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CBSE Class 10 Science 2025 — Paper 31/6/1

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

40 of its 45 questions also appear in 31/6/3 or 31/6/2; 5 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

    Also in 31/6/2 (Q3)

    In the following experimental setup of electrolysis of water, if P and Q are the gases collected in the test tubes enclosing the electrodes R and S, then select the option/options in which the matching is correct :
    1. (i)P - Oxygen gas , R - Anode
    2. (ii)P - Hydrogen gas , R - Cathode
    3. (iii)Q - Hydrogen gas , S - Cathode
    4. (iv)Q - Oxygen gas , S - Anode
    Battery P Q Test Tube Water R S
    1. (a)(i) and (ii)
    2. (b)(iii) and (iv)
    3. (c)(i) and (iii)
    4. (d)(ii) and (iv)
    Show answer
    (c) (i) and (iii)
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/6/3 (Q2)

    You have three aqueous solutions A, B and C as given below : A - Potassium nitrate B - Ammonium chloride C - Sodium carbonate The ascending order of the pH of these solutions is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (d)
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/6/2 (Q4)

    Select from the following a statement which is not true about burning of magnesium ribbon in air :
    1. (a)It burns with a dazzling white flame.
    2. (b)A white powder is formed on burning.
    3. (c)It is an endothermic reaction.
    4. (d)It is an example of a combination reaction.
    Show answer
    (c) It is an endothermic reaction.
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/6/2 (Q1), 31/6/3 (Q5)

    A Hydrocarbon which does not belong to the same homologous series of carbon compounds is
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  5. Q5 1 mark

    Also in 31/6/2 (Q7), 31/6/3 (Q4)

    The colour of the solution observed after about 1 hour of placing iron nails in copper sulphate solution is
    1. (a)Blue
    2. (b)Pale green
    3. (c)Yellow
    4. (d)Reddish brown
    Show answer
    (b) Pale green
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/6/3 (Q7)

    Juice of tamarind turns blue litmus to red. It is because of the presence of a chemical compound called
    1. (a)Acetic acid
    2. (b)Methanoic acid
    3. (c)Oxalic acid
    4. (d)Tartaric acid
    Show answer
    (d) Tartaric acid
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/6/2 (Q5), 31/6/3 (Q6)

    The water of crystallization is present in
    1. (i)Bleaching Powder
    2. (ii)Plaster of Paris
    3. (iii)Washing Soda
    4. (iv)Baking Soda
    1. (a)(ii) and (iv)
    2. (b)(ii) and (iii)
    3. (c)(i) and (iii)
    4. (d)(i) and (iv)
    Show answer
    (b) (ii) and (iii)
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/6/3 (Q9)

    A tall pea plant with round seeds (TTRR) is crossed with a short pea plant with wrinkled seeds (ttrr). The generation will be
    1. (a)25% tall with round seeds
    2. (b)50% tall with wrinkled seeds
    3. (c)75% tall with wrinkled seeds
    4. (d)100% tall with round seeds
    Show answer
    (d) 100% tall with round seeds
    Step-by-step solution →
  9. Q9 1 mark

    Also in 31/6/2 (Q11)

    A pair of endocrine glands located in the human brain is
    1. (a)Parathyroid and Pituitary
    2. (b)Pineal and Thymus
    3. (c)Hypothalamus and Thymus
    4. (d)Hypothalamus and Pineal
    Show answer
    (d) Hypothalamus and Pineal
    Step-by-step solution →
  10. Q10 1 mark
    Select the option having correct matching of the organism given in Column I with the mode of reproduction in Column II :
    1. (a)P-4, Q-2, R-1, S-3
    2. (b)P-3, Q-4, R-5, S-2
    3. (c)P-3, Q-4, R-1, S-2
    4. (d)P-4, Q-3, R-2, S-1
    Show answer
    (c) P-3, Q-4, R-1, S-2
    Step-by-step solution →
  11. Q11 1 mark

    Also in 31/6/2 (Q9), 31/6/3 (Q10)

    The basic filtration unit of the excretory system in human beings is :
    1. (a)Nephron
    2. (b)Urethra
    3. (c)Neuron
    4. (d)Urinary bladder
    Show answer
    (a) Nephron
    Step-by-step solution →
  12. Q12 1 mark

