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CBSE Class 10 Science 2025 — Paper 31/2/3

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

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

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Questions 1–20 · 1 mark each

  1. Q1 1 mark
    In the electrolysis of water, the mass ratio of hydrogen and oxygen gases liberated at the electrodes is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/2/1 (Q3)

    The body of human beings works within the pH range of :
    1. (a) to
    2. (b) to
    3. (c) to
    4. (d) to
    Show answer
    (c) to
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/2/1 (Q2), 31/2/2 (Q1)

    The warning sign shown in the given figure must invariably be displayed/pasted on the containers which contain hydroxide of :
    1. (a)Aluminium
    2. (b)Calcium
    3. (c)Sodium
    4. (d)Magnesium
    Show answer
    (c) Sodium
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/2/1 (Q5), 31/2/2 (Q4)

    Two metals zinc and tin are dissolved separately in definite proportions in molten copper (the primary metal) to obtain two different alloys respectively known as :
    1. (a)Bronze and Brass
    2. (b)Brass and Solder
    3. (c)Brass and Bronze
    4. (d)Solder and Bronze
    Show answer
    (c) Brass and Bronze
    Step-by-step solution →
  5. Q5 1 mark

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

    Aluminium powder is used in thermit welding because :
    1. (a)Its reaction with iron is highly exothermic.
    2. (b)When it is heated with iron (III) oxide, molten iron is obtained.
    3. (c)When it is heated with iron (III) oxide, molten aluminium oxide is obtained to join railway tracks.
    4. (d)Its melting point is low as compared to iron and a molten alloy of iron and aluminium is formed on heating which is used to join railway tracks.
    Show answer
    (b) When it is heated with iron (III) oxide, molten iron is obtained.
    Step-by-step solution →
  6. Q6 1 mark
    Select the correct option from the following statements about the functioning of the human heart.
    1. (a)Right atrium receives deoxygenated blood from different parts of the body and sends it to pulmonary veins.
    2. (b)Left atrium sends oxygenated blood to right ventricle which pumps it to different parts of the body.
    3. (c)Right atrium receives deoxygenated blood from the body and sends it to the right ventricle.
    4. (d)Left atrium receives oxygenated blood from the pulmonary arteries and sends it to the left ventricle.
    Show answer
    (c) Right atrium receives deoxygenated blood from the body and sends it to the right ventricle.
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/2/1 (Q8), 31/2/2 (Q10)

    Observe the given figures A and B. When chhui-mui (sensitive) plant is touched, its leaves fold. This is due to :
    Figure A Figure B
    Figure A and Figure B
    1. (a)Hormonal effect
    2. (b)Thermal effect
    3. (c)Change in amount of water in cells
    4. (d)Electromagnetic effect
    Show answer
    (c) Change in amount of water in cells
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/2/1 (Q7), 31/2/2 (Q8)

    One-cell thick blood vessels are known as :
    1. (a)Alveoli
    2. (b)Capillaries
    3. (c)Arteries
    4. (d)Veins
    Show answer
    (b) Capillaries
    Step-by-step solution →
  9. Q9 1 mark
    Which of the following is the correct sequence of parts of female reproductive system of flowering plants in terms of their placement ?
    1. (a)Stigma, ovule, ovary, style
    2. (b)Ovule, stigma, ovary, style
    3. (c)Style, stigma, ovule, ovary
    4. (d)Stigma, style, ovary, ovule
    Show answer
    (d) Stigma, style, ovary, ovule
    Step-by-step solution →
  10. Q10 1 mark

    Also in 31/2/1 (Q10), 31/2/2 (Q7)

    The number of chromosomes in a cell division is halved. This kind of cell division is observed in :
    1. (a)Only testis
    2. (b)Only ovary
    3. (c)Ovary and testis both
    4. (d)All cells of the body
    Show answer
    (c) Ovary and testis both
    Step-by-step solution →
  11. Q11 1 mark

    Also in 31/2/1 (Q14)

