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CBSE Class 10 Science 2024 — Paper 31/1/3

80 marks · 3 hours · 39 questions (44 with the OR choices)

32 of its 44 questions also appear in 31/1/1, 31/1/2 and other papers; 12 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark

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

    Select from the following a decomposition reaction in which source of energy for decomposition is light :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/1/1 (Q1), 31/1/2 (Q7)

    When 2 mL of sodium hydroxide solution is added to few pieces of granulated zinc in a test tube and then warmed, the reaction that occurs can be written in the form of a balanced chemical equation as :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  3. Q3 1 mark

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

    The reaction given above is a redox reaction because in this case :
    1. (a) is oxidised and is reduced.
    2. (b) is oxidised.
    3. (c) is reduced.
    4. (d) is reduced and is oxidised.
    Show answer
    (d) is reduced and is oxidised.
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/1/1 (Q5), 31/1/2 (Q3)

    Consider the following compounds : ; ; ; The compound having maximum number of water of crystallisation in its crystalline form in one molecule is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (d)
    Step-by-step solution →
  5. Q5 1 mark

    Also in 31/1/1 (Q10), 31/1/2 (Q9)

    In a nerve cell, the site where the electrical impulse is converted into a chemical signal is known as :
    1. (a)Axon
    2. (b)Dendrites
    3. (c)Neuromuscular junction
    4. (d)Cell body
    Show answer
    (a) Axon
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/1/1 (Q3), 31/1/2 (Q5)

    A metal and a non-metal that exists in liquid state at the room temperature are respectively :
    1. (a)Bromine and Mercury
    2. (b)Mercury and Iodine
    3. (c)Mercury and Bromine
    4. (d)Iodine and Mercury
    Show answer
    (c) Mercury and Bromine
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/1/1 (Q12), 31/1/2 (Q13)

    At what distance from a convex lens should an object be placed to get an image of the same size as that of the object on a screen ?
    1. (a)Beyond twice the focal length of the lens.
    2. (b)At the principal focus of the lens.
    3. (c)At twice the focal length of the lens.
    4. (d)Between the optical centre of the lens and its principal focus.
    Show answer
    (c) At twice the focal length of the lens.
    Step-by-step solution →
  8. Q8 1 mark

    Also in 31/1/1 (Q4)

    Carbon compounds :
    1. (i)are good conductors of electricity.
    2. (ii)are bad conductors of electricity.
    3. (iii)have strong forces of attraction between their molecules.
    4. (iv)have weak forces of attraction between their molecules. The correct statements are :
    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(ii) and (iv)
    4. (d)(i) and (iii)
    Show answer
    (c) (ii) and (iv)
    Step-by-step solution →
  9. Q9 1 mark

    Also in 2025 · 31/S/3 (Q5), 31/1/1 (Q6), 31/1/2 (Q2)

    Oxides of aluminium and zinc are :
    1. (a)acidic
    2. (b)basic
    3. (c)amphoteric
    4. (d)neutral
    Show answer
    (c) amphoteric
    Step-by-step solution →
  10. Q10 1 mark

    Also in 31/1/1 (Q9), 31/1/2 (Q10)

    Chromosomes :
    1. (i)carry hereditary information from parents to the next generation.
    2. (ii)are thread like structures located inside the nucleus of an animal cell.
    3. (iii)always exist in pairs in human reproductive cells.
    4. (iv)are involved in the process of cell division. The correct statements are :
    1. (a)(i) and (ii)
    2. (b)(iii) and (iv)
    3. (c)(i), (ii) and (iv)
    4. (d)(i) and (iv)
    Show answer
    (c) (i), (ii) and (iv)
    Step-by-step solution →
  11. Q11 1 mark

    Also in 31/1/1 (Q8)

    Consider the following statements :
    1. (i)The sex of a child is determined by what it inherits from the mother.
    2. (ii)The sex of a child is determined by what it inherits from the father.
    3. (iii)The probability of having a male child is more than that of a female child.
    4. (iv)The sex of a child is determined at the time of fertilisation when male and female gametes fuse to form a zygote. The correct statements are :
    1. (a)(i) and (iii)
    2. (b)(ii) and (iv)
    3. (c)(iii) and (iv)
    4. (d)(i), (iii) and (iv)
    Show answer
    (b) (ii) and (iv)
    Step-by-step solution →
  12. Q12 1 mark
    Which one of the following organ is NOT a part of human female reproductive system ?
    1. (a)Ovary
    2. (b)Uterus
    3. (c)Vas deferens
    4. (d)Fallopian tube
    Show answer
    (c) Vas deferens
    Step-by-step solution →
  13. Q13 1 mark