    Also in 31/6/2 (Q13), 31/6/3 (Q12)

    In human alimentary canal, the digestive juice secreted by the gastric glands are
    1. (a)Bile, Trypsin, Pepsin
    2. (b)Hydrochloric acid, Pepsin, Mucus
    3. (c)Lipase, Bile, Mucus
    4. (d)Salivary amylase, Pepsin, Bile
    Show answer
    (b) Hydrochloric acid, Pepsin, Mucus
    Step-by-step solution →
  13. Q13 1 mark

    Also in 31/6/2 (Q12), 31/6/3 (Q14)

    Identify from the following the ray diagram which shows the correct path of the reflected ray for the ray incident on a concave mirror as shown :
    C F P C F P (A) C F P (B) C F P (C) C F P (D)
    1. (a)The reflected ray crosses the principal axis between F and the pole P, and runs on below the axis.
    2. (b)The reflected ray crosses the principal axis at the centre of curvature C.
    3. (c)The reflected ray crosses the principal axis at the principal focus F.
    4. (d)The reflected ray travels back to the left above the principal axis without ever meeting it.
    Show answer
    (c) The reflected ray crosses the principal axis at the principal focus F.
    Step-by-step solution →
  14. Q14 1 mark
    The part of human eye which controls the amount of light entering into it.
    1. (a)Iris
    2. (b)Cornea
    3. (c)Ciliary muscles
    4. (d)Pupil
    Show answer
    (a) Iris
    Step-by-step solution →
  15. Q15 1 mark

    Also in 31/6/3 (Q16)

    Consider the following food chain : Grass → Grasshopper → Frog → Snake → Eagle If the amount of energy available at third trophic level is 50 kJ, the available energy at the producer level was :
    1. (a)0.5 kJ
    2. (b)5 kJ
    3. (c)500 kJ
    4. (d)5000 kJ
    Show answer
    (d) 5000 kJ
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/6/2 (Q15)

    The incorrect statement about ozone is
    1. (a)It is a deadly poisonous gas.
    2. (b)It shields the surface of the earth from UV radiation from sun.
    3. (c)It is used as a refrigerant and in fire-extinguishers.
    4. (d)It is formed by combining oxygen molecule with free oxygen atom.
    Show answer
    (c) It is used as a refrigerant and in fire-extinguishers.
    Step-by-step solution →
  17. Q17 1 mark

    Also in 31/6/2 (Q19), 31/6/3 (Q18)

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

    Assertion (A): Carbon and its compounds are our major sources of fuels.

    Reason (R): Most of the carbon compounds on burning release a large amount of heat and light.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation of the 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 the Assertion (A).
    Step-by-step solution →
  18. Q18 1 mark

    Also in 31/6/2 (Q17), 31/6/3 (Q17)

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

    Assertion (A): Xylem tissue moves water and minerals obtained from the soil by the roots.

    Reason (R): Xylem tissue is found only in the roots of a plant.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (c) Assertion (A) is true, but Reason (R) is false.
    Step-by-step solution →
  19. Q19 1 mark

    Also in 31/6/2 (Q20), 31/6/3 (Q20)

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

    Assertion (A): In the common domestic circuits the earth wire is connected to a metallic plate buried deep inside the earth.

    Reason (R): Earth wire ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, so the user may not get a severe electric shock.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation of the 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 the Assertion (A).
    Step-by-step solution →
  20. Q20 1 mark

    Also in 31/6/2 (Q18), 31/6/3 (Q19)

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

    Assertion (A): Food web is a network of several food chains operating in an ecosystem.

    Reason (R): Food web decreases the stability of an ecosystem.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation of the Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (c) Assertion (A) is true, but Reason (R) is false.
    Step-by-step solution →

Questions 21–26 · 2 marks each

  1. Q21 2 marks
    A light green coloured solution of sulphate salt of metal 'P' is taken in a beaker, a rod of another metal 'Q' is put in this solution as shown the following figures : Identify the metals 'P' and 'Q' and write its chemical equation for the reaction that occurs. State the conclusion of this reaction in terms of reactivity series of metals.
    Q PSO4 (Light Green Solution) Q QSO4 (Colourless Solution) After about 1 hour
    Show answer

    P is iron and Q is zinc; zinc displaces iron from its salt solution, so zinc is more reactive than iron.