    An electric bulb is connected to a power supply of V. If the current drawn by the bulb from the supply is mA, the power of the bulb is :
    1. (a) W
    2. (b) W
    3. (c) W
    4. (d) W
    Show answer
    (b) W
    Step-by-step solution →
  12. Q12 1 mark

    Also in 31/2/1 (Q15), 31/2/2 (Q11)

    Four identical resistors of each are connected in series to form a square ABCD as shown in the figure. The resistance of the network between the two points and is :
    1 2 12 Ω 12 Ω 12 Ω 12 Ω A B C D
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  13. Q13 1 mark
    The part of the human eye which can modify the curvature of the eye lens to some extent is :
    1. (a)Pupil
    2. (b)Cornea
    3. (c)Ciliary muscles
    4. (d)Aqueous humour
    Show answer
    (c) Ciliary muscles
    Step-by-step solution →
  14. Q14 1 mark

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

    If the absolute refractive indices of two media X and Y are and respectively, then the refractive index of Y with respect to X will be :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (a)
    Step-by-step solution →
  15. Q15 1 mark

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

    An object is placed at a distance of 30 cm from the pole of a concave mirror. If its real and inverted image is formed at 60 cm in front of the mirror, the focal length of the mirror is :
    1. (a) cm
    2. (b) cm
    3. (c) cm
    4. (d) cm
    Show answer
    (b) cm
    Step-by-step solution →
  16. Q16 1 mark

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

    Identify from the following a group containing all non-biodegradable substances.
    1. (a)Leather, Glass, Plastic
    2. (b)Cotton, Wood, Nylon
    3. (c)DDT, Polyester, Glass
    4. (d)Leather, Silk, Wool
    Show answer
    (c) DDT, Polyester, Glass
    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): Ductility is that property of metals which enables copper to be used in making cooking utensils.

    Reason (R): Copper is a metal which is ductile as well as malleable.

    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 31/2/1 (Q18), 31/2/2 (Q19)

    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer from the codes given below.

    Assertion (A): A mango seed will germinate to form a mango tree.

    Reason (R): Heredity determines the process by which traits and characteristics are reliably inherited from parents to offspring.

    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
    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): The bending of a wire in the form of a coil of several turns does not affect its electrical resistance.

    Reason (R): The electrical resistance of a wire is directly proportional to its electrical resistivity.

    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 31/2/1 (Q20), 31/2/2 (Q20)

    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer from the codes given below.

    Assertion (A): Animals will not get energy if they eat (consume) coal as food.

    Reason (R): Specific enzymes are needed for the breakdown of a particular food.

    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 →

Questions 21–26 · 2 marks each

  1. Q21 2 marks

    Also in 31/2/1 (Q21)

    1. (a)In common practice silver is recovered from silver nitrate solution by the use of copper metal. Name the type of reaction that takes place in this process and give the chemical equation of the reaction involved.
    2. (b)Name the method used for refining silver.
    Show answer

    It is a displacement reaction, ; silver is refined by electrolytic refining.

    (a) It is a displacement reaction (copper, being more reactive, displaces silver from its salt): ; (b) Silver is refined by electrolytic refining.
    Step-by-step solution →
  2. Q22 2 marks
    What are micelles ? Draw the structure.
    Show answer

    Micelles are clusters of soap molecules in water, with hydrocarbon tails and oily dirt inside and ionic ends facing the water.

    Micelles are clusters of soap molecules formed in water, in which the hydrophobic hydrocarbon tails are in the interior of the cluster and the ionic (hydrophilic) ends, , lie on its surface facing the water; oily dirt is collected in the centre. Structure: a sphere of soap molecules with the ionic heads on the outside and the hydrocarbon tails pointing inwards around the oily dirt, as in the figure.
    Step-by-step solution →
  3. Q23 2 marks
    A person is making a list to purchase few things from a nearby market. Explain how the fore-brain plays an important role in performing this activity.
    Show answer

    The fore-brain takes in the information, thinks and decides using memory, and its motor areas direct the hand muscles that write the list.

