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CBSE Class 10 Science 2025 — Paper 31(B)/S

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

1 of its 44 questions has been set in other papers too (each one says where); the other 43 are only in this paper.

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Questions 1–20 · 1 mark each

  1. Q1 1 mark
    The chemical reaction of solid calcium oxide with water is :
    1. (a)Exothermic and the pH of the solution formed is less than .
    2. (b)Endothermic and the pH of the solution formed is less than .
    3. (c)Endothermic and the pH of the solution formed is more than .
    4. (d)Exothermic and the pH of the solution formed is more than .
    Show answer
    (d) Exothermic and the pH of the solution formed is more than .
    Step-by-step solution →
  2. Q2 1 mark
    In which one of the following decomposition reactions energy is released ?
    1. (a)Electrolysis of water
    2. (b)Decomposition of silver chloride
    3. (c)Decomposition of vegetable matter into compost
    4. (d)Decomposition of lead nitrate
    Show answer
    (c) Decomposition of vegetable matter into compost
    Step-by-step solution →
  3. Q3 1 mark
    Juice of tamarind turns blue litmus to red. It is because of the presence of an acid called :
    1. (a)Oxalic acid
    2. (b)Lactic acid
    3. (c)Methanoic acid
    4. (d)Tartaric acid
    Show answer
    (d) Tartaric acid
    Step-by-step solution →
  4. Q4 1 mark
    Which one of the following oxides reacts with HCl as well as NaOH to form corresponding salt and water ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  5. Q5 1 mark
    Consider the following cases/reactions :
    1. (i)
    2. (ii)
    3. (iii)
    4. (iv) The cases/reactions in which new products will form are :
    1. (a)(i) and (iv)
    2. (b)(i) and (ii)
    3. (c)(ii) and (iii)
    4. (d)(i), (ii) and (iv)
    Show answer
    (a) (i) and (iv)
    Step-by-step solution →
  6. Q6 1 mark
    A hydrocarbon which is different from the other three is :
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (b)
    Step-by-step solution →
  7. Q7 1 mark
    One formula unit of which one of the following salts has seven molecules of water of crystallisation ?
    1. (a)
    2. (b)
    3. (c)
    4. (d)
    Show answer
    (c)
    Step-by-step solution →
  8. Q8 1 mark
    Select the correct statements from the following about the human heart :
    1. (i)Right atrium receives oxygenated blood from lungs through pulmonary artery.
    2. (ii)Right atrium receives deoxygenated blood through vena cava from upper and lower body.
    3. (iii)Left atrium transfers oxygenated blood to left ventricle which sends it to various parts of the body.
    4. (iv)Left atrium transfers oxygenated blood to aorta which sends it to various parts of the body.
    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(iii) and (iv)
    4. (d)(i) and (iv)
    Show answer
    (b) (ii) and (iii)
    Step-by-step solution →
  9. Q9 1 mark
    In the human alimentary canal, when the process of digestion is completed, the
    1. (i)carbohydrates,
    2. (ii)fats, and
    3. (iii)proteins get converted respectively into :
    1. (a)(i) glucose, (ii) fatty acids and (iii) amino acids
    2. (b)(i) glucose, (ii) fatty acids and glycerol and (iii) amino acids
    3. (c)(i) fatty acids and glycerol, (ii) amino acids and (iii) glucose
    4. (d)(i) amino acids, (ii) fatty acids and glycerol and (iii) sugars
    Show answer
    (b) (i) glucose, (ii) fatty acids and glycerol and (iii) amino acids
    Step-by-step solution →
  10. Q10 1 mark
    A gland which does not occur as a pair in the body of human beings is :
    1. (a)Adrenal
    2. (b)Ovary
    3. (c)Pituitary
    4. (d)Testis
    Show answer
    (c) Pituitary
    Step-by-step solution →
  11. Q11 1 mark
    The part of a seed which is a source of food during its germination is :
    1. (a)Radicle
    2. (b)Cotyledon
    3. (c)Plumule
    4. (d)Embryo
    Show answer
    (b) Cotyledon
    Step-by-step solution →
  12. Q12 1 mark

