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CBSE Class 10 Science 2026 — Paper 31/7/3/SB

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

30 of its 45 questions also appear in 31/7/1/SB or 31/7/2/SB; 15 are only in this paper.

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Questions 1–9 · 1 mark each

  1. Q1 1 mark

    Also in 31/7/1/SB (Q5), 31/7/2/SB (Q6)

    From the following, identify the sexually transmitted diseases caused by bacteria.
    1. (i)Warts
    2. (ii)Gonorrhoea
    3. (iii)AIDS
    4. (iv)Syphilis Choose the correct option :
    1. (a)(iii) and (iv)
    2. (b)(i) and (iii)
    3. (c)(i) and (ii)
    4. (d)(ii) and (iv)
    Show answer
    (d) (ii) and (iv)
    Step-by-step solution →
  2. Q2 1 mark

    Also in 31/7/1/SB (Q3), 31/7/2/SB (Q1)

    The growth of tendril in pea plant is due to which of the following ?
    1. (a)Effect of chemotropism
    2. (b)Effect of gravity
    3. (c)Rapid growth of part of the tendril away from the point of contact with the support (object)
    4. (d)Rapid growth of the part of the tendril in contact with the support (object)
    Show answer
    (c) Rapid growth of part of the tendril away from the point of contact with the support (object)
    Step-by-step solution →
  3. Q3 1 mark

    Also in 31/7/1/SB (Q6), 31/7/2/SB (Q5)

    Mendel crossed pea plants having yellow seeds (YY) with pea plants having green seeds (yy). Yellow seeds are obtained in progeny which undergoes self-pollination to produce progeny. Select the option showing the result of progeny.
    1. (a)3 green : 1 yellow
    2. (b)1 yellow : 1 green
    3. (c)3 yellow : 1 green
    4. (d)2 green : 2 yellow
    Show answer
    (c) 3 yellow : 1 green
    Step-by-step solution →
  4. Q4 1 mark

    Also in 31/7/1/SB (Q2), 31/7/2/SB (Q4)

    Which of the following is not a function of transpiration ?
    1. (a)It helps in regulation of temperature.
    2. (b)It helps in upward movement of water.
    3. (c)It transports food to different parts of the plant body.
    4. (d)It helps in movement of minerals from the roots to the leaves.
    Show answer
    (c) It transports food to different parts of the plant body.
    Step-by-step solution →
  5. Q5 1 mark
    The capillary net-like (web-like) structure in the kidney associated with a cup-shaped structure is called :
    1. (a)Nephron
    2. (b)Collecting duct
    3. (c)Glomerulus
    4. (d)Bowman's capsule
    Show answer
    (c) Glomerulus
    Step-by-step solution →
  6. Q6 1 mark

    Also in 31/7/1/SB (Q1)

    Name the enzyme that acts on the protein-rich diet inside our stomach :
    1. (a)Pepsin
    2. (b)Amylase
    3. (c)Trypsin
    4. (d)Lipase
    Show answer
    (a) Pepsin
    Step-by-step solution →
  7. Q7 1 mark

    Also in 31/7/1/SB (Q4)

    Identify the hormone that regulates carbohydrate, protein and fat metabolism in the body :
    1. (a)Growth hormone
    2. (b)Adrenaline
    3. (c)Thyroxin
    4. (d)Testosterone
    Show answer
    (c) Thyroxin
    Step-by-step solution →
  8. Q8 1 mark
    For Questions number 8 and 9, 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): In early teenage years, body appearance changes in both boys and girls.

    Reason (R): Body undergoes sexual maturity during puberty.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
    Step-by-step solution →
  9. Q9 1 mark

    Also in 31/7/1/SB (Q8)

    For Questions number 8 and 9, 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): Walls of the ventricles are thicker than those of the atria.

    Reason (R): Thick walls of the ventricles help in preventing the backflow of blood.

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

Questions 10–12 · 2 marks each

  1. Q10 2 marks

    Also in 31/7/1/SB (Q12), 31/7/2/SB (Q11(a))

    Following are the diagrammatic representations of different organisms performing a process. (i) Identify the organisms
    1. (a)and
    2. (b). (ii) Name the process commonly shown in both the figures. (iii) How is this process advantageous to the organisms ?
    (a) (b)
    Show answer

    (a) Hydra and (b) Bryophyllum; both show asexual reproduction, which needs one parent and gives genetically identical offspring quickly.

    (i) (a) Hydra; (b) Bryophyllum (a leaf of Bryophyllum). (ii) Asexual reproduction: budding in (a) and vegetative propagation in (b). (iii) Only one parent is needed, so no mate has to be found and no gametes or fertilisation are involved; the process is quick and simple, so many offspring are produced in a short time, and every offspring is genetically identical to the parent, so characters that already suit the surroundings are passed on unchanged. Plants raised this way, as in (b), bear flowers and fruits earlier than plants grown from seed, and plants that have lost the capacity to produce seeds can still be multiplied.
    Step-by-step solution →
  2. Q11(a) 2 marks
    What will be the impact on an ecosystem if all the bacteria, fungi or microorganisms are removed from the environment ?
    Show answer

    Without decomposers, dead matter and wastes would pile up and nutrients would not return to the soil, so plants and food chains would fail.

