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CBSE Class 10 Science 2024 — Paper 31/2/2

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

29 of its 45 questions also appear in 31/2/1, 31/2/3 and other papers; 16 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

    Also in 31/2/1 (Q7), 31/2/3 (Q6)

    Consider the following statements about homologous series of carbon compounds :
    1. (a)All succeeding members differ by unit.
    2. (b)Melting point and boiling point increases with increasing molecular mass.
    3. (c)The difference in molecular masses between two successive members is 16 u.
    4. (d) and are NOT the successive members of alkyne series. The correct statements are –
    1. (a)(a) and (b)
    2. (b)(b) and (c)
    3. (c)(a) and (c)
    4. (d)(c) and (d)
    Show answer
    (a) (a) and (b)
    Step-by-step solution →
  2. Q2 1 mark
    The number of shells required to write the electronic configuration of Potassium (At. No. 19)
    1. (a)1
    2. (b)2
    3. (c)3
    4. (d)4
    Show answer
    (d) 4
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/2/1 (Q3)

    Select from the following a process in which a combination reaction is involved :
    1. (a)Black and White photography
    2. (b)Burning of coal
    3. (c)Burning of methane
    4. (d)Digestion of food
    Show answer
    (b) Burning of coal
    Step-by-step solution →
  4. Q4 1 mark

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

    The oxide which can react with as well as to give corresponding salt and water is
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  5. Q5 1 mark
    Which of the following is an alloy of copper and tin ?
    1. (a)Nichrome
    2. (b)Brass
    3. (c)Constantan
    4. (d)Bronze
    Show answer
    (d) Bronze
    Step-by-step solution →
  6. Q6 1 mark
    Tooth decay begins at the pH of :
    1. (a)5.1
    2. (b)5.8
    3. (c)6.5
    4. (d)8.0
    Show answer
    (a) 5.1
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/2/1 (Q1)

    Solid Calcium oxide reacts vigorously with water to form Calcium hydroxide accompanied by the liberation of heat. From the information given above it may be concluded that this reaction
    1. (a)is endothermic and pH of the solution formed is more than 7.
    2. (b)is exothermic and pH of the solution formed is 7.
    3. (c)is endothermic and pH of the solution formed is 7.
    4. (d)is exothermic and pH of the solution formed is more than 7.
    Show answer
    (d) is exothermic and pH of the solution formed is more than 7.
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/2/1 (Q10), 31/2/3 (Q10)

    In human respiratory system, when a person breathes in, the position of ribs and diaphragm will be :
    1. (a)lifted ribs and curve/dome shaped diaphragm.
    2. (b)lifted ribs and flattened diaphragm.
    3. (c)relaxed ribs and flattened diaphragm.
    4. (d)relaxed ribs and curve/dome shaped diaphragm.
    Show answer
    (b) lifted ribs and flattened diaphragm.
    Step-by-step solution →
  9. Q9 1 mark

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

    Select out of the following a gland which does NOT occur as a pair in the human body :
    1. (a)Pituitary
    2. (b)Ovary
    3. (c)Testis
    4. (d)Adrenal
    Show answer
    (a) Pituitary
    Step-by-step solution →
  10. Q10 1 mark

    Also in 31/2/1 (Q8), 31/2/3 (Q11)

    Which of the following statement(s) is (are) true about human heart ?
    1. (a)Right atrium receives oxygenated blood from lungs through pulmonary artery.
    2. (b)Left atrium transfers oxygenated blood to left ventricle which sends it to various parts of the body.
    3. (c)Right atrium receives deoxygenated blood through vena cava from upper and lower body.
    4. (d)Left atrium transfers oxygenated blood to aorta which sends it to different parts of the body.
    1. (a)(a)
    2. (b)(a) and (d)
    3. (c)(b) and (c)
    4. (d)(b) and (d)
    Show answer
    (c) (b) and (c)
    Step-by-step solution →
  11. Q11 1 mark
    Which one of the following organism is represented by this diagram ?
    Spores
    1. (a)Spirogyra
    2. (b)Planaria
    3. (c)Yeast
    4. (d)Rhizopus
    Show answer
    (d) Rhizopus
    Step-by-step solution →
  12. Q12 1 mark