    Also in 31/1/1 (Q16), 31/1/2 (Q12)

    In which of the following organisms, multiple fission is a means of asexual reproduction ?
    1. (a)Yeast
    2. (b)Leishmania
    3. (c)Paramoecium
    4. (d)Plasmodium
    Show answer
    (d) Plasmodium
    Step-by-step solution →
  14. Q14 1 mark
    In bifocal lenses used for the correction of presbyopia :
    1. (a)the upper portion is of convex lens for the near vision and lower part is of concave lens for the distant vision.
    2. (b)the upper portion is of convex lens for the distant vision and lower part is of concave lens for the near vision.
    3. (c)the upper portion is of concave lens is for the near vision and lower part is of convex lens for the distant vision.
    4. (d)the upper portion is of concave lens for the distant vision and lower part is of convex lens for the near vision.
    Show answer
    (d) the upper portion is of concave lens for the distant vision and lower part is of convex lens for the near vision.
    Step-by-step solution →
  15. Q15 1 mark

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

    The pattern of the magnetic field produced inside a current carrying solenoid is :
    (a) (b) (c) (d)
    1. (a)Four parallel, equally spaced straight lines, each with an arrowhead pointing the same way (to the right).
    2. (b)Three concentric closed loops round a central dot, with arrowheads showing a clockwise sense.
    3. (c)The same four parallel straight lines with arrowheads to the right, plus a curved line above them and a curved line below them whose arrowheads also point to the right.
    4. (d)Two small circles with dots at their centres, above and below a straight central line; curved lines between them bend towards the central line, with arrowheads pointing to the left.
    Show answer
    (a) Four parallel, equally spaced straight lines, each with an arrowhead pointing the same way (to the right).
    Step-by-step solution →
  16. Q16 1 mark

    Also in 31/1/1 (Q15), 31/1/2 (Q16)

    Identify the food chain in which the organisms of the second trophic level are missing :
    1. (a)Grass, goat, lion
    2. (b)Zooplankton, Phytoplankton, small fish, large fish
    3. (c)Tiger, grass, snake, frog
    4. (d)Grasshopper, grass, snake, frog, eagle
    Show answer
    (c) Tiger, grass, snake, frog
    Step-by-step solution →
  17. Q17 1 mark

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

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

    Assertion (A): The rainbow is a natural spectrum of sunlight in the sky.

    Reason (R): Rainbow is formed in the sky when the sun is overhead and water droplets are also present in air.

    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, but 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 →
  18. Q18 1 mark

    Also in 31/1/1 (Q17), 31/1/2 (Q20)

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

    Assertion (A): Hydrogen gas is not evolved when zinc reacts with nitric acid.

    Reason (R): Nitric acid oxidises the hydrogen gas produced to water and itself gets reduced.

    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, but 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 →
  19. Q19 1 mark

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

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

    Assertion (A): Accumulation of harmful chemicals is maximum in the organisms at the highest trophic level of a food chain.

    Reason (R): Harmful chemicals are sprayed on the crops to protect them from diseases and pests.

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

    Also in 31/1/1 (Q19), 31/1/2 (Q17)

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

    Assertion (A): The rate of breathing in aquatic organisms is much faster than in terrestrial organisms.

    Reason (R): The amount of oxygen dissolved in water is very high as compared to the amount of oxygen in air.

    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, but 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(a) 2 marks
    1. (i)Write the significance of peripheral nervous system in human beings.
    2. (ii)How is human brain protected from mechanical injuries and shocks ?
    Show answer

    The peripheral nervous system links the central nervous system with the body; the brain is protected by the cranium and fluid-filled membranes.

    (i) The peripheral nervous system, made of the cranial nerves arising from the brain and the spinal nerves arising from the spinal cord, links the central nervous system with all the other parts of the body. It carries information from the receptors in the sense organs to the brain and spinal cord, and carries their messages to the effectors, the muscles and glands. (ii) The brain sits inside a bony box, the cranium (skull), which protects it from mechanical injury; inside this box it is contained in a fluid-filled balloon (membranes holding cerebrospinal fluid) that absorbs shocks.
    Step-by-step solution →
    OR
    Q21(b) 2 marks
    Name one directional growth movement each in response to chemicals and water in plants. Write an example for each of them.
    Show answer

    Chemotropism, for example the pollen tube growing towards the ovule; hydrotropism, for example roots growing towards water.