    The fore-brain is the main thinking part of the brain. While the list is being made, its sensory areas receive impulses from the eyes (for example, on seeing which items at home are finished); its association areas interpret this information by putting it together with information already stored in the brain, the memory of what is needed and what the market sells, and a decision is made about what to buy. The decision is then passed to the motor areas of the fore-brain, which control the voluntary muscles of the hand that write the list. Thinking, memory and the voluntary action of writing are all directed by the fore-brain.
    Step-by-step solution →
  4. Q24(a) 2 marks

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

    Write one function each of the following, in a seed :
    1. (i)Seed coat
    2. (ii)Cotyledon
    3. (iii)Radicle
    4. (iv)Plumule
    Show answer

    The seed coat protects the embryo, the cotyledon stores food, the radicle forms the root and the plumule forms the shoot.

    (i) The seed coat is the tough outer covering that protects the embryo inside from mechanical injury, drying and micro-organisms; (ii) The cotyledon stores the food that nourishes the embryo and the young seedling until it can photosynthesise; (iii) The radicle is the embryonic root and grows downwards to form the root system, anchoring the plant and absorbing water and minerals; (iv) The plumule is the embryonic shoot and grows upwards to form the stem and the leaves.
    Step-by-step solution →
    OR
    Q24(b) 2 marks

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

    Write the main steps to culture yeast in the laboratory.
    Show answer

    Dissolve sugar in water, add yeast, keep it warm for one to two hours, then observe a drop under a microscope.

    Dissolve some sugar in about mL of water in a clean beaker or test tube to make a dilute sugar solution; add a pinch of dry yeast granules and stir; put a cotton plug on the mouth of the vessel and keep it undisturbed in a warm place at about to for one to two hours, during which the yeast feeds on the sugar and multiplies; then place a drop of the culture on a clean glass slide, cover it with a cover slip and observe it under a microscope, where yeast cells with buds are seen.
    Step-by-step solution →
  5. Q25(a) 2 marks

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

    Out of the two lenses, one concave and the other convex, state which one will diverge a parallel beam of light falling on it. Draw a ray diagram to show the principal focus of the lens.
    Show answer

    A concave lens diverges a parallel beam; the refracted rays appear to come from its virtual principal focus .

    The concave lens diverges a parallel beam of light. After refraction the rays spread outwards, and when they are produced backwards they appear to come from a single point F on the principal axis on the same side as the incoming light. F is the principal focus of the concave lens; it is a virtual focus, so its focal length is negative.
    Step-by-step solution →
    OR
    Q25(b) 2 marks

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

    A ray of light after refraction from a convex lens emerges parallel to its principal axis.
    1. (i)Draw a labelled ray diagram to show it.
    2. (ii)In this case, the incident ray before refraction from the lens passes through a point on its principal axis. Name the point.
    Show answer

    A ray passing through the principal focus emerges parallel to the axis after a convex lens; the point is the principal focus.

    (i) The ray diagram shows a ray that crosses the principal axis at the principal focus on the incident side, strikes the convex lens and emerges parallel to the principal axis. (ii) The point is the principal focus of the convex lens (the focus on the side from which the light comes).
    Step-by-step solution →
  6. Q26 2 marks
    How and why is an electric fuse used in an electric circuit ? Briefly describe its function.
    Show answer

    A fuse wire in series in the live wire melts on excess current, breaking the circuit and protecting the appliances.

    How: a fuse — a short piece of wire of a metal or alloy of appropriate melting point, usually in a porcelain cartridge — is connected in series with the circuit or appliance, in the live wire, so that the whole current passes through it. Why: to protect the circuit and the appliances from an unduly high current caused by overloading or short-circuiting. Function: if the current rises above the rated value, the heat produced in the fuse wire () raises its temperature until it melts; this breaks the circuit and stops the current before the wiring or the appliances are damaged.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27(a) 3 marks

    Also in 31/2/1 (Q27(a)), 31/2/2 (Q27(a))

    1. (i)Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity.
    2. (ii)Decomposition of vegetable matter into compost is considered an exothermic reaction. Why ?
    Show answer

    A decomposition reaction breaks one reactant into simpler products; composting releases heat to the surroundings, so it is exothermic.