    P is iron (Fe), whose sulphate gives the light green solution, and Q is zinc (Zn), whose sulphate is colourless. The reaction is . Conclusion: zinc displaces iron from its salt solution, so zinc is more reactive than iron and lies above it in the reactivity series — a more reactive metal displaces a less reactive one from its salt solution.
    Step-by-step solution →
  2. Q22 2 marks

    Also in 31/6/2 (Q23), 31/6/3 (Q25)

    1. (a)How is brain protected in our body ?
    2. (b)A doctor finds in one of his patients that he is not maintaining a proper posture and balance of his body. State the region of brain and also the part of brain which is responsible for it.
    Show answer

    A bony cranium, three meninges and cerebrospinal fluid protect the brain; the cerebellum of the hind-brain maintains posture and balance.

    (a) The brain is protected by a bony box, the cranium of the skull; inside it the brain is covered by three membranes, the meninges, and it is contained in a fluid filled balloon of cerebrospinal fluid, which cushions it against shocks. (b) Region of the brain: the hind-brain. Part responsible: the cerebellum, which maintains posture and balance of the body.
    Step-by-step solution →
  3. Q23(a) 2 marks

    Also in 31/6/2 (Q22(a)), 31/6/3 (Q24(a))

    “Proteins control the expression of various characters.” Explain this statement by taking an example of “tallness” as a characteristic in plants.
    Show answer

    Tallness depends on a hormone made by an enzyme (a protein) coded by a gene; an altered gene gives a short plant.

    A gene carries the information for making a protein, so a character can only be expressed through a protein. The height of a plant depends on the amount of plant growth hormone it makes, and that hormone is made by an enzyme, which is a protein. If the gene works efficiently, plenty of the active enzyme is made, plenty of hormone is produced and the plant is tall. If that gene is altered so that the enzyme is less efficient, less hormone is made and the plant remains short. Tallness is therefore controlled by the protein the gene codes for, which is what is meant by saying that proteins control the expression of characters.
    Step-by-step solution →
    OR
    Q23(b) 2 marks

    Also in 31/6/2 (Q22(b)), 31/6/3 (Q24(b))

    Explain the mechanism of inheritance used by sexually reproducing organisms to ensure the stability of DNA of the species.
    Show answer

    Gametes carry half the chromosomes and fertilisation restores the full number, so the species' DNA stays stable from generation to generation.

    In sexually reproducing organisms the germ cells, or gametes, are made by a special cell division that halves the chromosome number, so a gamete carries only one set of chromosomes, , while a body cell carries two, . At fertilisation a male gamete fuses with a female gamete, so the zygote receives one set from each parent and the full number is restored. In a human, . The halving and the doubling cancel exactly, so the amount of DNA and the number of chromosomes of the species stay stable from generation to generation, even though each offspring gets a fresh combination of genes.
    Step-by-step solution →
  4. Q24 2 marks

    Also in 31/6/2 (Q25), 31/6/3 (Q22)

    Study the figure in which the path of a ray of light going from Medium 1 to Medium 2 is shown.
    1. (a)Out of the two Media – Medium 1 and Medium 2, in which is the speed of light more ?
    2. (b)State reason of bending of the refracted ray away from the normal.
    3. (c)Express refractive index of Medium 2 with respect to Medium 1 in terms of speed of light in two media.
    Incident Ray Medium 1 Medium 2 Refracted Ray
    Show answer

    Light is faster in Medium 2; the ray bends away from the normal in the rarer medium; .

    (a) The speed of light is more in Medium 2. (b) The ray bends away from the normal because it passes from an optically denser medium into an optically rarer medium, in which light travels faster; the change of speed at the boundary changes the direction of the ray, so the angle of refraction is greater than the angle of incidence. (c) , where and are the speeds of light in Medium 1 and Medium 2 respectively.
    Step-by-step solution →
  5. Q25(a) 2 marks

    Also in 31/6/2 (Q24(a)), 31/6/3 (Q23(a))

    Give reasons :
    1. (i)The sky appears dark to passengers flying at very high altitude.
    2. (ii)‘Danger’ signal lights are red in colour.
    Show answer

    (i) Thin air at high altitude scatters little light, so the sky looks dark; (ii) red is scattered the least, so it is seen farthest.