    In response to chemicals: chemotropism. Example: the pollen tube grows from the stigma through the style towards the ovule. In response to water: hydrotropism. Example: the roots of a plant grow towards water in the soil. In both, the plant part grows towards the stimulus.
    Step-by-step solution →
  2. Q22 2 marks
    1. (i)Give reason why herbivorous animals have longer, small intestine than carnivorous animals ?
    2. (ii)Although ‘Pepsin’ and ‘Trypsin’ are both protein digesting enzymes yet they differ from each other. Justify this statement by giving one difference between them.
    Show answer

    Herbivores digest cellulose in a longer small intestine; pepsin acts in the acidic stomach, trypsin in the alkaline small intestine.

    (i) Herbivores eat grass, which is rich in cellulose, and cellulose takes a long time to digest. A longer small intestine keeps the food in the gut for longer, allowing the cellulose to be digested; meat is easier to digest, so carnivores have a shorter small intestine. (ii) Pepsin is secreted by the gastric glands of the stomach and acts in the stomach in an acidic medium, provided by hydrochloric acid, whereas trypsin is secreted by the pancreas and acts in the small intestine in an alkaline medium. Any one of these differences, in the site of secretion, the site of action or the medium, answers the question.
    Step-by-step solution →
  3. Q23 2 marks
    Translate the following statement into a balanced chemical equation. “When barium chloride reacts with aluminium sulphate, aluminium chloride and barium sulphate are formed.” State the type of this reaction giving reason to justify your answer.
    Show answer

    The balanced equation is , a double displacement reaction.

    Balanced equation: . Type: double displacement reaction (also a precipitation reaction). Reason: the two reactants exchange ions, barium ions combining with sulphate ions and aluminium ions with chloride ions, and the insoluble barium sulphate separates out as a white precipitate.
    Step-by-step solution →
  4. Q24 2 marks

    Also in 31/1/1 (Q26), 31/1/2 (Q21)

    1. (i)Two magnetic field lines do not intersect each other. Why ?
    2. (ii)How is a uniform magnetic field in a given region represented ? Draw a diagram in support of your answer.
    Show answer

    (i) The field has only one direction at a point, so lines cannot cross; (ii) a uniform field is shown by parallel, equally spaced lines.

    (i) The direction of the magnetic field at a point is the direction in which a compass needle placed there points, and there is only one such direction. If two field lines intersected, the field at the point of intersection would have two directions, so a compass needle there would point in two directions at once, which is not possible. Hence two field lines never intersect. (ii) A uniform magnetic field is represented by parallel, straight, equally spaced field lines, all with arrows pointing in the same direction (as inside a long current-carrying solenoid). The diagram shows such parallel, equidistant lines with arrows in one direction.
    Step-by-step solution →
  5. Q25 2 marks
    Draw the pattern of the magnetic field lines due to a straight current carrying conductor indicating the direction of current in the conductor and the direction of the corresponding magnetic field lines.
    Show answer

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

    The magnetic field lines of a straight current-carrying conductor are concentric circles centred on the conductor, in planes perpendicular to it, drawn farther apart as the distance from the wire increases. Their direction is given by the right-hand thumb rule: for a current flowing vertically upwards, the field lines run anticlockwise when seen from above, as in the drawing; if the current is reversed, they run clockwise.
    Step-by-step solution →
  6. Q26 2 marks

    Also in 31/1/1 (Q24)

    An object is placed at a distance of cm from a convex mirror of focal length cm. Find the position of the image formed by the mirror.
    Show answer

    cm: the image is formed 6 cm behind the mirror, virtual, erect and diminished.

    cm: the image is formed cm behind the mirror. It is virtual, erect and diminished ().
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27(a) 3 marks

    Also in 31/1/1 (Q33(a)), 31/1/2 (Q31(a))

    Plants Deer Lion In the given food chain, what will be the impact of removing all the organisms of second trophic level on the first and third trophic level? Will the impact be the same for the organisms of the third trophic level in the above food chain if they were present in a food web? Justify.
    Show answer

    Removing deer makes plants increase and lions starve; in a food web the impact on lions is less, as they have other prey.