    The bacteria, fungi and other microorganisms are the decomposers of an ecosystem, so removing them would stop decomposition altogether. (i) The dead bodies of plants and animals and all their waste products would no longer be broken down and would go on accumulating in the environment. (ii) The simple inorganic nutrients locked inside that dead matter would never be released back into the soil and the air, so the cycling of nutrients would stop; the plants would be left without the raw materials they absorb, and with the producers failing the food chains and the energy flow of the whole ecosystem would break down.
    Step-by-step solution →
    OR
    Q11(b) 2 marks
    Why do food chains in an ecosystem generally consist of only three or four steps ?
    Show answer

    Only about of energy passes to the next level, so after three or four steps too little energy remains for another level.

    Because energy is transferred between trophic levels according to the ten per cent law: only about of the energy available at a level reaches the level above it, while the remaining is spent in the organism's own life processes and lost as heat, or stays in parts that are not eaten or not digested. The energy therefore falls very steeply along the chain — of J captured by the producers about J reaches the herbivores, J the next level and J the one after — so after three or four steps too little energy is left to support a further trophic level.
    Step-by-step solution →
  3. Q12 2 marks

    Also in 31/7/1/SB (Q11(a)), 31/7/2/SB (Q10)

    In a tobacco plant, the male gametes have twenty-four chromosomes. The number of chromosomes in the female gametes is also twenty-four. What is the number of chromosomes in the zygote produced ? How is the chromosome number in the zygote re-established ?
    Show answer

    The zygote has 48 chromosomes; gametes carry half (24) and fertilisation restores the full number.

    The zygote has chromosomes ( from the male gamete from the female gamete). The number is re-established because the gametes are produced by a special reduction division that halves the chromosome number of the parent plant from to ; when a male and a female gamete fuse at fertilisation the two half sets come together, so the zygote regains the full number of characteristic of the tobacco plant, and this halving-and-fusing keeps the chromosome number constant from generation to generation.
    Step-by-step solution →

Questions 13–14 · 3 marks each

  1. Q13 3 marks

    Also in 31/7/1/SB (Q14), 31/7/2/SB (Q13)

    1. (a)Improper waste disposal is a curse to the environment. Explain.
    2. (b)Why are crop fields considered as artificial ecosystems ?
    3. (c)Farmers are using pesticides on the crop plants, but the chances of accumulation of harmful chemicals is greatest in human beings. Why ? Name the phenomenon.
    Show answer

    Waste pollutes and spreads disease; crop fields are made and maintained by people; pesticides build up by biological magnification, most in humans.

    (a) When waste is not segregated at source, it simply accumulates, and much of it is non-biodegradable, such as plastics, glass, metals, chemicals and pesticides. Decomposers cannot break these down, so they remain in the environment for years: they pollute the soil and water, choke drains and water bodies and kill animals that swallow them. Biodegradable waste left in the open rots, gives off foul smells and harmful gases and breeds flies, mosquitoes and micro-organisms that spread disease, while harmful chemicals seep into the soil and groundwater and enter food chains. Burning the waste instead releases poisonous smoke and gases into the air. (b) A crop field is a man-made ecosystem. It is created and maintained by human beings: people decide which crop is grown, plough the land, sow selected seeds, irrigate the field, add manure, fertilisers and pesticides and weed out all other plants, so very few species live there and the field could not sustain itself without this care, unlike a natural ecosystem such as a forest, a lake or a pond. It is still an ecosystem, because the living organisms in it interact with each other and with the abiotic components such as soil, water, air and sunlight. Being made and kept going by people, it is called an artificial ecosystem. (c) Pesticides are non-biodegradable. They are washed into the soil and water and are absorbed by the crop plants along with water and minerals, so they enter the food chain. No organism can break them down or excrete them, so the whole amount taken in with the food stays in the body, and since each consumer eats many organisms of the level below, the concentration increases at every trophic level. Human beings are at the top of the food chain and feed from several food chains, so the greatest concentration collects in their bodies. The phenomenon is called biological magnification (biomagnification).
    Step-by-step solution →
  2. Q14 3 marks
    1. (a)List any two factors on which the amount of water reabsorbed from the filtrate depends in the human kidneys.
    2. (b)Write any two methods used by plants to get rid of excretory products.
    Show answer

    Water reabsorption depends on excess body water and dissolved waste; plants store wastes in vacuoles and as resins and gums in old xylem.