    Also in 2025 · 31(B)/S (Q12), 31/2/1 (Q12), 31/2/3 (Q12)

    A cross made between two pea plants produces 50% tall and 50% short pea plants. The gene combination of the parental pea plants must be
    1. (a)Tt and Tt
    2. (b)TT and Tt
    3. (c)Tt and tt
    4. (d)TT and tt
    Show answer
    (c) Tt and tt
    Step-by-step solution →
  13. Q13 1 mark

    Also in 31/2/1 (Q16)

    Strength of magnetic field produced by a current carrying solenoid DOES NOT depend upon :
    1. (a)number of turns in the solenoid
    2. (b)direction of the current flowing through it
    3. (c)radius of solenoid
    4. (d)material of core of the solenoid
    Show answer
    (b) direction of the current flowing through it
    Step-by-step solution →
  14. Q14 1 mark
    S.I. unit of electrical resistivity is
    1. (a)ohm per metre
    2. (b)ohm per metre
    3. (c)ohm . metre
    4. (d)ohm . metre
    Show answer
    (c) ohm . metre
    Step-by-step solution →
  15. Q15 1 mark
    The minimum resistance which can be made using five resistors each of resistance is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/2/1 (Q13)

    Consider the following statements in the context of human eye :
    1. (a)The diameter of the eye ball is about 2.3 cm.
    2. (b)Iris is a dark muscular diaphragm that controls the size of the pupil.
    3. (c)Most of the refraction for the light rays entering the eye occurs at the crystalline lens.
    4. (d)While focusing on the objects at different distances the distance between the crystalline lens and the retina is adjusted by ciliary muscles. The correct statements are –
    1. (a)(a) and (b)
    2. (b)(a), (b) and (c)
    3. (c)(b), (c) and (d)
    4. (d)(a), (c) and (d)
    Show answer
    (a) (a) and (b)
    Step-by-step solution →
  17. Q17 1 mark

    Also in 31/2/1 (Q20), 31/2/3 (Q18)

    These questions consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :

    Assertion (A): The deflection of a compass needle placed near a current carrying wire decreases when the magnitude of an electric current in the wire is increased.

    Reason (R): Strength of the magnetic field at a point due to a current carrying conductor increases on increasing the current in the conductor.

    1. (a)Both (A) and (R) are true and (R) is the correct explanation of (A).
    2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of (A).
    3. (c)(A) is true, but (R) is false.
    4. (d)(A) is false, but (R) is true.
    Show answer
    (d) (A) is false, but (R) is true.
    Step-by-step solution →
  18. Q18 1 mark

    Also in 31/2/1 (Q18), 31/2/3 (Q20)

    These questions consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :

    Assertion (A): Human female has a perfect pair of sex chromosome.

    Reason (R): Sex chromosome contributed by the human male in the zygote decides the sex of a child.

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

    Also in 31/2/1 (Q19)

    These questions consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :

    Assertion (A): Myopic eye cannot see distant objects distinctly.

    Reason (R): For the correction of myopia converging lenses of appropriate power are prescribed by eye-surgeons.

    1. (a)Both (A) and (R) are true and (R) is the correct explanation of (A).
    2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of (A).
    3. (c)(A) is true, but (R) is false.
    4. (d)(A) is false, but (R) is true.
    Show answer
    (c) (A) is true, but (R) is false.
    Step-by-step solution →
  20. Q20 1 mark
    Q. Nos. 17 to 20 are Assertion – Reason based questions : These questions consist of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below :

    Assertion (A): Metals in the middle of activity series are found in nature as sulphides or carbonates.

    Reason (R): The sulphide ores are calcinated whereas carbonate ores are roasted to extract metals from them.

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

Questions 21–26 · 2 marks each

  1. Q21(a) 2 marks
    Define a decomposition reaction. Write an equation to show thermal decomposition of ferrous sulphate crystals.
    Show answer

    A decomposition reaction gives two or more simpler products from one reactant; heated ferrous sulphate gives , and .