    On removing the deer (second trophic level): the plants (first trophic level) are no longer eaten, so their number increases; the lions (third trophic level) lose their only food, so they starve and their number decreases, and they may die out. In a food web the impact on the lions would not be the same; it would be much less severe. A food web is a network of interconnected food chains, so the lions can feed on other herbivores (for example zebra or buffalo) and survive.
    Step-by-step solution →
    OR
    Q27(b) 3 marks

    Also in 31/1/1 (Q33(b)), 31/1/2 (Q31(b))

    A gas ‘X’ which is a deadly poison is found at the higher levels of atmosphere and performs an essential function. Name the gas and write the function performed by this gas in the atmosphere. Which chemical is linked to the decrease in the level of this gas? What measures have been taken by an international organization to check the depletion of the layer containing this gas?
    Show answer

    Ozone () shields Earth from UV radiation; CFCs deplete it; UNEP's 1987 agreement froze CFC production at 1986 levels.

    Gas X is ozone (). Function: At the higher levels of the atmosphere, the ozone layer shields the surface of the Earth from the ultraviolet (UV) radiation of the Sun, which is highly damaging to organisms (for example, it causes skin cancer in human beings). Chemical: Chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers, are linked to the decrease in ozone. Measures: In 1987 the United Nations Environment Programme (UNEP) forged an agreement to freeze CFC production at 1986 levels, and it is now mandatory for all manufacturing companies to make CFC-free refrigerators throughout the world.
    Step-by-step solution →
  2. Q28 3 marks

    Also in 31/1/1 (Q32)

    Name and state the rule to determine the direction of a :
    1. (i)magnetic field produced around a current carrying straight conductor.
    2. (ii)force experienced by a current carrying straight conductor placed in a magnetic field which is perpendicular to it.
    Show answer

    Right-hand thumb rule gives the field direction around a straight current; Fleming's left-hand rule gives the force direction.

    (i) Right-hand thumb rule: Imagine that you are holding the current-carrying straight conductor in your right hand such that the thumb points in the direction of the current. Then the fingers wrapped around the conductor point in the direction of the field lines of the magnetic field. (ii) Fleming's left-hand rule: Stretch the thumb, forefinger and middle finger of your 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, then the thumb points in the direction of the force (motion) acting on the conductor.
    Step-by-step solution →
  3. Q29 3 marks

    Also in 31/1/1 (Q31)

    Study the diagram given below and answer the questions that follow :
    1. (i)Name the defect of vision represented in the diagram. Give reason for your answer.
    2. (ii)List two causes of this defect.
    3. (iii)With the help of a diagram show how this defect of vision is corrected.
    N N′
    Show answer

    Hypermetropia, caused by a long eye-lens focal length or a small eyeball, is corrected by a convex lens.

    (i) Hypermetropia (far-sightedness). Reason: the rays of light from the nearby point N′ are brought to a focus at a point behind the retina, as the dashed lines show, so a nearby object is not seen distinctly; such an eye sees distant objects clearly. (ii) Causes: (1) The focal length of the eye lens is too long. (2) The eyeball has become too small. (iii) It is corrected by using a convex (converging) lens of appropriate power. The convex lens makes the rays from the nearby object N′ appear to come from the farther point N, which the eye can focus, so the image forms on the retina (see the diagram).
    Step-by-step solution →
  4. Q30 3 marks

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

    Define reflex action. With the help of a flow chart show the path of a reflex action such as sneezing.
    Show answer

    A reflex action is a sudden, automatic, involuntary response to a stimulus, as in sneezing when dust irritates the nose.

    Reflex action: A sudden, automatic and involuntary response of the body to a stimulus, made without conscious thinking; for example, sneezing when dust enters the nose. Flow chart: Stimulus (dust or pepper irritating the nose) → Receptors in the lining of the nose → Sensory neuron → Relay neuron in the central nervous system (for sneezing, the hind-brain) → Motor neuron → Effectors (muscles of the chest and the diaphragm) → Response (sneezing: air is forced out through the nose and mouth).
    Step-by-step solution →
  5. Q31 3 marks
    1. (i)Which organisms have a three-chambered heart ? Why do they have three-chambered hearts ?
    2. (ii)List two functions of lymph.
    Show answer

    Amphibians and many reptiles have three-chambered hearts because their energy needs are low; lymph carries absorbed fat and returns excess fluid to blood.