    (a) The amount of water reabsorbed from the filtrate depends on (i) the amount of excess water in the body at that time, and (ii) the amount of dissolved waste that has to be excreted, since the wastes can only be carried out dissolved in water. (b) (i) Many plant waste products are stored in the vacuoles of cells; (ii) Other waste products are stored as resins and gums in the old xylem. (Wastes are also stored in leaves that later fall off and in dead tissues; in addition, plants lose excess water by transpiration and release some wastes into the soil around them.)
    Step-by-step solution →

Question 15 · 4 marks

  1. Q15 4 marks

    Also in 31/7/1/SB (Q15), 31/7/2/SB (Q15)

    Read the following passage and answer the questions that follow : Indian scientists helped crack the DNA code of ‘bread-wheat’ genome, which would help improve global food security. It will allow faster breeding for climate-resilient varieties with drought, heat and frost tolerance, disease-resistant and superior quality grain. It may also help people suffering from allergy to wheat gluten. The information generated will help identify genes controlling yield, grain quality, resistance to diseases, etc. A variety of grains can thus be produced.
    1. (a)Write the full form of DNA.[1]
    2. (b)What type of cell division is involved in the formation of gametes ?[1]
    3. (c)(i)Variations confer an advantage to an individual to survive. Justify.[2]
    4. (c)(ii)OR : How do genes control characteristics or traits ?[2]
    Show answer

    DNA is deoxyribonucleic acid; meiosis forms gametes; suitable variations help survival; genes control traits by controlling protein synthesis.

    (a) DNA is deoxyribonucleic acid. (b) Meiosis, the reduction division: each gamete receives half the number of chromosomes of the parent cell, so the full number is restored when two gametes fuse at fertilisation. (c)(i) Individuals of a species are not all alike, and these differences are variations; depending on its nature, a variation suits its carrier to particular conditions. India has a variety of climatic conditions, so variations among wheat varieties, such as drought, heat or frost tolerance and disease resistance, let farmers grow the variety best suited to the climate of each region, which gives the benefit of survival and better grain quality. In the same way, when the surroundings change, for example a sudden rise in temperature, a population of bacteria is mostly killed, but the few with a heat-resistant variation survive and multiply. A variation therefore gives the individual that carries it a better chance of survival, and it is the basis of evolution. OR (c)(ii) Traits are brought about by proteins, and each protein is made according to the information in a gene, which is a section of DNA. For example, the height of a plant depends on the amount of a plant hormone it makes; the amount of that hormone depends on the efficiency of the enzyme that produces it, and an enzyme is a protein. The gene coding for that enzyme therefore decides how much hormone is made and so how tall the plant grows. If the gene is altered, a less efficient enzyme is made, less hormone is produced and the plant is short. Genes thus control characteristics by controlling the synthesis of proteins.
    Step-by-step solution →

Question 16 · 5 marks

  1. Q16(a) 5 marks

    Also in 31/7/1/SB (Q16(b)), 31/7/2/SB (Q16(a))

    1. (i)Reflex arcs have evolved in animals because the thinking process of the brain is not fast enough for all situations. Draw a flow chart showing the path of a reflex arc.
    2. (ii)Given below are some actions/activities. Identify the part of brain involved in controlling them : (I) Taking a walk in the garden (II) Dancing (III) Blood pressure
    Show answer

    The reflex message turns round at the spinal cord; walking uses the cerebrum, dancing the cerebellum, blood pressure the medulla.

    (i) Path of a reflex arc: Stimulus (for example, a hot object) Receptor (in the skin) Sensory neuron Spinal cord (relay neuron) Motor neuron Effector (muscle of the arm) Response (the hand is withdrawn). The message is switched across in the spinal cord and not in the brain, which is what makes the response immediate. (ii)(I) Taking a walk in the garden: fore-brain (cerebrum), which directs voluntary movement of the leg muscles; the cerebellum helps by maintaining posture and balance. (ii)(II) Dancing: cerebellum (hind-brain), which is responsible for the precision of voluntary actions and for maintaining the posture and balance of the body. (ii)(III) Blood pressure: medulla (hind-brain), which controls involuntary actions.
    Step-by-step solution →
    OR
    Q16(b) 5 marks

    Also in 31/7/1/SB (Q16(a)), 31/7/2/SB (Q16(b))

    1. (i)What are the limitations of electrical impulses in multicellular organisms ?
    2. (ii)(I) In a ‘Touch-me-not’ plant, movement is seen on touch. Compare it with that of animals. (II) Name the type of movement shown by the growth of pollen tube towards an ovule.
    Show answer

    Impulses reach only connected cells and need resetting; touch-me-not folds by water loss while animals use muscle; pollen tube growth is chemotropism.

