic contractions of the muscles of the uterus. Role of the placenta: the placenta is a disc of special tissue embedded in the uterine wall, with villi on the embryo’s side surrounded by blood spaces on the mother’s side; this large surface lets glucose and oxygen pass from the mother’s blood to the embryo, and lets the waste substances produced by the embryo pass into the mother’s blood for removal. If the egg is not fertilised, it lives for about one day; the thick, spongy lining the uterus prepared is not needed, so it breaks down and comes out through the vagina as blood and mucus — menstruation, which takes place roughly every month and usually lasts about two to eight days.
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  3. Q36(a) 5 marks

    Also in 31/4/1 (Q34(a))

    A carbon compound 'A' on heating with excess conc. forms a compound 'B', which on addition of one mole of hydrogen gas in the presence of nickel catalyst forms a compound 'C'. 'C' on combustion in air forms 2 moles of carbon dioxide and 3 moles of water. Identify 'A', 'B' and 'C' and write their structures. Give chemical equations of the reactions involved. Also state the role of concentrated sulphuric acid in the formation of 'B' from 'A'.
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    A is ethanol, B ethene, C ethane; conc. sulphuric acid acts as a dehydrating agent, removing water from ethanol.

    'A' is ethanol, ; 'B' is ethene, ; and 'C' is ethane, . Structures: ethanol has two carbons joined by a single bond with an on the end carbon and hydrogen filling every other valency; ethene has the two carbons joined by a double bond with two hydrogens on each; ethane has them joined by a single bond with three hydrogens on each. Equations: ; ; . Role of concentrated sulphuric acid: it acts as a dehydrating agent, removing a molecule of water from ethanol to give the unsaturated compound ethene.
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    OR
    Q36(b) 5 marks

    Also in 31/4/1 (Q34(b))

    A carbon compound 'A' is widely used as a preservative in pickles and has a molecular formula . This compound reacts with ethanol to form a sweet smelling compound 'B'.
    1. (i)Identify the compound 'A' and write its structure.
    2. (ii)Write the chemical equation for the reaction of 'A' with ethanol to form compound 'B'. State the role of presence of an acid in the reaction.
    3. (iii)How can we get compound 'A' back from 'B'?
    4. (iv)How can 'A' be obtained from ethanol?
    5. (v)Name the gas produced when compound 'A' reacts with washing soda.
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    'A' is ethanoic acid; it forms the ester ethyl ethanoate with ethanol, is recovered by hydrolysis, and gives carbon dioxide with washing soda.

    (i) 'A' is ethanoic acid (acetic acid), ; its structure is a methyl group joined to a carboxyl group, the carboxyl carbon being doubly bonded to one oxygen and singly bonded to an group. (ii) ; 'B' is ethyl ethanoate. The acid acts as a catalyst and, being a dehydrating agent, removes the water formed and drives the reversible reaction towards the ester. (iii) By hydrolysing 'B': warming it with gives sodium ethanoate and ethanol (saponification), and adding dilute to the sodium ethanoate liberates ethanoic acid; warming 'B' with water and a dilute mineral acid does it in one step. (iv) By oxidising ethanol: (acidified works equally well). (v) Carbon dioxide: .
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Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/4/1 (Q39), 31/4/2 (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.
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  2. Q38 4 marks

    Also in 31/4/1 (Q37), 31/4/2 (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.
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  3. Q39 4 marks

    Also in 31/4/1 (Q38), 31/4/2 (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]
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    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.
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