    (i) A decomposition reaction is one in which a single reactant breaks down into two or more simpler products. Heat: ; Light: ; Electricity: . (ii) Micro-organisms decompose the vegetable matter and energy is released to the surroundings as heat, so the compost heap warms up; a reaction that gives out energy is exothermic.
    Step-by-step solution →
    OR
    Q27(b) 3 marks

    Also in 31/2/1 (Q27(b)), 31/2/2 (Q27(b))

    Why are decomposition reactions called the opposite of combination reactions ? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.
    Show answer

    Decomposition reverses combination: calcium oxide and water combine to give calcium hydroxide, whereas calcium carbonate breaks into calcium oxide and carbon dioxide.

    In a combination reaction two or more substances join to form a single product, whereas in a decomposition reaction a single substance breaks up into two or more products; combination usually gives out energy while decomposition needs energy to be supplied. Each change is therefore the reverse of the other, which is why decomposition is called the opposite of combination. Combination: , that is calcium oxide (quick lime) and water giving calcium hydroxide (slaked lime). Decomposition: , that is calcium carbonate (limestone) giving calcium oxide (quick lime) and carbon dioxide.
    Step-by-step solution →
  2. Q28 3 marks
    Two substances ‘A’ and ‘B’ are burnt in air separately. For ‘A’, ashes are collected and dissolved in water to get solution ‘X’, while for ‘B’, fumes produced are passed through water to get solution ‘Y’. Both the solutions ‘X’ and ‘Y’ are then tested with pH paper.
    1. (a)If ‘X’ gives light blue colour and ‘Y’ gives orange colour to the pH paper, then write the nature and range of pH of ‘X’ and ‘Y’.
    2. (b)Which one of the two – A and B, is a metal ? Justify your answer.
    Show answer

    X is basic (pH above 7) and Y is acidic (pH below 7); A is the metal because metal oxides are basic.

    (a) X is basic (alkaline) in nature: light blue on pH paper means a pH above , roughly to . Y is acidic in nature: orange means a pH below , roughly to . (b) A is the metal. The ash of A is its oxide, and it dissolves in water to give a basic solution, X; metal oxides are basic (apart from a few amphoteric ones such as aluminium oxide and zinc oxide), while the oxides of non-metals are generally acidic, which is why the fumes of B, a non-metal, give the acidic solution Y. For example, magnesium burns to give magnesium oxide, which forms magnesium hydroxide in water, while sulphur burns to give sulphur dioxide, which forms sulphurous acid.
    Step-by-step solution →
  3. Q29 3 marks

    Also in 31/2/1 (Q29)

    1. (a)Define hormone.
    2. (b)“Hormones should be secreted in precise quantities. We have a feedback mechanism through which this is done.” With the help of an example justify the statement.
    Show answer

    A hormone is a chemical messenger from a ductless gland; feedback keeps its quantity precise, as falling blood glucose reduces insulin secretion.

    (a) A hormone is a chemical messenger secreted in very small amounts by an endocrine (ductless) gland, released directly into the blood and carried to specific target organs, where it controls and coordinates growth, development and metabolism. (b) The amount matters because both excess and deficiency are harmful, so secretion is regulated by feedback. Example: after a meal the blood glucose level rises; the cells of the pancreas detect this and secrete more insulin; insulin makes the cells take up glucose and the liver store it as glycogen, so the glucose level falls; as it falls, insulin secretion is reduced. The level being controlled thus decides how much hormone is released, which is why hormones are secreted in precise quantities.
    Step-by-step solution →
  4. Q30 3 marks
    A pure pea plant bearing terminal flowers was cross-pollinated with a pure plant having axillary flowers. In generation, plants with axial flowers only were obtained. generation plants are self-pollinated and generation is obtained.
    1. (a)Work out the pattern of inheritance in this case.
    2. (b)What will be the ratio of plants obtained in generation ?
    Show answer

    Axial is dominant over terminal; is all and shows axial : terminal (genotypes ).