    (i) Limitations of electrical impulses: (1) they reach only those cells that are connected by nervous tissue, not each and every cell of the body; (2) once a nerve cell has generated and transmitted an impulse it must reset its mechanism before it can send another, so impulses cannot be created and transmitted continuously; (3) the impulse is short-lived, so it cannot by itself bring about a lasting effect such as growth. Multicellular organisms therefore also use chemical communication, which is slower but reaches every cell and lasts longer. (ii)(I) In the Touch-me-not plant there is no nervous tissue and no muscle. The touch is passed from cell to cell as an electrochemical signal, and the cells in the swelling at the base of the leaflet lose water, become flaccid and shrink, so the leaflet folds; the movement is independent of growth and happens at a point different from the point of touch (the swelling at the base of the leaflet), which is why the information of touch has to be passed on from cell to cell. In animals the touch is detected by a receptor, carried along a sensory neuron to the spinal cord or brain and out along a motor neuron, and a muscle contracts; animal cells change shape by means of special proteins in muscle cells, not by changing their water content. Both are rapid responses carried by an electrical signal, but the plant uses no nerves, no muscle and no central nervous system. (ii)(II) Chemotropism.
    Step-by-step solution →

Questions 17–24 · 1 mark each

  1. Q17 1 mark
    Which of the following metals has a very low melting point ?
    1. (a)Copper
    2. (b)Sodium
    3. (c)Aluminium
    4. (d)Caesium
    Show answer
    (d) Caesium
    Step-by-step solution →
  2. Q18 1 mark
    Which of the following compounds is formed on heating limestone with the release of gas ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  3. Q19 1 mark
    Butanoic acid is a four-carbon compound with the functional group :
    1. (a)Carboxylic acid
    2. (b)Aldehyde
    3. (c)Ketone
    4. (d)Alcohol
    Show answer
    (a) Carboxylic acid
    Step-by-step solution →
  4. Q20 1 mark
    Which of the following compound is a member of homologous series of alkene ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  5. Q21 1 mark
    ‘Phenolphthalein’ is an example of :
    1. (a)Natural indicator
    2. (b)Synthetic indicator
    3. (c)Olfactory indicator
    4. (d)Universal indicator
    Show answer
    (b) Synthetic indicator
    Step-by-step solution →
  6. Q22 1 mark

    Also in 31/7/1/SB (Q21), 31/7/2/SB (Q20)

    The soap molecule has a :
    1. (a)Hydrophilic head and a hydrophobic tail
    2. (b)Hydrophobic head and a hydrophilic tail
    3. (c)Hydrophilic head and a hydrophilic tail
    4. (d)Hydrophobic head and a hydrophobic tail
    Show answer
    (a) Hydrophilic head and a hydrophobic tail
    Step-by-step solution →
  7. Q23 1 mark

    Also in 31/7/1/SB (Q20), 31/7/2/SB (Q21)

    Which of the following are exothermic processes ? I. The decomposition of vegetable matter into compost II. Dilution of an acid III. Evaporation of water IV. Sublimation of camphor Options :
    1. (a)I and II
    2. (b)II and III
    3. (c)I and IV
    4. (d)III and IV
    Show answer
    (a) I and II
    Step-by-step solution →
  8. Q24 1 mark

    Also in 31/7/1/SB (Q24), 31/7/2/SB (Q24)

    For Question number 24, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Burning of natural gas is an endothermic reaction.

    Reason (R): Methane gas reacts with oxygen to produce and .

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

Question 25 · 2 marks

  1. Q25 2 marks
    Answer the following :
    1. (a)Why does distilled water not conduct electricity, whereas rainwater does ?
    2. (b)While diluting an acid, why is it recommended that the acid should be added to water and not water to acid ?
    Show answer

    Distilled water has practically no free ions but rainwater has; dilution is strongly exothermic, so acid is added to water.

    (a) Distilled water is very nearly pure water - on distillation the dissolved salts stay behind in the flask and the dissolved gases are driven off by boiling - so it has practically no free ions and cannot carry a current. Rainwater dissolves carbon dioxide from the air as it falls (and sulphur dioxide and oxides of nitrogen where the air is polluted); the carbon dioxide forms carbonic acid, which ionises to give and ions, and these free, moving ions carry the current, so rainwater conducts electricity. (b) Diluting a concentrated acid is a strongly exothermic process. If the acid is added slowly, with stirring, to a large volume of water, the heat given out is shared by all that water and the temperature rises only a little. If water were added to the acid, all that heat would be released in a very small volume of liquid; the mixture can boil at once and splash concentrated acid out of the container (which may also crack), causing severe burns.
    Step-by-step solution →

Questions 26–27 · 3 marks each

  1. Q26(a) 3 marks

    Also in 31/7/1/SB (Q26(a)), 31/7/2/SB (Q27(a))

    A neutral organic compound ‘A’ (molecular formula ) reacts with acidified and on oxidation gives an acidic compound ‘B’. Compound ‘A’ reacts with compound ‘B’ on warming, in the presence of concentrated to give a sweet smelling substance ‘C’. What are A, B and C ? Also write their structures.
    Show answer

    A is ethanol, B is ethanoic acid and C is the ester ethyl ethanoate, .

    A is ethanol, (); B is ethanoic acid (acetic acid), ; C is the sweet-smelling ester ethyl ethanoate (ethyl acetate), . Structures: A (ethanol) B (ethanoic acid) C (ethyl ethanoate)
    Step-by-step solution →
    OR
    Q26(b) 3 marks

    Also in 31/7/1/SB (Q26(b)), 31/7/2/SB (Q27(b))

    Give reasons for the following :
    1. (i)Unsaturated hydrocarbons show addition reactions.
    2. (ii)Detergents are better cleansing agents than soaps.
    3. (iii)Carbon forms only covalent compounds.
    Show answer

    Multiple bonds open to add atoms; detergents give no scum in hard water; carbon shares electrons as gaining or losing four is difficult.