    (i) Amphibians, such as frogs, and many reptiles, such as lizards, have three-chambered hearts (two atria and one ventricle). They do not use energy to maintain their body temperature, which depends on the temperature of the surroundings, so their energy needs are lower than those of birds and mammals. They can therefore tolerate some mixing of oxygenated and deoxygenated blood in the single ventricle and do not need the complete separation a four-chambered heart gives. (ii) Two functions of lymph: (1) it carries digested and absorbed fat from the intestine; (2) it drains excess fluid from the intercellular spaces back into the blood.
    Step-by-step solution →
  6. Q32 3 marks
    A compound which is prepared from gypsum has the property of hardening when water is mixed in right quantity with it :
    1. (i)Write common name and the chemical name of this compound.
    2. (ii)Give chemical equation for its preparation.
    3. (iii)List its two uses.
    Show answer

    Plaster of Paris, calcium sulphate hemihydrate, is made by heating gypsum at 373 K and is used for supporting fractured bones and making toys.

    (i) Common name: Plaster of Paris; chemical name: calcium sulphate hemihydrate, . (ii) It is prepared by heating gypsum at 373 K: . (iii) Any two uses: as plaster for supporting fractured bones in the right position; for making toys; for making materials for decoration; for making surfaces smooth.
    Step-by-step solution →
  7. Q33 3 marks
    1. (i)Define a decomposition reaction. Write chemical equation for the reaction that occurs when lead nitrate is heated strongly in a boiling tube.
    2. (ii)In electrolytic decomposition of water two gases are liberated at the electrodes. Give the mass ratio of the gas liberated at the cathode and at the anode.
    Show answer

    Heated lead nitrate gives lead oxide, nitrogen dioxide and oxygen; the mass ratio of hydrogen (cathode) to oxygen (anode) is .

    (i) A decomposition reaction is a reaction in which a single reactant breaks down to give simpler products. When lead nitrate is heated strongly: ; brown fumes of nitrogen dioxide are given off. (ii) Hydrogen is liberated at the cathode and oxygen at the anode, . Mass of hydrogen : mass of oxygen (by volume the ratio is ).
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks
    (i) State whether the currents and potential difference in all the bulbs will be same or different when in a circuit three bulbs of :
    1. (a)same wattage are connected in series.
    2. (b)same wattage are connected in parallel.
    3. (c)different wattage are connected in series.
    4. (d)different wattage are connected in parallel. (ii) Two identical resistors of each are connected to a battery of V. Calculate the ratio of the power consumed by the resulting combinations with (a) minimum resistance and (b) maximum resistance.
    Show answer

    Series bulbs share the current and parallel bulbs share the potential difference; the power ratio (minimum to maximum resistance) is .

    (i) Taking the bulbs as rated for the same voltage, so that equal wattage means equal resistance: (a) same wattage in series: current same, potential difference same; (b) same wattage in parallel: potential difference same, current same; (c) different wattage in series: current same, potential difference different; (d) different wattage in parallel: potential difference same, current different. (ii) Minimum resistance (parallel) , so W; maximum resistance (series) , so W. Ratio of the powers (a) : (b) .
    Step-by-step solution →
    OR
    Q34(b) 5 marks
    Draw a schematic diagram of a circuit consisting of a battery of six V cells, a resistor, a resistor and a resistor and a plug key all connected in series. Calculate the following (when key is closed) :
    1. (i)Electric current flowing in the circuit.
    2. (ii)Potential difference across resistor.
    3. (iii)Electric power consumed in resistor.
    Show answer

    Six 2 V cells give V across , so A; the resistor has V across it and uses W.

    Circuit: six V cells in series (a V battery), a plug key and the , and resistors, all in one loop, as in the figure. Total resistance . (i) A A. (ii) V. (iii) W.
    Step-by-step solution →
  2. Q35(a) 5 marks

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

    1. (i)Define a homologous series of carbon compounds.
    2. (ii)Why is the melting and boiling points of higher than that of or ?
    3. (iii)Why do we NOT see any gradation in chemical properties of a homologous series compounds ?
    4. (iv)Write the name and structures of (i) aldehyde and (ii) ketone with molecular form .
    Show answer

    Chemical properties are similar because the functional group is the same; heavier melts and boils higher; propanal and propanone fit .