    (i) At a very high altitude the atmosphere is extremely thin, so there are very few air molecules and dust particles to scatter sunlight. Hardly any scattered light reaches the passengers' eyes, so the sky appears dark to them instead of blue. (ii) Red light has the longest wavelength of the visible colours, and scattering falls off steeply as the wavelength grows, so red is scattered the least by fog, smoke and dust. It therefore travels the farthest and is visible from the greatest distance, which is what a danger signal needs.
    Step-by-step solution →
    OR
    Q25(b) 2 marks

    Also in 31/6/2 (Q24(b)), 31/6/3 (Q23(b))

    What is a rainbow ? “We see a rainbow in the sky only after the rainfall.” Why ?
    Show answer

    A rainbow is a spectrum formed by dispersion of sunlight in water droplets, so it is seen only after rain.

    A rainbow is a natural spectrum, an arc of seven colours, that appears in the sky after a rain shower. It is caused by the dispersion of sunlight by the tiny water droplets suspended in the atmosphere: each droplet acts as a small prism, so sunlight entering it is refracted and dispersed, then reflected internally at the inner surface at the back of the drop, and refracted again as it leaves. The colours emerge separated, red at about and violet at about to the direction of the sunlight, and the rainbow is always seen opposite the Sun. We see it only after rainfall because only then are tiny water droplets suspended in the air to act as those prisms; without the droplets there is nothing to disperse the sunlight.
    Step-by-step solution →
  6. Q26 2 marks
    We do not clean natural ponds or lakes whereas an aquarium or a swimming pool needs to be cleaned regularly. Why ?
    Show answer

    A pond has decomposers and cleans itself; an aquarium is an artificial, incomplete ecosystem, so wastes collect and it needs cleaning.

    A pond or a lake is a natural, complete and self-sustaining ecosystem. Along with its producers and consumers it contains decomposers, the bacteria and fungi, which break down the dead remains and the wastes of the organisms in it and return the nutrients to the water for the producers to use again. Nothing accumulates, so the pond cleans itself. An aquarium or a swimming pool is an artificial and incomplete ecosystem with few or no decomposers and no natural cycling of materials, so the wastes and dead matter simply collect in it and have to be removed by us. That is why they must be cleaned regularly.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks

    Also in 31/6/2 (Q30)

    Write balanced chemical equation for the reactions that occur when
    1. (a)steam is passed over red hot iron.
    2. (b)natural gas is burnt in air.
    3. (c)glucose reacts with oxygen in the cells of our body and provides energy.
    Show answer

    Balanced equations: iron and steam give and hydrogen, methane burns to and water, and glucose gives , water and energy.

    (a) ; (b) with heat and light; (c) with release of energy.
    Step-by-step solution →
  2. Q28(a) 3 marks

    Also in 31/6/3 (Q28(a))

    State the chemical property in each case on which the following uses of baking soda are based upon :
    1. (i)as an anti-acids
    2. (ii)as a constituent in making baking powder
    3. (iii)in soda-acid fire-extinguishers
    Show answer

    Baking soda is a mild base that neutralises stomach acid, and reacts with acid to give carbon dioxide, which raises dough and smothers fires.

    (i) It is a mild base, so it neutralises the excess hydrochloric acid in the stomach: . (ii) It reacts with the mild edible acid of the baking powder, and also decomposes on heating, to liberate carbon dioxide, whose bubbles make the dough rise. (iii) With sulphuric acid it liberates a large volume of carbon dioxide at once, and this gas, being heavier than air and non-supporter of combustion, smothers the fire.
    Step-by-step solution →
    OR
    Q28(b) 3 marks

    Also in 31/6/3 (Q28(b))

    Write chemical equations to show what happens when an acid reacts with a
    1. (i)metal
    2. (ii)base and
    3. (iii)carbonate Write the name of the main product formed in each case.
    Show answer

    An acid with a metal, base or carbonate forms a salt: zinc sulphate or sodium chloride, with hydrogen, water, or water and carbon dioxide.