    (i) Unsaturated hydrocarbons contain a carbon-carbon double or triple bond, so their carbon atoms are not bonded to the maximum number of atoms. One part of the multiple bond opens and the new atoms add on with nothing expelled, as in ; (ii) Soaps form insoluble calcium and magnesium salts - a scum - with the and ions of hard water, so they are wasted and do not lather, whereas the charged ends of detergents form no such precipitate and go on cleaning even in hard water; (iii) Carbon has four electrons in its outermost shell, and it can neither gain four electrons (a nucleus of six protons cannot hold ten electrons) nor lose four (far too much energy is needed), so it completes its shell by sharing its four electrons and forms only covalent bonds.
    Step-by-step solution →
  2. Q27 3 marks

    Also in 31/7/1/SB (Q27), 31/7/2/SB (Q26)

    1. (a)Identify the type of reaction taking place and mention why does the beaker become hot.
    2. (b)Write the balanced chemical equation.
    Beaker Water Quick lime (CaO) (Hot to touch)
    Show answer

    It is a combination reaction in which quick lime and water form slaked lime; the beaker gets hot because the reaction is exothermic.

    (a) The reaction is a combination reaction — calcium oxide (quick lime) and water combine to form a single product, calcium hydroxide (slaked lime). The beaker becomes hot because this reaction is exothermic: a large amount of heat energy is released as the reaction takes place, and that heat raises the temperature of the water and of the beaker. (b)
    Step-by-step solution →

Question 28 · 4 marks

  1. Q28 4 marks

    Also in 31/7/1/SB (Q28), 31/7/2/SB (Q28)

    Read the following passage and answer the questions that follow : Salts are formed by reactions of acids and bases. Strong acids react with strong bases to form neutral salts. Weak acids react with strong bases to form basic salts. Acidic salts are formed by strong acids and weak bases. and are salts of strong base () and weak acid (). Bleaching powder is prepared when slaked lime reacts with dry chlorine gas. Crystalline salts are hydrated and lose water on heating. Gypsum on heating at 373 K gives Plaster of Paris.
    1. (a)What is the formula of POP (Plaster of Paris) ? What is its chemical name ?[1]
    2. (b)Name the compound of sodium which is used for softening hard water. What is its chemical formula ?[1]
    3. (c)(i)(I) What is water of crystallization ? (II) Write the preparation of bleaching powder.[2]
    4. (c)(ii)OR : (I) Name an indicator used by visually impaired children. (II) Give one use of bleaching powder.[2]
    Show answer

    POP is ; washing soda softens hard water; dry slaked lime and chlorine give bleaching powder.

    (a) Plaster of Paris is ; its chemical name is calcium sulphate hemihydrate (calcium sulphate half-hydrate). (b) Washing soda, that is sodium carbonate decahydrate; its formula is . (c)(i)(I) Water of crystallization is the fixed number of water molecules present in one formula unit of a salt, held in its crystal — for example holds five such molecules. (II) Bleaching powder is prepared by the action of dry chlorine on dry slaked lime: . OR (c)(ii)(I) Vanilla essence, an olfactory indicator (onion or clove oil is equally acceptable), since it is identified by smell and not by colour. (II) Bleaching powder is used for disinfecting drinking water (also for bleaching cotton and linen, for bleaching wood pulp in the paper industry, and as an oxidising agent).
    Step-by-step solution →

Question 29 · 5 marks

  1. Q29(a) 5 marks

    Also in 31/7/1/SB (Q29(a)), 31/7/2/SB (Q29(a))

    1. (i)Write the balanced chemical equation involved in the extraction of : (I) Hg from HgS (II) Zn from (III) Na from NaCl
    2. (ii)Why does sodium chloride : (I) not conduct electricity in solid state ? (II) have very high melting point ?
    Show answer

    Hg is roasted and heated, Zn calcined and reduced, Na electrolysed; solid NaCl has fixed ions, and strong ionic forces give a high melting point.