    A decomposition reaction is a reaction in which a single reactant breaks down to give two or more simpler products. Thermal decomposition of ferrous sulphate: , that is, ferrous sulphate gives ferric oxide, sulphur dioxide and sulphur trioxide.
    Step-by-step solution →
    OR
    Q21(b) 2 marks
    What is meant by a balanced chemical equation ? Why is it necessary for the equation to be balanced ?
    Show answer

    A balanced equation has equal atoms of each element on both sides, as mass is conserved in a chemical reaction.

    A balanced chemical equation is one in which the number of atoms of each element is the same on the reactant side and on the product side, for example . An equation must be balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction, so every atom present in the reactants must appear in the products.
    Step-by-step solution →
  2. Q22 2 marks
    Two test tubes A and B are taken, each containing one mL of starch solution. Add 1 mL of saliva to test tube ‘A’ only and leave both the test tubes undisturbed for a few minutes. Now add a few drops of dilute iodine solution to both the test tubes.
    1. (a)Which one of the two test tubes shows change in colour ? Write the changed colour observed in this test tube.
    2. (b)What can we conclude from this activity ?
    Show answer

    Test tube B turns blue-black as its starch remains; saliva contains amylase, which breaks starch into sugar, so digestion starts in the mouth.

    (a) Test tube B shows the change in colour: it turns blue-black, because its starch is still present. Test tube A does not turn blue-black, because its starch has been broken down. (b) Saliva contains the enzyme salivary amylase, which breaks down starch, a complex molecule, into simple sugar; the digestion of starch therefore begins in the mouth.
    Step-by-step solution →
  3. Q23 2 marks
    Name two types of germ cells present in human beings. List two structural differences between the two.
    Show answer

    The germ cells are the sperm and the egg; the sperm is very small with a tail, the egg much larger with no tail.

    The two germ cells are the sperm (male germ cell) and the egg or ovum (female germ cell). Structural differences (any two): the sperm is very small, while the egg is much larger; the sperm has a long tail and is motile, while the egg has no tail and does not move on its own; the egg contains stored food, while the sperm carries mainly genetic material and no food store.
    Step-by-step solution →
  4. Q24(a) 2 marks

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

    State two laws of refraction of light.
    Show answer

    The incident ray, refracted ray and normal lie in one plane, and is constant (Snell's law).

    (1) The incident ray, the refracted ray and the normal to the interface of the two transparent media at the point of incidence all lie in the same plane. (2) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light of a given colour and for the given pair of media, (Snell's law of refraction); this constant is the refractive index of the second medium with respect to the first.
    Step-by-step solution →
    OR
    Q24(b) 2 marks

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

    Define the term absolute refractive index of a medium. A ray of light enters from vacuum to glass of absolute refractive index . Find the speed of light in glass. The speed of light in vacuum is m/s.
    Show answer

    The absolute refractive index is (vacuum speed over medium speed); light travels at m/s in glass.

    The absolute refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium, . For glass, m/s.
    Step-by-step solution →
  5. Q25 2 marks

    Also in 31/2/1 (Q25), 31/2/3 (Q24)

    Use Ohm’s law to determine the potential difference across the resistor in the circuit shown in the following diagram when key is closed :
    K 2 V + − 1 Ω 2 Ω 3 Ω
    Show answer

    The potential difference across the resistor is 1 V.

    The resistors are in series, so and the current is A. By Ohm's law, the potential difference across the resistor is V.
    Step-by-step solution →
  6. Q26 2 marks

    Also in 31/2/1 (Q26), 31/2/3 (Q26)

    Name the term used for the materials which cannot be broken down by biological processes. Give two ways by which they harm various components of an ecosystem.
    Show answer

    They are called non-biodegradable substances; they persist and pollute soil and water, and some, like pesticides, magnify along food chains.