    (i) ; main product zinc sulphate, a salt, together with hydrogen gas. (ii) ; main product sodium chloride, a salt, together with water. (iii) ; main product sodium chloride, a salt, together with water and carbon dioxide.
    Step-by-step solution →
  3. Q29 3 marks

    Also in 31/6/3 (Q31)

    Name the blood vessel that brings
    1. (i)oxygenated blood
    2. (ii)deoxygenated blood, to the human heart. Also name that chamber of the heart which receives deoxygenated blood and state how deoxygenated blood from this chamber is sent to lungs for oxygenation.
    Show answer

    Pulmonary vein brings oxygenated blood; vena cava brings deoxygenated blood to the right atrium, then right ventricle, pulmonary artery and lungs.

    (i) The pulmonary vein brings oxygenated blood to the heart, from the lungs into the left atrium. (ii) The vena cava, that is the superior and inferior venae cavae, brings deoxygenated blood from the body to the heart. The chamber that receives this deoxygenated blood is the right atrium (right auricle). From it the blood passes through the tricuspid valve into the right ventricle, and the right ventricle contracts and pumps the blood through the pulmonary artery to the lungs, where it is oxygenated.
    Step-by-step solution →
  4. Q30 3 marks

    Also in 31/6/2 (Q27)

    The gene combination of purple flowered pea plants is denoted as (WW) and that of white flowered pea plants as (ww), when these two plants are crossed generation is obtained.
    1. (a)List two observations made by Mendel in generation plants.
    2. (b)Give the (i) percentage of white flowered plants and (ii) ratio of the gene combinations WW, Ww and ww in generation. (c) Write one difference between dominant and recessive trait.
    Show answer

    All plants were purple with no blending; 25% of plants are white, and .

    (a) All the F1 plants bore purple flowers, so only one of the two parental traits was expressed and not one white flowered plant appeared; and no plant of an intermediate colour was seen, so the two traits had not blended. The white character was not lost but only hidden, as its reappearance in F2 shows: purple is dominant and white recessive, and the F1 plants are Ww although they look like the purple parent. (b) (i) White flowered plants in F2 . (ii) . (c) A dominant trait shows itself when even one copy of its allele is present, as in Ww, whereas a recessive trait shows only when both alleles are recessive, as in ww.
    Step-by-step solution →
  5. Q31 3 marks

    Also in 31/6/2 (Q32), 31/6/3 (Q29)

    A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below : Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :
    1. (a)What is the focal length of the convex lens used ? Give reason to justify your answer.
    2. (b)Which one of the sets of observations is not correct and why ?
    3. (c)Draw ray diagram to show image formation for any correct set of observation.
    Observations of the candle flame and its image for a convex lens
    S.No.Distance of flame from the lens (cm)Distance of the image from the lens
    1
    2
    3
    4
    5
    6
    7
    8
    Show answer

    cm, as object and image are both at 30 cm (); set 8 is wrong because the flame is inside the focal length.

    (a) cm. In set 4 the flame and its image are both 30 cm from the lens; for a convex lens the two distances are equal only when the object is at 2F, so cm and cm. (b) Set 8 is not correct. The flame there is 12 cm from the lens, which is less than the focal length of 15 cm, so it lies between the optical centre and the focus. A convex lens then gives a virtual, erect and magnified image on the same side as the flame, which cannot be caught on a screen, so an image distance of cm is impossible. (c) For set 4 the ray diagram shows the object at 2F and a real, inverted image of the same size at 2F on the other side, with cm and .
    Step-by-step solution →
  6. Q32 3 marks
    A person uses lenses of D power in his spectacles for the correction of his vision.
    1. (a)Name the defect of vision the person is suffering from.
    2. (b)List two causes of this defect.
    3. (c)Determine the focal length of the lenses used in the spectacles.
    Show answer

    The person has hypermetropia, since D is a convex lens; cm.

    (a) Hypermetropia, that is far-sightedness or long-sightedness, since a positive power means a convex, converging lens. (b) Two causes: the converging power of the eye lens has become too small, that is its focal length is too long; and the eyeball has become too short, so the eye lens sits too close to the retina. In either case the image of a near object falls behind the retina. (c) m cm.
    Step-by-step solution →
  7. Q33 3 marks

    Also in 31/6/2 (Q31), 31/6/3 (Q33)

    1. (a)Explain the statement "Potential difference between two points is 1 volt".
    2. (b)What do the symbols given below represent in an electric circuit ? Write one function of each.
    A + − (i) (ii)
    Show answer

    1 volt means 1 joule of work per coulomb; the symbols are an ammeter (measures current) and a rheostat (varies current).