    (i) A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain; successive members differ by a unit ( u), share one general formula and have similar chemical properties. (ii) , and are successive members of the alkene series, with molecular masses u, u and u. Melting and boiling points increase with molecular mass in a homologous series, so , the largest and heaviest molecule, has the highest melting and boiling points. (iii) Chemical properties are determined by the functional group, and every member of a homologous series has the same functional group, so the members show similar chemical properties with no gradation. (iv) Aldehyde: propanal, ; ketone: propanone, . Both have the formula (full structures in the figure).
    Step-by-step solution →
    OR
    Q35(b) 5 marks

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

    1. (i)Write the name and structure of an organic compound ‘X’ having two carbon atoms in its molecule and its name is suffixed with ‘–ol’.
    2. (ii)What happens when ‘X’ is heated with excess concentrated sulphuric acid at 443 K? Write chemical equation for the reaction stating the conditions for the reaction. Also state the role played by concentrated sulphuric acid in the reaction.
    3. (iii)Name and draw the electron dot structure of hydrocarbon produced in the above reaction.
    Show answer

    X is ethanol; hot concentrated sulphuric acid dehydrates it at 443 K to ethene, whose carbon atoms share a double bond.

    (i) 'X' is ethanol, ; its structure is (the first carbon carries three H atoms, the second carbon two H atoms and the group). (ii) Ethanol is dehydrated to ethene: . Conditions: heating at K with excess concentrated sulphuric acid. Concentrated sulphuric acid acts as a dehydrating agent: it removes water from ethanol. (iii) The hydrocarbon is ethene, (). In its electron dot structure the two carbon atoms share two pairs of electrons (a double bond) and each carbon shares one pair with each of its two hydrogen atoms, so every carbon has an octet (see the figure).
    Step-by-step solution →
  3. Q36(a) 5 marks

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

    1. (i)Name three techniques/devices used by human females to avoid pregnancy. Mention the side effects caused by each.
    2. (ii)What will happen if in a human female (a) fertilisation takes place, (b) an egg is not fertilised ?
    Show answer

    Pills, Copper-T and tubectomy prevent pregnancy; after fertilisation the embryo develops in the uterus, otherwise the lining is shed as menstruation.

    (i) Three methods used by women: (1) Oral contraceptive pills, which change the hormonal balance so that no egg is released; side effect: hormonal imbalance, because they change the body's hormonal balance. (2) A Copper-T or loop placed in the uterus; side effect: irritation of the uterus. (3) Surgical blocking of the fallopian tubes (tubectomy), so that the egg cannot reach the uterus; side effect: the surgery can cause infection and other problems if it is not performed properly. (ii) (a) If fertilisation takes place, the zygote divides and is implanted in the thick, blood-rich lining of the uterus; the embryo develops there, getting nutrition from the mother's blood through the placenta, the lining is kept (no menstruation) and the pregnancy continues for about nine months. (b) If the egg is not fertilised, it lives for about one day; the thickened lining of the uterus is not needed, so it breaks down and comes out through the vagina as blood and mucus. This is menstruation, which lasts about two to eight days and recurs roughly every month.
    Step-by-step solution →
    OR
    Q36(b) 5 marks

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

    (i) Draw a diagram showing spore formation in Rhizopus and label the
    1. (a)reproductive and
    2. (b)non-reproductive parts. Why does Rhizopus not multiply on a dry slice of bread ? (ii) Name and explain the process by which reproduction takes place in Hydra.
    Show answer

    Spores grow only on moist surfaces, so Rhizopus does not multiply on dry bread; Hydra reproduces by budding.

    (i) Diagram (see the figure): a moist slice of bread covered by thread-like hyphae; upright stalks rise from them, each ending in a round sporangium full of spores. (a) Reproductive parts: the sporangia and the spores inside them (the blob-on-a-stick structures). (b) Non-reproductive parts: the thread-like hyphae. Rhizopus does not multiply on a dry slice of bread because its spores begin to grow only when they come into contact with a moist surface; a dry slice gives them no moisture, so the spores do not germinate and no hyphae grow. (ii) Hydra reproduces by budding, a kind of asexual reproduction. Its regenerative cells divide repeatedly at one specific site and form an outgrowth, the bud. The bud grows into a tiny individual with a mouth and tentacles and, when fully mature, detaches from the parent to become a new, independent Hydra.
    Step-by-step solution →

Questions 37–39 · 4 marks each

  1. Q37 4 marks

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

    Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
    1. (i)When Mendel crossed pea plants with pure tall and pure short characteristics to produce progeny, which two observations were made by him in plants?[1]
    2. (ii)Write one difference between dominant and recessive trait.[1]
    3. (iii)(a)In a cross with two pairs of contrasting characters (Round Yellow) (Wrinkled Green) Mendel observed types of combinations in generation. By which method did he obtain generation ? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment.[2]
    4. (iii)(b)OR : Justify the statement : “It is possible that a trait is inherited but may not be expressed.”[2]
    Show answer

    plants are all tall; selfing them gives in and shows independent inheritance; a recessive trait can stay hidden.