    (i) (I) Hg from HgS — roasting, then thermal decomposition: and . (II) Zn from — calcination, then reduction with carbon: and . (III) Na from NaCl — electrolytic reduction of the molten salt. At the cathode: ; at the anode: ; overall . (ii) (I) Sodium chloride is ionic, and in the solid its and ions are held in fixed positions in the rigid crystal lattice by strong electrostatic forces. There are no free ions and no free electrons to carry charge, so the solid does not conduct; molten or dissolved, the lattice breaks down, the ions become free to move, and it conducts. (II) Every ion in the crystal is held by strong electrostatic forces of attraction from the oppositely charged ions all around it, and these forces run through the whole lattice. A large amount of heat energy is needed to overcome them, so sodium chloride melts only at a very high temperature (about K).
    Step-by-step solution →
    OR
    Q29(b) 5 marks

    Also in 31/7/1/SB (Q29(b)), 31/7/2/SB (Q29(b))

    During extraction of metals, electrolytic refining is used to obtain pure metals.
    1. (i)Which material will be used as cathode and anode for refining of zinc metal by this process ?
    2. (ii)Name the electrolyte used in this process. Where do we get pure zinc after passing electric current ?
    3. (iii)How does zinc prevent rusting of iron ? Also, name the process.
    Show answer

    Impure zinc anode, pure zinc cathode and acidified zinc sulphate electrolyte give pure zinc at the cathode; a zinc coat protects iron by galvanisation.

    (i) Both electrodes are made of zinc: the anode is a thick block (plate) of impure zinc, connected to the positive terminal of the battery, and the cathode is a thin strip (sheet) of pure zinc, connected to the negative terminal. (ii) The electrolyte is a solution of zinc sulphate, , acidified with a little dilute sulphuric acid — that is, a solution of a salt of the same metal. On passing the current, zinc from the impure anode dissolves into the solution and pure zinc is deposited at the cathode, so we get the pure zinc on the cathode. The insoluble impurities settle below the anode as anode mud. (iii) A layer of zinc is coated on the iron article. It protects the iron in two ways: it acts as a barrier that keeps air (oxygen) and moisture away from the iron surface, and because zinc is more reactive than iron it is oxidised in preference to iron, so even a scratched coating goes on protecting the iron beneath (sacrificial protection). The process is called galvanisation.
    Step-by-step solution →

Questions 30–32 · 1 mark each

  1. Q30 1 mark

    Also in 31/7/1/SB (Q30), 31/7/2/SB (Q31)

    Pushkar was studying a light phenomenon and wrote down the following statements in that context :
    1. (i)A concave mirror can be used as a shaving mirror because it can form an enlarged, virtual image when the object is placed between the pole and focus.
    2. (ii)A convex lens always forms real, inverted images, regardless of the position of the object.
    3. (iii)The relationship between object distance (), image distance () and focal length () of the lens is given by . Choose the option enlisting all the correct statements :
    1. (a)(i) and (ii)
    2. (b)(i) and (iii)
    3. (c)(ii) and (iii)
    4. (d)(i), (ii) and (iii)
    Show answer
    (b) (i) and (iii)
    Step-by-step solution →
  2. Q31 1 mark

    Also in 31/7/1/SB (Q31), 31/7/2/SB (Q30)

    During a lab experiment, Bhavna uses a solenoid connected to a battery. She notes down the following facts :
    1. (i)The pattern of magnetic field lines inside the solenoid is similar to a bar magnet.
    2. (ii)The north pole and south pole of the current-carrying solenoid can be identified by applying the right hand thumb rule.
    3. (iii)A solenoid can be used to magnetise a piece of a soft iron, making it an electromagnet. Choose the option enlisting all the correct statements :
    1. (a)(i) and (iii)
    2. (b)(ii) and (iii)
    3. (c)(i) and (ii)
    4. (d)(i), (ii) and (iii)
    Show answer
    (d) (i), (ii) and (iii)
    Step-by-step solution →
  3. Q32 1 mark

    Also in 31/7/1/SB (Q32), 31/7/2/SB (Q32)

    For Question number 32, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): For a medium of given refractive index, the value of the angle of refraction corresponding to an angle of incidence is not a constant.

    Reason (R): With change in angle of incidence, the value of angle of refraction changes. Hence, the ratio of sine of angle of incidence to sine of angle of refraction remains constant.

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

Questions 33–34 · 2 marks each

  1. Q33 2 marks
    1. (a)Copy the diagrams ‘a’ and ‘b’ and mark the direction of the magnetic fields produced around the current-carrying conductors.
    2. (b)Compare the strength of magnetic field at points (i) P and Q, and (ii) Q and R.
    2 cm O P I = 1 A ‘a’ 2 cm 4 cm O Q R I = 2 A ‘b’
    Show answer

    Fields circle anticlockwise in a and clockwise in b (seen from above); the field at Q is twice that at P and R.

    (a) The field lines are the circles already drawn round each conductor; the right-hand thumb rule fixes which way round they go. In diagram ‘a’ the current flows upwards, so seen from above the arrows run anticlockwise round the wire - along the near side of the loop they point to the right. In diagram ‘b’ the current flows downwards, so seen from above the arrows run clockwise, opposite to ‘a’, and both of its circles are marked the same way as each other. (b) Using : (i) at P the current is A at cm while at Q it is A at the same cm, so - the field at Q is twice as strong as at P. (ii) At Q and R the current is the same A, and cm is twice cm, so - the field at Q is twice as strong as at R. Taken together, , so the fields at P and R are equal.
    Step-by-step solution →
  2. Q34(a) 2 marks

    Also in 31/7/1/SB (Q33(a)), 31/7/2/SB (Q33(a))

    Calculate the total resistance of the given circuit and find the total current flowing in it.
    R1 = 1 Ω R3 = 6 Ω 12 V R2 = 10 Ω R4 = 4 Ω
    Show answer

    The total resistance is and the total current is 2 A.