    (a) Axial (axillary) flower position is dominant over terminal. Let stand for the factor for axial flowers and for terminal. The pure parents are axillary and terminal ; their gametes are and , so every plant is and has axial flowers, the terminal trait staying hidden. On self-pollination each plant forms gametes and in equal numbers, and these combine to give , , and in the , so the terminal trait reappears in the plants. The two factors of a pair separate when gametes form, and the recessive trait shows only when both factors are recessive. (b) In the , axial : terminal by appearance (phenotypic ratio); by genes (genotypic ratio).
    Step-by-step solution →
  5. Q31 3 marks
    A convex mirror used for rear view on an automobile has a focal length of m. If a m high bus is located at m from the mirror, use mirror formula to determine the position and size of the image of the bus as seen in the mirror.
    Show answer

    The image of the bus is 1.2 m behind the mirror and 0.6 m high: virtual, erect and diminished.

    Position: m, that is, the image is m behind the mirror. Size: m, so the image of the bus is m high, one-fifth of the height of the bus (). The image is virtual, erect and diminished.
    Step-by-step solution →
  6. Q32 3 marks

    Also in 31/2/1 (Q32)

    Three resistors of , and are connected in
    1. (i)series, and
    2. (ii)parallel. Draw the arrangements of the resistors and find the equivalent resistance of each arrangement.
    Show answer

    In series the equivalent resistance is ; in parallel it is .

    In series the three resistors lie end to end in one path and the equivalent resistance is . In parallel all three are joined between the same two points and , so .
    Step-by-step solution →
  7. Q33 3 marks
    Some harmful chemicals get accumulated in human bodies through the food chain. Name this phenomenon. Explain the reason of maximum concentration of these chemicals found in our bodies.
    Show answer

    The phenomenon is biological magnification; non-biodegradable chemicals build up at each trophic level, so humans at the top have the most.

    The phenomenon is biological magnification. Reason: harmful chemicals such as pesticides are washed into the soil and water bodies and are taken up by plants and aquatic organisms, so they enter the food chain. Being non-biodegradable, they are not broken down or readily excreted, so they accumulate in the organisms, and their concentration increases progressively at each trophic level as each organism eats a large quantity of food from the level below. Human beings occupy the top level of the food chains they belong to, so the maximum concentration of these chemicals accumulates in our bodies.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks

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

    The given figure shows the current passing through the straight conductor XY.
    1. (i)Copy the diagram and draw the magnetic field lines when current flows from conductor X to Y.
    2. (ii)Name and state the rule used in determining the direction of the magnetic field lines in the situation given above.
    3. (iii)State Fleming’s left hand rule. Using this rule, determine the direction of force applied on an electron entering a uniform magnetic field as shown in the figure.
    Q X Y Variable resistance A - + + - K
    Straight conductor XY passing through a cardboard sheet Q, in a circuit with a variable resistance, an ammeter, a battery of two cells and a key.
    Magnetic field e
    An electron entering a uniform magnetic field directed to the right.
    Show answer

    Field lines are clockwise circles (seen from above) by the right-hand thumb rule; the force on the electron is out of the paper.

    (i) The magnetic field lines are concentric circles centred on the conductor and lying in the plane of the cardboard sheet, that is at right angles to XY; they are close together near the wire and farther apart away from it. With the current flowing from X to Y (downwards) the circles run clockwise when seen from above. (ii) The rule is the right-hand thumb rule (Maxwell's corkscrew rule): if a current carrying straight conductor is held in the right hand with the thumb pointing in the direction of the current, then the fingers wrapped round the conductor give the direction of the magnetic field lines. (iii) Fleming's left hand rule: stretch the thumb, forefinger and central finger of the left hand mutually perpendicular; if the forefinger points along the magnetic field and the central finger along the current, the thumb gives the direction of the force. Here the electron moves upwards, so the conventional current is downwards, and the field is towards the right; the thumb then points out of the plane of the paper, so the force on the electron is directed perpendicular to the paper, outwards, towards the reader.
    Step-by-step solution →
    OR
    Q34(b) 5 marks