    (i) (1) All the plants were tall. (2) No plant was short or of a medium height: only one parental trait (tallness) was expressed, not a mixture of the two. (ii) A dominant trait is expressed even when only one copy of its factor is present (as tallness in plants), whereas a recessive trait is expressed only when both copies are recessive (as shortness in plants). (iii)(a) The generation was obtained by self-pollination of the plants (all , round yellow). The ratio is round yellow : round green : wrinkled yellow : wrinkled green , so the parental combinations, round yellow and wrinkled green, appear in the ratio . Conclusions: round and yellow are dominant, and since new combinations (round green, wrinkled yellow) appear, the traits of seed shape and seed colour are inherited independently. (iii)(b) In a cross of pure tall () and pure short () plants, all the plants are tall (): they carry the factor for shortness but do not express it, because tallness is dominant. On self-pollination of the plants, short () plants reappear in (about one in four), so the short trait was inherited by the plants without being expressed.
    Step-by-step solution →
  2. Q38 4 marks

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

    Study the data given below showing the focal length of three concave mirrors A, B and C and the respective distances of objects placed in front of the mirrors :
    CaseMirrorFocal Length (cm)Object Distance (cm)
    1A2045
    2B1530
    3C3020
    1. (i)In which one of the above cases the mirror will form a diminished image of the object ? Justify your answer.[1]
    2. (ii)List two properties of the image formed in case 2.[1]
    3. (iii)(a)What is the nature and size of the image formed by mirror C ? Draw ray diagram to justify your answer.[2]
    4. (iii)(b)OR : An object is placed at a distance of cm from the pole of a concave mirror of focal length cm. Find the position of the image formed in this case.[2]
    Show answer

    Case 1 gives a diminished image; case 2 a real, same-size image at C; mirror C an enlarged virtual image, ; or cm.

    (i) Case 1 (mirror A). Its focal length is cm, so C is at cm; the object at cm is beyond C, and a concave mirror then forms a real, inverted image between F and C that is smaller than the object ( cm, ). (ii) In case 2 the object is at C ( cm ). The image is formed at C; it is real and inverted, and of the same size as the object. (iii)(a) For mirror C the object ( cm) is between P and F ( cm): the image is virtual and erect, and enlarged, formed behind the mirror ( cm, , three times the size of the object), as the ray diagram shows. (iii)(b) With cm and cm, , so cm. The image is formed cm in front of the mirror, on the side of the object; it is real, inverted and magnified ().
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  3. Q39 4 marks

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

    The metals produced by various reduction processes are not very pure. They contain impurities, which must be removed to obtain pure metals. The most widely used method for refining impure metals is electrolytic refining.
    1. (i)What is the cathode and anode made of in the refining of copper by this process ?[1]
    2. (ii)Name the solution used in the above process and write its formula.[1]
    3. (iii)(a)How copper gets refined when electric current is passed in the electrolytic cell ?[2]
    4. (iii)(b)OR : You have two beakers ‘A’ and ‘B’ containing copper sulphate solution. What would you observe after about hours if you dip a strip of zinc in beaker ‘A’ and a strip of silver in beaker ‘B’? Give reason for your observations in each case.[2]
    Show answer

    Impure copper anode and pure copper cathode in acidified copper sulphate; zinc displaces copper from its solution but silver does not.

    (i) Cathode: a thin strip of pure copper; anode: impure copper (a thick block or rod). (ii) Acidified copper sulphate solution; formula (acidified with dilute ). (iii)(a) On passing current, copper from the impure anode dissolves into the solution as ions (), and an equivalent amount of ions from the solution is deposited on the cathode as pure copper (). Soluble impurities go into the solution and insoluble impurities settle below the anode as anode mud, so pure copper collects on the cathode. (iii)(b) Beaker 'A': the blue colour of the solution fades and a reddish-brown coating of copper forms on the zinc strip, because zinc is more reactive than copper and displaces it (). Beaker 'B': no change, because silver is less reactive than copper and cannot displace it from copper sulphate solution.
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