    (a) The potential difference between two points is 1 volt when 1 joule of work is done in moving a charge of 1 coulomb from one point to the other; that is, and . (b) (i) An ammeter. It is connected in series in the circuit and measures the electric current flowing through it, its plus terminal being joined towards the positive terminal of the cell. (ii) A variable resistance, that is a rheostat. It is connected in series and is used to change the resistance of the circuit and so to vary the current flowing in it.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks

    Also in 31/6/2 (Q35(a)), 31/6/3 (Q36(a))

    Name an alcohol and a carboxylic acid having two carbon atoms in their structures. Draw their structures and state how this alcohol can be converted into a carboxylic acid. What happens when these two compounds react in the presence of an acid ? Write chemical equations for the reactions involved in the two cases mentioned above.
    Show answer

    Ethanol, , is oxidised to ethanoic acid, ; the two react with an acid catalyst to form the ester ethyl ethanoate and water.

    The alcohol is ethanol, , and the carboxylic acid is ethanoic acid, , in which the second carbon is doubly bonded to one oxygen and singly bonded to an group. Ethanol is converted into ethanoic acid by oxidation, on warming with alkaline potassium permanganate (or acidified potassium dichromate): . When the two compounds are warmed together in the presence of a little concentrated sulphuric acid they undergo esterification, forming ethyl ethanoate, an ester with a sweet fruity smell, and water: .
    Step-by-step solution →
    OR
    Q34(b) 5 marks

    Also in 31/6/2 (Q35(b)), 31/6/3 (Q36(b))

    What are soaps ? Write the structure of a soap molecule. Explain the cleansing action of a soap. Why are soaps not considered suitable for washing clothes in a region where water is hard ? How is this problem overcomed ?
    Show answer

    Soaps are salts of long-chain carboxylic acids that clean by forming micelles; they form scum in hard water, so detergents are used.

    Soaps are the sodium or potassium salts of long-chain carboxylic acids, made by heating a fat or an oil with alkali. A soap molecule, for example sodium stearate , has a long hydrocarbon tail that is hydrophobic and dissolves in oil, and an ionic head that is hydrophilic and dissolves in water. In cleansing, the tails of many molecules enter the oily dirt while the heads remain in the water, so the molecules form a micelle with the dirt trapped inside; agitation frees the micelles from the cloth, they repel one another and remain suspended, and the dirt is rinsed away with the water. Soaps are unsuitable in hard water because hard water contains calcium and magnesium ions, and the calcium and magnesium salts of fatty acids are insoluble: they separate as a white curdy scum, so soap is wasted before any lather forms and the scum sticks to the clothes, as in . The problem is overcome by using detergents, the ammonium or sulphonate salts of long-chain hydrocarbons, whose calcium and magnesium salts are soluble in water, so they lather well even in hard water; the water may also be softened first.
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  2. Q35(a) 5 marks

    Also in 31/6/2 (Q34(a))

    1. (a)Define Puberty. List any two changes seen in boys at the time of puberty.
    2. (b)Why are testes in human males located outside the abdominal cavity in scrotum ?
    3. (c)List any three techniques of contraception used by humans. Which one of these is not meant for males ?
    Show answer

    Puberty is when the reproductive organs mature; the scrotum keeps the testes cooler for sperm formation; oral pills are not meant for males.

    (a) Puberty is the period during adolescence at which the reproductive organs mature and become functional, so that the body becomes capable of sexual reproduction. Two changes seen in boys are the growth of thick hair on the face as a beard and moustache, and in the armpits and pubic region; and the cracking and deepening of the voice as the larynx enlarges. (b) The formation of sperms requires a temperature about two to three degrees Celsius below the normal body temperature. The scrotum, lying outside the abdominal cavity, is cooler than the interior of the body and so provides that lower temperature. (c) Three techniques are the barrier method, such as the condom; the chemical or hormonal method, that is oral contraceptive pills; and the surgical method, that is vasectomy in males or tubectomy in females. Of these, the oral contraceptive pill is not meant for males, since it is taken only by females.
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    OR
    Q35(b) 5 marks

    Also in 31/6/2 (Q34(b))

    (a) Name the part performing following functions in human female reproductive system :
    1. (i)production of eggs
    2. (ii)site of fertilization
    3. (iii)site of implantation
    4. (iv)entry of the sperms (b) What changes are observed in the uterus : (i) subsequent to implantation of zygote and (ii) if an egg does not get fertilized ?
    Show answer

    The parts are the ovary, fallopian tube, uterus and vagina; the lining thickens after implantation or is shed as menstruation if no fertilisation occurs.