    Such materials are called non-biodegradable substances. Two ways in which they harm the components of an ecosystem: (1) They are not broken down by decomposers, so they persist in the environment for a long time and accumulate in the soil and in water bodies, polluting them. (2) Some of them, such as pesticides, enter food chains through plants and, being non-degradable, get accumulated progressively at each trophic level (biological magnification), so organisms at the higher trophic levels, including human beings, receive the highest concentration.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27(a) 3 marks

    Also in 31/2/1 (Q29(a)), 31/2/3 (Q30(a))

    Give reasons for the following :
    1. (i)Alveoli in lungs are richly supplied with blood capillaries.
    2. (ii)Respiratory pigment in the blood takes up oxygen and not carbon dioxide.
    3. (iii)During anaerobic respiration, a 3-carbon molecule is formed as an end product instead of in human beings.
    Show answer

    Capillaries give alveoli a large exchange surface; haemoglobin binds oxygen strongly; without oxygen, pyruvate becomes lactic acid, not carbon dioxide.

    (i) Alveoli are the sites where gases are exchanged: oxygen from the alveolar air passes into the blood and carbon dioxide from the blood passes into the alveoli. The rich network of blood capillaries in their thin walls provides a large surface for this exchange, making it quick and efficient. (ii) The respiratory pigment, haemoglobin, has a very high affinity for oxygen, so it takes up oxygen in the lungs and carries it to the tissues; carbon dioxide is more soluble in water than oxygen and is mostly transported in the dissolved form in the blood. (iii) In human muscle cells, when there is a lack of oxygen, the pyruvate (a 3-carbon molecule) formed from glucose is converted into lactic acid, which is also a 3-carbon molecule, instead of being broken down into carbon dioxide and water; that complete break-down needs oxygen and takes place in the mitochondria.
    Step-by-step solution →
    OR
    Q27(b) 3 marks

    Also in 31/2/1 (Q29(b)), 31/2/3 (Q30(b))

    1. (i)Name the movements that occur all along the gut in human digestive system. How do they help in digestion ?
    2. (ii)Where is bile juice stored in human body ? List two roles of bile juice.
    Show answer

    Peristalsis pushes food along the gut; bile, stored in the gall bladder, makes food alkaline and emulsifies fats.

    (i) The movements are peristaltic movements (peristalsis): rhythmic contractions of the muscles lining the alimentary canal, occurring all along the gut. They push the food forward in a regulated manner, so that it is processed properly in each part of the canal, and they help to mix the food with the digestive juices. (ii) Bile juice is made in the liver and stored in the gall bladder. Two roles of bile: (1) it makes the acidic food coming from the stomach alkaline, so that the pancreatic enzymes can act on it; (2) its bile salts break large fat globules into smaller globules (emulsification), increasing the efficiency of the enzymes that digest fats.
    Step-by-step solution →
  2. Q28 3 marks
    1. (a)In angiosperms why fertilisation cannot take place in flowers if pollination does not take place ? Where is zygote located in a flower after fertilisation ? What does it develop into ?
    2. (b)Write the names of those parts of a flower which serve the same function as the following do in animals : (i) testis (ii) ovary
    Show answer

    Fertilisation needs pollination so the pollen tube can reach the ovule; the zygote in the ovule forms an embryo.

    (a) The male germ cells are inside the pollen grains made in the anther, while the female germ cell (egg) is inside the ovule in the ovary. Only after pollination, the transfer of pollen to the stigma, can a pollen tube grow from the pollen grain through the style to the ovule and deliver the male germ cell; without pollination the two germ cells never meet, so fertilisation cannot take place. After fertilisation the zygote is located inside the ovule, within the ovary, and it divides to form an embryo; the ovule becomes the seed and the ovary the fruit. (b) (i) Testis: the anther of the stamen, which produces pollen grains carrying the male germ cells. (ii) Ovary: the ovary of the pistil (carpel), which contains the ovules with the egg cells.
    Step-by-step solution →
  3. Q29 3 marks
    1. (a)State any two observations when an electric current is passed through acidulated water, in a container having each electrode covered by test tubes filled with water.
    2. (b)Write the ratio of the mass of the gas collected at the cathode to the mass of the gas collected at the anode.
    Show answer

    Gas bubbles collect at both electrodes, with twice the volume at the cathode; the mass ratio of hydrogen to oxygen is .