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

    (i) Define the term solenoid. Draw the pattern of the magnetic field lines in and around a current carrying straight solenoid. Mark on the pattern the
    1. (i)direction of current,
    2. (ii)direction of field lines near the ends of the solenoid, and
    3. (iii)region where the magnetic field is uniform. (ii) How would you make an electromagnet using a current carrying solenoid ?
    Show answer

    A solenoid is a closely wound coil with a bar-magnet-like field, uniform inside; a soft iron core inside it, with current, makes an electromagnet.

    (i) A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. Its magnetic field pattern is like that of a bar magnet: inside the solenoid the field lines are straight, parallel and equally spaced, running from the south end to the north end, while outside they emerge from the north end, curve round and enter at the south end, forming closed loops that never cross. On the pattern, mark (i) the direction of the current with arrows on the turns and the leads, (ii) the field lines coming out at the end that is the north pole and going in at the end that is the south pole, and (iii) the central region inside the coil, where the equally spaced parallel lines show that the field is uniform. (ii) To make an electromagnet, place a rod of soft iron inside the solenoid and pass a current through the coil. The soft iron is strongly magnetised and the arrangement acts as a powerful magnet as long as the current flows; the strength can be increased by increasing the current or the number of turns, and the magnetism is lost when the current is switched off.
    Step-by-step solution →
  2. Q35(a) 5 marks

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

    1. (i)A compound ‘X’ having two carbon atoms in its molecule turns blue litmus red and solution of ‘X’ in water is widely used as a preservative. Identify the compound ‘X’ and write its structure.
    2. (ii)Compare its pH nature with a mineral acid.
    3. (iii)‘X’ on reacting with alcohols produces sweet smelling compounds, used in making perfumes. Name the reaction and write its chemical equation.
    4. (iv)When sodium carbonate is added to ‘X’, a colourless gas is produced which turns lime water milky. Write the chemical equation for the reaction giving the name of the salt produced.
    Show answer

    'X' is ethanoic acid, a weaker acid than HCl; it forms esters with alcohols and sodium ethanoate with sodium carbonate.

    (i) X is ethanoic acid (acetic acid), ; its structure is , that is a methyl group joined to a carboxyl group in which the carbon is double bonded to one oxygen and single bonded to an O-H. (ii) Ethanoic acid is a weak acid and ionises only partially, so at the same concentration it gives fewer hydrogen ions and has a higher pH (about ) than a mineral acid such as hydrochloric acid (about ), which is a strong acid and ionises completely. (iii) The reaction is esterification: . (iv) ; the salt produced is sodium ethanoate (sodium acetate).
    Step-by-step solution →
    OR
    Q35(b) 5 marks

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

    1. (i)Differentiate between saturated and unsaturated hydrocarbons by giving one example each, with a structural formula.
    2. (ii)Write the method of converting an unsaturated hydrocarbon into a saturated hydrocarbon. Name the industry where this reaction is commonly used.
    3. (iii)Write the name and structure of a hydrocarbon having double bond and four carbon atoms in its one molecule.
    Show answer

    Saturated hydrocarbons have only single bonds, unsaturated ones a double or triple bond; nickel hydrogenation makes vanaspati ghee; the four-carbon alkene is but-1-ene.

    (i) A saturated hydrocarbon has only single bonds between its carbon atoms, general formula ; example ethane, , structure . An unsaturated hydrocarbon has at least one double or triple bond between carbon atoms; example ethene, , structure . Saturated hydrocarbons give substitution reactions and burn with a clean blue flame, unsaturated ones give addition reactions and burn with a yellow sooty flame. (ii) The method is hydrogenation: the unsaturated hydrocarbon is heated with hydrogen in the presence of a nickel (or palladium) catalyst, . It is commonly used in the vegetable oil industry, to convert vegetable oils into vanaspati ghee. (iii) But-1-ene (butene), , structure .
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  3. Q36(a) 5 marks

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

    1. (i)“The length of the small intestine in various animals depends on the food they eat.” Justify the statement.
    2. (ii)Discuss the role of the pancreas and bile juice in the digestion of food in human beings.
    3. (iii)How is the small intestine designed to absorb digested food ?
    Show answer

    Herbivores need a longer small intestine; bile makes food alkaline and emulsifies fat, pancreatic enzymes digest the food, and villi absorb it.