    Total resistance of the circuit , and the total current flowing in it A.
    Step-by-step solution →
    OR
    Q34(b) 2 marks

    Also in 31/7/1/SB (Q33(b)), 31/7/2/SB (Q33(b))

    How would you join three resistors each of resistance , so that the equivalent resistance of the combination is
    1. (i), and
    2. (ii) ?
    Show answer

    For , join two in parallel and the third in series with them; for , join two in series and the third across them.

    (i) : join two of the resistors in parallel and connect the third in series with that pair — . (ii) : join two of the resistors in series and connect the third in parallel across that pair — .
    Step-by-step solution →

Questions 35–37 · 3 marks each

  1. Q35 3 marks

    Also in 31/7/1/SB (Q37), 31/7/2/SB (Q36)

    What is the use of fuse in domestic electric circuits ? How is its role different from the role of a green coloured wire given in domestic electric circuits ? What will happen if an electrician installs a fuse with higher-rated value than prescribed for a given electric circuit ?
    Show answer

    A fuse breaks the circuit on excess current; the earth wire protects the user from shock; an over-rated fuse fails to protect.

    Use of the fuse: a fuse is a safety device connected in series with the live wire of a domestic circuit. It is a short piece of wire of high resistivity and low melting point. If the current exceeds the rated value — because of overloading or a short circuit — the heat produced () melts the fuse wire, the circuit is broken and the current stops, so the wiring and the appliances are protected from damage and from fire. Difference from the green wire: the green wire is the earth wire, connected to a metal plate buried in the ground and to the metal body of the appliance. It normally carries no current. If the live wire touches the metal body, it provides a low resistance path for that current to flow into the earth, keeping the body at the potential of the earth. So the fuse protects the **circuit** and the **appliances** from excessive current by breaking the circuit, while the earth wire protects the **user** from electric shock by conducting leakage current safely away; the fuse is in series in the live wire, the earth wire is attached to the metal casing. Higher-rated fuse: it will not melt at the current for which the circuit was designed, so during an overload or a short circuit the large current goes on flowing. The wiring, switches and appliances are overheated, the insulation may melt, the appliances may be damaged and a fire may be caused — the fuse fails to protect the circuit at all.
    Step-by-step solution →
  2. Q36 3 marks
    1. (a)What is meant by Tyndall effect ?
    2. (b)How is scattering of light different from reflection of light ?
    3. (c)What will happen if white light (sunlight) is scattered from (i) very small particles of medium, and (ii) very large particles of medium ?
    Show answer

    Tyndall effect is scattering by colloidal particles; scattering depends on particle size and colour, unlike reflection; small particles give blue, large give white.

    (a) The Tyndall effect is the scattering of a beam of light by the very fine, colloidal particles suspended in a medium, which makes the path of the beam visible from the side - as when sunlight enters a smoke-filled room through a small hole, or shines through the mist in a dense forest. (b) Scattering of light depends on the size of the scattering particles and on the wavelength (colour) of the light, whereas reflection of light is independent of both. In reflection the light meets a surface and is turned back into the same medium along one definite direction, obeying the laws of reflection (angle of reflection = angle of incidence), and its colour is unchanged; in scattering the light is taken up by fine particles suspended in the medium and sent out again in all directions, and very fine particles scatter mainly the shorter wavelengths, so the scattered light can differ in colour from the incident light. (c) (i) Very small particles scatter mainly the shorter wavelengths, so the scattered light appears **blue** - this is why the clear sky is blue. (ii) Very large particles scatter all wavelengths almost equally, so the scattered light appears **white** - this is why clouds and fog look white.
    Step-by-step solution →
  3. Q37 3 marks

    Also in 31/7/1/SB (Q35)

    State the reasons for myopia. With the help of a ray diagram, show the :
    1. (a)Image formation by myopic eye, and
    2. (b)Correction of myopic eye by using an appropriate lens.
    Show answer

    Myopia arises from excessive lens curvature or an elongated eyeball; a concave lens of suitable power brings the image back onto the retina.