    (a) Any two of: bubbles of gas form at both electrodes and collect in the test tubes, pushing the water out of them; the volume of gas collected over the cathode is twice the volume collected over the anode; the gas from the cathode burns with a pop sound when a burning candle is brought near (hydrogen), while the gas from the anode makes the candle burn more brightly (oxygen). (b) The gas at the cathode is hydrogen and the gas at the anode is oxygen. From , mass of hydrogen : mass of oxygen .
    Step-by-step solution →
  4. Q30 3 marks

    Also in 31/2/1 (Q28)

    Draw a labelled diagram to show electrolytic refining of copper. State what happens when electric current is passed through the electrolyte taken in this case.
    Show answer

    The impure copper anode dissolves and pure copper deposits on the cathode; insoluble impurities settle as anode mud.

    Diagram: a tank containing acidified copper sulphate solution as the electrolyte; a thick block of impure copper as the anode, joined to the positive terminal of the battery; a thin strip of pure copper as the cathode, joined to the negative terminal; anode mud collecting at the bottom below the anode. When electric current is passed, copper from the impure anode dissolves into the electrolyte as copper ions (), and an equivalent amount of pure copper from the electrolyte is deposited on the cathode (). The soluble impurities go into the solution, while the insoluble impurities settle down at the bottom of the anode as anode mud. The anode becomes thinner and the cathode becomes thicker with pure copper.
    Step-by-step solution →
  5. Q31(a) 3 marks
    An object is placed in front of a concave mirror of focal length 12 cm. If distance of the object from the pole of the mirror is 8 cm, then use mirror formula to determine the position of the image formed. Draw a labelled ray diagram to justify your answer in this case.
    Show answer

    The image is 24 cm behind the mirror, virtual, erect and three times the size of the object.

    The image is formed 24 cm behind the mirror ( cm). It is virtual and erect, and three times the size of the object (). Ray diagram: a ray from the top of the object parallel to the principal axis is reflected through F, and a ray along the line from C through the top of the object is reflected back along itself. The two reflected rays diverge, and their backward extensions meet 24 cm behind the mirror, where the erect, enlarged image is seen.
    Step-by-step solution →
    OR
    Q31(b) 3 marks
    1. (i)The image of an object formed by a mirror is real, inverted and is of magnification . If the image is at a distance of 30 cm from the mirror, where is the object placed ? Give reason to justify your answer.
    2. (ii)Where would the image be if the object is moved 15 cm towards the mirror ? Draw ray diagram for the new position of the object to justify your answer.
    Show answer

    The object is at C, 30 cm from the mirror; moved 15 cm closer, it is at F and the image forms at infinity.

    (i) The object is placed 30 cm in front of the mirror ( cm), at its centre of curvature C. Reason: only a concave mirror forms a real, inverted image, and means cm; then , so cm and cm. A concave mirror forms a real, inverted image of the same size only when the object is at C, and the image then forms at C too. (ii) Moving the object 15 cm towards the mirror puts it at cm, the principal focus F. Then , so the image is formed at infinity: real, inverted and highly enlarged. In the ray diagram the two reflected rays leave the mirror parallel to each other.
    Step-by-step solution →
  6. Q32 3 marks
    1. (a)State Fleming’s left hand rule. Apply this rule to determine the direction of force experienced by a straight current carrying conductor AB placed in a uniform magnetic field as shown.
    2. (b)What will happen to an electron which enters in the same field in the same direction in which the current is flowing in the conductor AB ? Give reason to justify your answer.
    A B Electric Current Magnetic field
    Show answer

    The force on AB is into the page; the electron is deflected, with a force out of the page, opposite to that on AB.