    (i) Herbivores such as cattle and deer eat grass, which is rich in cellulose and difficult to digest, so they need a longer small intestine to complete digestion and absorption; carnivores such as tigers eat meat, which is easily and quickly digested, so their small intestine is much shorter. The length therefore depends on how hard the food is to digest, which justifies the statement. (ii) Bile juice, made in the liver and stored in the gall bladder, makes the acidic food coming from the stomach alkaline, because pancreatic enzymes act only in an alkaline medium, and it emulsifies fat, breaking large globules into small droplets so that lipase can act; bile contains no enzymes. The pancreas secretes pancreatic juice containing trypsin, which digests proteins into amino acids, pancreatic amylase, which breaks starch into sugars, and lipase, which breaks emulsified fats into fatty acids and glycerol. (iii) The small intestine is very long and its inner lining bears a very large number of finger-like villi, which greatly increase the surface area for absorption; each villus has a wall only one cell thick and a rich supply of blood vessels, so the digested food passes easily into the blood and is carried to every cell of the body.
    Step-by-step solution →
    OR
    Q36(b) 5 marks

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

    1. (i)State the role of rings of cartilage present in the throat.
    2. (ii)Discuss the role of the ribs and diaphragm when air is taken in during the breathing cycle.
    3. (iii)Why do we get muscle cramps during heavy exercise ? Explain.
    Show answer

    Cartilage rings keep the trachea open; the diaphragm and ribs enlarge the chest so air rushes in; oxygen shortage makes lactic acid, causing cramps.

    (i) The C-shaped rings of cartilage in the wall of the wind pipe keep the air passage permanently open; they prevent the trachea from collapsing when the pressure of the air inside it falls, so air can pass freely at all times. (ii) During inhalation the muscles of the diaphragm contract, so the domed diaphragm flattens and moves downwards, and at the same time the muscles between the ribs contract and lift the ribs up and outwards. Both movements increase the volume of the chest cavity, the lungs expand and the pressure inside them falls below atmospheric pressure, so air rushes in through the nostrils and trachea and fills the alveoli. (iii) During heavy exercise the supply of oxygen to the muscle cells falls short of the demand, so the cells respire anaerobically: glucose is broken down to pyruvate in the cytoplasm and the pyruvate is then converted into lactic acid, releasing only a little energy. The lactic acid accumulates in the muscle fibres and causes the pain and cramps; rest and deep breathing supply the oxygen that clears it away.
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Questions 37–39 · 4 marks each

  1. Q37 4 marks

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

    When a girl is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these start maturing. One matured egg is released every month by one of the ovaries. The two oviducts unite into an elastic bag-like structure known as uterus.
    1. (a)Write the site of fertilization in human female.[1]
    2. (b)How does the uterus prepare itself to receive and nurture the growing embryo ? Explain.[1]
    3. (c)(i)What happens when the egg is not fertilized ?[2]
    4. (c)(ii)OR : How does the developing embryo get nutrition from the mother's blood ? Explain.[2]
    Show answer

    Fertilisation occurs in the oviduct; the uterine lining thickens for implantation; an unfertilised egg leads to menstruation; the placenta nourishes the embryo.