    Reasons for myopia: the image of a distant object is formed in front of the retina, because of (i) excessive curvature of the eye lens, which gives it too short a focal length and too much converging power, and / or (ii) elongation of the eyeball, which puts the retina too far behind the lens. The far point of the eye then lies at a finite distance instead of at infinity, so distant objects are seen blurred while near objects are seen clearly. (a) Ray diagram of the myopic eye: parallel rays from a distant object are converged by the eye lens and meet at a point in front of the retina, then spread out again and fall on the retina as a blurred patch. (b) Ray diagram of the correction: a concave (diverging) lens of suitable power is placed in front of the eye. It diverges the parallel rays so that they appear to come from the far point of the eye; the eye lens then brings them to a sharp focus on the retina. The focal length of the correcting lens equals the distance of the far point, so its power is negative — , .
    Step-by-step solution →

Question 38 · 4 marks

  1. Q38 4 marks

    Also in 31/7/1/SB (Q38), 31/7/2/SB (Q38)

    Read the following passage and answer the questions that follow : Two students, A and B were performing an activity on refraction through glass slab and refraction through prism respectively. Student ‘A’ observed that the emergent light ray is parallel to the direction of incident ray, while student ‘B’ observed that the emergent ray is making a certain angle with respect to the direction of the incident ray in the prism, called angle of deviation. He also noted that the angle of deviation is different for different colours of light.
    1. (a)When white light passes through a glass slab, it does not show dispersion, while it shows dispersion when it passes through a glass prism. Why ?[1]
    2. (b)Why do we get spectrum when white light passes through tiny water droplets present in air ?[1]
    3. (c)(i)For a given pair of mediums, if we change the angle of incidence gradually, will it change the speed of light in that medium ? Explain.[2]
    4. (c)(ii)OR : Calculate the speed of light in a given medium if angle of incidence in air is and angle of refraction in the medium is . Given that speed of light in air is m/s.[2]
    Show answer

    Parallel slab faces cancel dispersion, inclined prism faces add it; droplets act as prisms; speed does not depend on ; OR m/s.

    (a) White light is dispersed at the first refracting surface in both cases, because glass has a slightly different refractive index for each colour. The two faces of a glass **slab** are parallel, so the refraction at the second face is equal and opposite to that at the first; the colours recombine and the emergent beam is white, parallel to the incident beam and only shifted sideways. The two faces of a **prism** are inclined to each other, so the deviations at the two faces add instead of cancelling, each colour leaves in a different direction, and a spectrum is seen. (b) The tiny water droplets act as tiny prisms. Sunlight entering a droplet is refracted and dispersed at its surface, the separated colours are reflected internally at the back surface of the drop, and they are refracted and dispersed again as they leave the drop. Each colour is deviated by a different total amount, so the light emerging is spread into a band of colours — the spectrum we see as a rainbow. (c)(i) No. The speed of light in a medium is , and the refractive index is a property of the medium and of the colour of the light, not of the angle of incidence. Changing the angle of incidence changes the angle of refraction in just the way that keeps equal to the same , so the light travels in a different direction inside the medium but at the same speed. (c)(ii) , so m/s.
    Step-by-step solution →

Question 39 · 5 marks

  1. Q39(a) 5 marks
    1. (i)What is meant by resistance in an electric circuit ?
    2. (ii)In a series electric circuit comprising a resistor made of a metallic wire, the ammeter reads 500 mA. If the length of wire is doubled and area of cross-section is made half, what will be the new reading of ammeter ?
    3. (iii)How will the resistivity of a wire be affected if both, the length of the wire and its area of cross-section, are doubled ? Justify your answer.
    Show answer

    Resistance opposes current, ; the ammeter reads 125 mA; resistivity is unchanged as it depends only on the material and temperature.

    (i) Resistance is the property of a conductor by which it opposes the flow of electric current through it. It is measured by the ratio of the potential difference across the conductor to the current through it, , and its SI unit is the ohm (); a conductor has a resistance of when a potential difference of V across it drives a current of A. (ii) For a wire, . With the length doubled and the area of cross-section halved, . The battery is unchanged, so the same potential difference acts and mA. The ammeter will read mA, that is A. (iii) The resistivity is not affected at all - it stays exactly the same. Resistivity is a property of the **material** of the wire (and of its temperature) only, not of the size or shape of the piece. (Here the resistance is unchanged too: doubling the length doubles it and doubling the area halves it, so keeps its value.)
    Step-by-step solution →
    OR
    Q39(b) 5 marks
    1. (i)State Ohm’s law.
    2. (ii)Derive an expression for equivalent resistance of parallel combination of three resistors having resistances , and .
    3. (iii)V-I graph is given for two wires having identical shapes and sizes, but made of two different materials A and B. Out of A and B, which one is a better choice for making a heating element ? Justify your answer.
    B A Potential difference (V) Current I (A)
    Show answer

    Ohm's law is ; for parallel resistors ; material B suits a heating element.

    (i) Ohm's law: at constant temperature, the potential difference across the ends of a metallic conductor is directly proportional to the current flowing through it; , so , where the constant is the resistance of the conductor. (ii) Draw a circuit diagram with , and connected between the same two points across a battery and a plug key, as in the figure, marking the main current and the branch currents , , . In this parallel combination the same potential difference acts across all three resistors and the main current divides, so . With , , and , this gives ; cancelling the common leaves . (iii) Material B. On the given graph the slope of each line is , which is the resistance of that wire, and B's line is the steeper, so . The two wires have the same length and the same area of cross-section, so from B has the greater resistivity - and a high resistivity is the property needed in the material of a heating element. For the same current, is also larger for B.
    Step-by-step solution →

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