    (a) Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, the thumb points in the direction of the force (motion) on the conductor. Here the field is from left to right and the current flows up the conductor from B to A, so the thumb points into the page: the force on AB is perpendicular to the plane of the page, directed into it (away from the reader). (b) The electron is deflected: it experiences a force perpendicular to both the field and its motion, directed out of the page, opposite to the force on AB. Reason: an electron is negatively charged, so an electron moving up the page is equivalent to a current flowing down the page, and for a downward current in a left-to-right field the left-hand rule gives a force out of the page.
    Step-by-step solution →
  7. Q33 3 marks

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

    Use of pesticides to protect our crops affect organisms at various trophic levels especially human beings. Name the phenomenon involved and explain how does it happen.
    Show answer

    Biological magnification: non-degradable pesticides enter the food chain and build up at each trophic level, reaching the maximum in human beings.

    The phenomenon is biological magnification. Pesticides used on crops are washed down into the soil or into water bodies. From the soil and water they are absorbed by plants along with water and minerals, and from water bodies they are taken up by aquatic plants and animals — this is how they enter the food chain. As these chemicals are not degradable, they get accumulated progressively at each trophic level, since each organism takes in the chemical from all the food it eats from the level below. Human beings occupy the top level in the food chain, so the maximum concentration of these chemicals gets accumulated in our bodies.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34 5 marks

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

    1. (a)Upper half of a convex lens is covered with a black paper. Draw a ray diagram to show the formation of image of an object placed at a distance of from such a lens. Mention the position and nature of the image formed. State the observable difference in the image obtained if the lens is uncovered. Give reason to justify your answer.
    2. (b)An object is placed at a distance of 30 cm from the optical centre of a concave lens of focal length 15 cm. Use lens formula to determine the distance of the image from the optical centre of the lens.
    Show answer

    (a) Image at , real, inverted, same size but dimmer; brighter when uncovered. (b) cm.

    (a) With the upper half covered, only rays passing through the lower half form the image; in the ray diagram the ray from the top of the object through emerges parallel to the axis and meets a second ray through the lower half at . Position: at , on the other side of the lens. Nature: real, inverted and of the same size as the object; the image is complete but less bright. If the lens is uncovered, the image is formed at the same place with the same size and nature, but it is brighter. Reason: every part of a lens forms a complete image, because rays from each point of the object pass through all parts of the lens and meet at the same image point; covering half the lens only cuts down the amount of light reaching the image. (b) cm and cm, so and cm. The image is 10 cm from the optical centre, on the same side as the object (virtual, erect and diminished, ).
    Step-by-step solution →
  2. Q35(a) 5 marks

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

    1. (i)Give reason why carbon can neither form cations nor anions but form covalent compounds.
    2. (ii)What is homologous series of carbon compound ? Write the molecular formula of any two consecutive members of homologous series of aldehydes.
    3. (iii)Draw the structure of the molecule of cyclohexane ().
    Show answer

    Carbon shares its four valence electrons; and are consecutive aldehydes; cyclohexane is a ring of six carbon atoms with single bonds.

    (i) Carbon (atomic number 6, configuration 2, 4) has four valence electrons. To form it would have to gain four electrons, and it would be difficult for a nucleus with six protons to hold ten electrons; to form it would have to lose four electrons, which needs a large amount of energy and would leave six protons holding only two electrons. Carbon therefore shares its four valence electrons with other atoms and forms covalent bonds, so its compounds are covalent. (ii) A homologous series is a series of carbon compounds with the same functional group and the same general formula, in which successive members differ by a unit (14 u); the members have similar chemical properties and show a gradation in physical properties. Two consecutive aldehydes: methanal, (molecular formula ), and ethanal, (molecular formula ). (iii) Cyclohexane, : six carbon atoms joined in a ring by single bonds, each carbon also bonded to two hydrogen atoms, as drawn in the structure.
    Step-by-step solution →
    OR
    Q35(b) 5 marks

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

    (i) Name a commercially important carbon compound having functional group and write its molecular formula. (ii) Write chemical equation to show its reaction with
    1. (1)Sodium metal
    2. (2)Excess conc. sulphuric acid
    3. (3)Ethanoic acid in the presence of an acid catalyst
    4. (4)Acidified potassium dichromate Also write the name of the product formed in each case.
    Show answer

    Ethanol () gives sodium ethoxide, ethene, ethyl ethanoate and ethanoic acid with sodium, conc. sulphuric acid, ethanoic acid and dichromate.