    (a) (i) Ovary; (ii) Fallopian tube, that is the oviduct; (iii) Uterus, in its lining; (iv) Vagina. (b) (i) After the zygote implants, the lining of the uterus becomes still thicker and richer in blood vessels, and a placenta develops between the embryo and the uterine wall, with villi on the embryo's side and blood spaces on the mother's side, through which glucose and oxygen pass to the embryo and its wastes pass back to the mother; the uterus enlarges as the embryo grows. (ii) If the egg is not fertilised, the thick lining is no longer needed and breaks down, and it is shed through the vagina as blood and mucus. This is menstruation, lasting about two to five days and recurring about every twenty-eight days.
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  3. Q36(a) 5 marks

    Also in 31/6/3 (Q34(a))

    What are magnetic field lines ? How is the direction of magnetic field at a point determined ? Draw the pattern of magnetic field lines of the magnetic field produced by a current carrying circular loop. Mark on it the direction of
    1. (i)current and
    2. (ii)magnetic field lines. Name the two factors on which the magnitude of the magnetic field due to a current carrying coil depends.
    Show answer

    Field lines are paths of a free north pole; a compass shows their direction; a coil's field depends on its current and turns.

    Magnetic field lines are the paths along which a free north magnetic pole would move in a magnetic field; equivalently, the tangent to a field line at any point gives the direction of the magnetic field at that point. They are closed continuous curves, run from north to south outside a magnet, never cross one another, and are crowded where the field is strong. The direction of the field at a point is found by placing a small compass needle there: the direction in which its north end points is the direction of the field; for a current-carrying conductor the right-hand thumb rule gives the same direction, the thumb along the current and the curled fingers along the field lines. For a circular loop the field lines are closed curves encircling the wire, growing larger with distance from it, and at the centre of the loop they appear as a straight line along the axis, perpendicular to the plane of the loop. On the diagram the current is marked by a dot for current out of the page and a cross for current into the page, and the arrowheads on the field lines run up through the middle of the loop, outwards beyond it and round the wire. The magnitude of the magnetic field of a current-carrying coil depends on (1) the strength of the current through it, , and (2) the number of turns of the coil, ; it is also inversely proportional to the radius of the coil.
    Step-by-step solution →
    OR
    Q36(b) 5 marks

    Also in 31/6/3 (Q34(b))

    Why can't two magnetic field lines cross each other ? Draw magnetic field lines showing the direction of the magnetic field due to a current carrying long straight solenoid. State the conclusion which can be drawn from the pattern of magnetic field lines inside the solenoid. Name any two factors on which the magnitude of the magnetic field due to this solenoid depends.
    Show answer

    Field lines cannot cross because the field has one direction at a point; inside a solenoid it is uniform.

    Two magnetic field lines cannot cross because, if they did, two tangents could be drawn at the point of crossing, so the magnetic field there would have two directions at the same time. A compass needle placed at that point can come to rest in one direction only, so the field at a point has a single direction and no crossing is possible. For a long straight current-carrying solenoid the field lines are straight, parallel and equally spaced inside the coil, running along its axis; at the ends they emerge, spread out, curve round outside and re-enter at the other end, each line being a closed curve. The end from which the lines emerge is the north pole and the end at which they enter is the south pole, so outside the solenoid the lines run from north to south, just as for a bar magnet. The conclusion from the pattern inside is that the magnetic field inside a current-carrying solenoid is uniform, that is, it has the same magnitude and the same direction at every point there. The magnitude of the field depends on (1) the strength of the current through the solenoid and (2) the number of turns per unit length of the solenoid, and also on the nature of the core, a soft iron core increasing it greatly.
    Step-by-step solution →

Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/6/2 (Q37), 31/6/3 (Q37)