    (a) Fertilisation takes place in the oviduct, also called the fallopian tube. (b) Every month the lining of the uterus becomes thick and spongy and is richly supplied with blood, so that a fertilised egg can be implanted in it and the growing embryo can be nourished and protected there. (c)(i) An unfertilised egg lives for only about one day. The thick, spongy lining that the uterus had prepared is then no longer needed, so it slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation; it happens roughly every month and usually lasts about two to eight days. (c)(ii) The embryo gets its nutrition from the mother's blood through the placenta, a special disc-shaped tissue embedded in the uterine wall. It bears villi on the embryo's side, and on the mother's side there are blood spaces that surround these villi. This gives a large surface area across which glucose and oxygen pass from the mother's blood to the embryo, while the waste substances the embryo generates are passed back into the mother's blood through the same placenta.
    Step-by-step solution →
  2. Q38 4 marks

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

    A person allowed a narrow beam of white light from the sun to enter a dark room through a small aperture and placed a glass prism in its path in such a manner that the beam falls on the face AB of the prism as shown in the figure. A screen S is placed on the other side of the prism, facing AC. On turning the prism slowly, a beautiful band of colours is obtained on the screen. It is the spectrum of sunlight.
    White light A B C S Screen
    White light falling on face AB of a glass prism, with screen S facing AC
    1. (a)Name the phenomenon due to which a prism splits the incident white light into a band of colours.[1]
    2. (b)State the reason of getting a band of seven colours in the above case.[1]
    3. (c)(i)Explain with the help of a labelled ray diagram, an experimental arrangement to show the recombination of the spectrum of white light.[2]
    4. (c)(ii)OR : Draw a labelled ray diagram to show the formation of a rainbow.[2]
    Show answer

    Dispersion; each colour has a different refractive index and bends differently; an inverted second prism recombines the colours; raindrops disperse sunlight into a rainbow.

    (a) Dispersion, that is, the dispersion of white light. (b) Sunlight is a mixture of seven colours, and the glass of the prism has a different refractive index for each of them, so each colour is deviated through a different angle - violet the most and red the least. The colours therefore emerge along different paths, become distinct and are seen as a band of seven colours (VIBGYOR). (c)(i) Newton's two-prism arrangement: a narrow beam of white light is dispersed by a first prism P1 held apex upwards, and a second identical prism P2 is placed close behind it in an inverted position so that all the colours of the spectrum pass through it. P2 deviates every colour by an equal angle in the opposite sense, so the deviations cancel colour by colour, the colours merge and a beam of white light emerges from P2 and is caught on the screen (see the labelled ray diagram). (c)(ii) A rainbow forms in tiny water droplets left hanging after a rain shower, with the Sun behind the observer. Sunlight entering a droplet is refracted and dispersed at the first surface, is reflected at the inside of the far surface, and is refracted again as it leaves the droplet. The colours emerge at slightly different angles - about for red and about for violet - so different droplets send different colours to the eye and the bow appears with red on the outside and violet on the inside (see the labelled ray diagram).
    Step-by-step solution →
  3. Q39 4 marks

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

    Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 – 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator. Answer the following questions :
    0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Scale
    pH Scale
    1. (a)Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q ?[1]
    2. (b)A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH.[1]
    3. (c)(i)When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration ? State the colour change shown by the pH paper.[2]
    4. (c)(ii)OR : A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach ? Write the common name of the medicines people use for remedy. Give the chemical name of “milk of magnesia” often used for this purpose.[2]
    Show answer

    P is near pH 0 and Q near pH 14; the hydronium ion concentration becomes 100 times greater, turning pH paper orange to red.

    (a) P, being a strong acid, is placed at the extreme left of the scale, close to 0 (about pH 0 to 2); Q, being a strong base, is placed at the extreme right, close to 14 (about pH 12 to 14). (b) (i) To increase the pH, add a base such as sodium hydroxide, ; (ii) to decrease the pH, add an acid such as hydrochloric acid, . (c)(i) The hydronium ion concentration rises from mol L to mol L, that is, it becomes times greater; the pH paper changes colour from orange (at pH 4) to red (at pH 2). (c)(ii) The fluid in the stomach is strongly acidic, of pH about 1.2 to 2. The medicines used for relief are called antacids, and the chemical name of milk of magnesia is magnesium hydroxide, .
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