    (i) Ethanol (ethyl alcohol); molecular formula (that is, ). (ii) (1) ; product: sodium ethoxide (and hydrogen gas). (2) ; product: ethene. (3) ; product: ethyl ethanoate (an ester). (4) ; product: ethanoic acid.
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  3. Q36(a) 5 marks

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

    (i) Distinguish between hormonal co-ordination in plants and animals. (ii) Which part of the brain is responsible for –
    1. (1)intelligence
    2. (2)riding a bicycle
    3. (3)vomiting
    4. (4)controlling hunger (iii) How is brain and spinal-cord protected against mechanical injuries ?
    Show answer

    Plant hormones lack glands while animal hormones come from endocrine glands; cerebrum, cerebellum, medulla and hypothalamus control the four activities; cranium and backbone protect.

    (i) Plants: hormones are made by cells in particular regions, such as auxin at the shoot tip, with no special glands. Animals: hormones are made by specialised endocrine glands, such as the pituitary, thyroid and adrenal glands. Plants: the hormones diffuse from cell to cell to the area where they act. Animals: the hormones are secreted directly into the blood and carried to target organs. Plants: they mainly control growth and development, so the responses are slow. Animals: they control growth, metabolism, reproduction and quick responses to emergencies, work with the nervous system, and are regulated by feedback mechanisms. (ii) (1) Intelligence: cerebrum (fore-brain); (2) Riding a bicycle: cerebellum (hind-brain); (3) Vomiting: medulla (hind-brain); (4) Controlling hunger: hypothalamus (fore-brain). (iii) The brain lies inside a bony box, the cranium (skull), and within it is enclosed in fluid-filled membranes that absorb shocks. The spinal cord is protected by the vertebral column (backbone), which surrounds it.
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    OR
    Q36(b) 5 marks

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

    (i) What are tropic movements ? Give an example of a plant hormone which
    1. (1)inhibits growth and
    2. (2)promotes cell division. (ii) Explain directional movement of a tendril in pea plant in response to touch. Name the hormone responsible for this movement.
    Show answer

    Tropic movements are directional growth responses to a stimulus; abscisic acid inhibits growth, cytokinin promotes cell division, and auxin makes the pea tendril coil.

    (i) Tropic movements are directional growth movements of a plant part in response to a stimulus such as light, gravity, water, chemicals or touch: the part grows towards the stimulus (positive) or away from it (negative), as when a shoot bends towards light. (1) A hormone that inhibits growth: abscisic acid. (2) A hormone that promotes cell division: cytokinin. (ii) The tendrils of a pea plant are sensitive to touch. When a tendril touches a support, the part in contact with the support grows more slowly than the part away from it. The faster growth of the far side makes the tendril curl towards the support, circle round it and cling to it. The hormone responsible for this movement is auxin.
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Questions 37–39 · 4 marks each

  1. Q37 4 marks

    Also in 31/2/1 (Q39), 31/2/3 (Q38)

    When electric current flows in a purely resistive circuit electrical energy gets fully converted into heat energy. The amount of heat produced () in the circuit is found to be directly proportional to
    1. (i)the square of current ()
    2. (ii)the resistance () of the conductor and
    3. (iii)the time () for which current flows. In other words . Electrical devices such an electric fuse, electric heater, electric iron etc. are all based on this effect called heating effect of electric current.
    1. (a)List two properties of heating elements.[1]
    2. (b)List two properties of electric fuse.[1]
    3. (c)(i)Name the principle on which an electric fuse works. Explain how a fuse wire is capable of saving electrical appliances from getting damaged due to accidently produced high currents.[2]
    4. (c)(ii)OR : The power of an electric heater is 1100 W. If the potential difference between the two terminals of the heater is 220 V, find the current flowing in the circuit. What will happen to an electric fuse of rating 5 A connected in this circuit ?[2]
    Show answer

    Heating elements need high resistivity and melting point; a fuse uses the heating effect to break the circuit; or the heater draws A.