    Many pure metals like copper, iron and gold are very soft and as such are considered unsuitable for certain uses. Metallic objects around us such as cooking utensils, statues, ornaments, guns etc. are actually not made up of pure metals. Instead of pure metals, alloys are used in the design of most of the useful objects. Making alloys enhances the basic properties of a metal which is the primary constituent (metal) of an alloy.
    1. (I)How does electrical conductivity and melting point of a metal change when it is converted to its alloy by mixing a small amount of an element in it ?[1]
    2. (II)Name an alloy used for welding two wires together in an electric circuit. Write its major constituents.[1]
    3. (III)(a)What are alloys ? How is ‘Brass’ (an alloy) prepared ?[2]
    4. (III)(b)OR : What is stainless steel ? How is it prepared ? Write one important property which makes it more useful in making cooking utensils as compared to its primary metal.[2]
    Show answer

    Alloying lowers conductivity and melting point; solder is lead and tin; brass is copper and zinc; stainless steel does not rust.

    (I) Both decrease: the electrical conductivity of the alloy is less than that of the pure metal, and so is its melting point. (II) Solder; its major constituents are lead (Pb) and tin (Sn), about half and half by mass. (III)(a) An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. Brass is made by melting copper and dissolving the required amount of zinc in the molten copper (about Cu and Zn), then cooling it. (III)(b) Stainless steel is an alloy of iron with chromium and nickel. It is made by melting iron and mixing in about chromium and nickel, then cooling. It does not rust or corrode, unlike pure iron, which is what makes it better for cooking utensils.
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  2. Q38 4 marks

    Also in 31/6/2 (Q38), 31/6/3 (Q38)

    The growth movements of plant parts in which the direction of the stimulus determines the direction of the response is known as tropic movements or tropism. Plants also have non-directional movements which may not be growth dependent.
    Potted plant lying on its side with roots X and shoot Y X Y
    1. (I)Name the movement which causes ‘X’ and ‘Y’ to grow downwards and upwards respectively. (Refer above figure)[1]
    2. (II)Write the name of a hormone that plays a major role in (i) falling of leaves (ii) rapid cell division[1]
    3. (III)(a)Leaves of the sensitive plant move very quickly in response to ‘touch’. How is this stimulus of touch communicated and explain how the movement takes place.[2]
    4. (III)(b)OR : Name the plant hormone which is synthesized at the shoot tip. How does this hormone helps the plant to bend towards light ?[2]
    Show answer

    Geotropism; abscisic acid and cytokinin; touch-sensitive leaves fold by losing water; auxin bends shoots towards light.

    (I) Geotropism (gravitropism): the root X shows positive geotropism by growing towards gravity and the shoot Y shows negative geotropism by growing away from it. (II) (i) Abscisic acid; (ii) Cytokinin. (III)(a) Plants have no nerves, so the touch is passed from cell to cell as an electrical and chemical signal. The cells at the base of the leaflets then lose water, shrink and lose their swollen shape, so the leaflets fold and droop; when water returns they swell again and the leaflets reopen. The movement is not caused by growth. (III)(b) Auxin. Made at the shoot tip, it diffuses towards the shaded side when light comes from one side. The cells on that shaded side then grow longer than those on the lit side, and this unequal growth bends the shoot towards the light.
    Step-by-step solution →
  3. Q39 4 marks

    Also in 31/6/2 (Q39), 31/6/3 (Q39)

    Study the circuit shown in which two resistors X and Y of resistances and respectively are joined in series with a battery of V.
    Series circuit printed with question 39 X 3 Ω 6 Ω Y V − + A − + − + 2V
    1. (I)Draw a circuit diagram showing the above two resistors X and Y joined in parallel with same battery and same ammeter and voltmeter.[1]
    2. (II)In which combination of resistors will the (i) potential difference across X and Y and (ii) current through X and Y, be the same ?[1]
    3. (III)(a)Find the current drawn from the battery by the series combination of the two resistors (X and Y).[2]
    4. (III)(b)OR : Determine the equivalent resistance of the parallel combination of the two resistors (X and Y).[2]
    Show answer

    Parallel gives equal potential difference and series gives equal current; the series current is A, or the parallel resistance is .

    (I) X and Y are drawn as two separate branches between the same pair of junctions, with the V battery and the plug key in the main circuit, the ammeter still in the main line and the voltmeter connected across the parallel pair (see the circuit diagram). (II) (i) The potential difference across X and Y is the same in the parallel combination; (ii) the current through X and Y is the same in the series combination. (III)(a) , so the current drawn is A. (III)(b) , so .
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