    (a) A heating element should have (1) a high resistivity, so that the heat is produced mainly in the element, and (2) a high melting point, so that it stays red-hot without melting; it should also not oxidise readily at high temperatures. (b) An electric fuse wire has (1) a low melting point and (2) a high resistance per unit length, being much thinner than the connecting wires that carry the same current, so that the heat produced by an excess current is concentrated in the fuse and quickly raises it to its melting point. It is connected in series with the appliance, in the live wire, and has a definite current rating (such as 1 A, 2 A, 3 A, 5 A or 10 A). (c)(i) A fuse works on the heating effect of electric current (Joule's law of heating, ). It is connected in series with the appliance, so the whole current flows through it. When an unduly high current flows, the heat produced in the fuse wire rises sharply (as ), the wire reaches its melting point and melts, and the circuit is broken before the high current can damage the appliance. OR (c)(ii) A. The fuse is rated 5 A and melts only if the current is more than 5 A, so it will not melt and the heater works normally; the fuse is then working at its limit, and any rise in the current above 5 A would melt it.
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  2. Q38 4 marks

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

    Salts play a very important role in our daily life. Sodium chloride which is known as common salt is used almost in every kitchen. Baking soda is also a salt used in faster cooking as well as in baking industry. The family of salts is classified on the basis of cations and anions present in them.
    1. (a)Identify the acid and base from which Sodium chloride is formed.[1]
    2. (b)Find the cation and the anion present in Calcium sulphate.[1]
    3. (c)(i)“Sodium chloride and washing soda both belong to the same family of salts.” Justify this statement.[2]
    4. (c)(ii)OR : Define the term pH scale. Name the salt obtained by the reaction of Potassium hydroxide and Sulphuric acid and give the pH value of its aqueous solution.[2]
    Show answer

    and form sodium chloride; calcium sulphate has and ; sodium chloride and washing soda share .

    (a) Sodium chloride is formed from hydrochloric acid, , and sodium hydroxide, : . (b) Cation: calcium ion, ; anion: sulphate ion, . (c)(i) Salts having the same positive or the same negative ion belong to one family. Sodium chloride (, ) and washing soda, (, ), have the same cation, , so both belong to the family of sodium salts. OR (c)(ii) The pH scale is a scale for measuring the hydrogen ion concentration in a solution, from 0 (very acidic) to 14 (very alkaline), with 7 as neutral. : the salt is potassium sulphate. It is the salt of a strong acid and a strong base, so its aqueous solution is neutral, .
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  3. Q39 4 marks

    Also in 31/2/1 (Q38), 31/2/3 (Q37)

    Asexual reproduction involves a single parent to produce offsprings without the formation of gametes. It occurs by the following ways : Fission, Budding, Fragmentation, Spore formation and Regeneration. In one of the methods like regeneration, Planaria A is cut horizontally into three pieces – L, M and N and Planaria B is cut vertically into two equal halves – O and P.
    A L M N B O P
    1. (a)Which of the cut pieces of the two Planaria could regenerate to form a complete organism ?[1]
    2. (b)Give an example of another organism which follows the same mode of reproduction as Planaria.[1]
    3. (c)(i)What is the meaning of ‘development’ in regeneration ?[2]
    4. (c)(ii)OR : Differentiate between regeneration and fragmentation.[2]
    Show answer

    All pieces (L, M, N, O, P) regenerate; Hydra is another example; development is the organised sequence of changes forming the lost parts.

    (a) All the pieces: L, M and N of Planaria A, and O and P of Planaria B, can each regenerate into a complete organism. (b) Hydra. (c)(i) In regeneration, specialised cells in the cut piece divide and make a large number of cells. From this mass of cells, different cells change into the various cell types and tissues of the missing parts. These changes take place in an organised sequence, and this organised sequence of changes is called development. OR (c)(ii) Regeneration: occurs in fully differentiated organisms (Hydra, Planaria) when the body is cut or broken; specialised cells divide and develop into the lost parts; it is not the normal way these organisms reproduce. Fragmentation: occurs in simple multicellular organisms (Spirogyra); the body breaks up into pieces by itself on maturing, and each piece simply grows by cell division into a new individual; it is a normal method of reproduction.
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