    Also in 2024 · 31/2/1 (Q12), 2024 · 31/2/2 (Q12), 2024 · 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
    An optical device always forms images of magnification . This device is :
    1. (a)Concave mirror
    2. (b)Plane mirror
    3. (c)Convex lens
    4. (d)Concave lens
    Show answer
    (b) Plane mirror
    Step-by-step solution →
  14. Q14 1 mark
    Select the correct statements from the following in the context of the human eye :
    1. (i)Most of the refraction of light rays entering the eye occurs at the crystalline lens.
    2. (ii)The diameter of the eye ball is about cm.
    3. (iii)Iris is a dark muscular diaphragm which controls the size of the pupil.
    4. (iv)While focusing on the objects at different distances, the distance between the lens and the retina is adjusted by the ciliary muscles.
    1. (a)(i) and (iv)
    2. (b)(ii) and (iii)
    3. (c)(i), (ii) and (iv)
    4. (d)(ii), (iii) and (iv)
    Show answer
    (b) (ii) and (iii)
    Step-by-step solution →
  15. Q15 1 mark
    Four wires a, b, c and d are made of the same materials. If denotes length and denotes radius, then the wire which offers least resistance to the flow of electric current has length and radius respectively as :
    1. (a) and
    2. (b) and
    3. (c) and
    4. (d) and
    Show answer
    (a) and
    Step-by-step solution →
  16. Q16 1 mark
    Select from the following, a food chain in which the organism of the second trophic level is missing :
    1. (a)small fish, zooplankton, phytoplankton, large fish
    2. (b)frog, snake, grass, grasshopper, eagle
    3. (c)grass, frog, tiger, snake
    4. (d)grass, goat, lion
    Show answer
    (c) grass, frog, tiger, snake
    Step-by-step solution →
  17. Q17 1 mark
    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Sodium sulphate is a neutral salt.

    Reason (R): Sodium sulphate is a salt formed by a strong acid and a strong base.

    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 →
  18. Q18 1 mark
    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): Ozone layer protects the surface of the earth from the harmful ultraviolet radiations.

    Reason (R): Chlorofluorocarbons (CFCs) are responsible for the depletion of the ozone layer present at the higher levels of the atmosphere.

    1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
    2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    3. (c)Assertion (A) is true, but Reason (R) is false.
    4. (d)Assertion (A) is false, but Reason (R) is true.
    Show answer
    (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
    Step-by-step solution →
  19. Q19 1 mark
    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

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

    Reason (R): The amount of oxygen dissolved in pond/river water is comparatively higher than the amount of oxygen present in air.

    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 →
  20. Q20 1 mark
    Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (a), (b), (c) and (d) as given below.

    Assertion (A): The magnification of the virtual images produced by concave mirrors is always more than .

    Reason (R): The virtual images formed by concave mirrors are erect and magnified.

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

Questions 21–26 · 2 marks each

  1. Q21 2 marks
    When magnesium ribbon is burnt in oxygen, an ash of white colour is produced. Name the type of reaction giving justification for your answer. Write the balanced chemical equation for the reaction that occurs.
    Show answer

    It is a combination reaction: magnesium and oxygen combine to form a single product, magnesium oxide, the white ash.

    It is a combination reaction, because two substances — magnesium and oxygen — combine to form a single product, magnesium oxide, the white ash. The balanced equation is . The same reaction is also an oxidation of magnesium, and it gives out heat and light, so it is exothermic.
    Step-by-step solution →
  2. Q22 2 marks
    Photosynthesis takes place in the leaves of a plant and the food prepared by the leaves reaches other parts of the plant. Name and explain the process involved in the phenomenon.
    Show answer

    Translocation by the phloem: food loaded into sieve tubes raises the pressure, which pushes the sap to roots, fruits, seeds and buds.

    The process is **translocation**, and it is carried out by the phloem. Sucrose and other soluble products of photosynthesis are loaded from the leaf into the sieve tubes by the companion cells using energy from ATP. The sap inside is then concentrated, so water enters it by osmosis from the xylem and the pressure in the tube rises. That pressure pushes the sap along to parts where the pressure is lower — roots, fruits, seeds and growing buds — where the food is unloaded and used or stored. Because a pressure difference is what drives it, translocation can carry food upwards as well as downwards, unlike the xylem, which moves water and minerals upwards only.
    Step-by-step solution →
  3. Q23(a) 2 marks
    Where are auxins synthesised ? How are they related to phototropism ?
    Show answer

    Auxins are synthesised at the shoot tip; they collect on the shaded side, making it grow longer, so the shoot bends towards light.

    Auxins are synthesised at the **tip** (the apical region) of a shoot, and they then move down to the region just behind the tip where the cells are elongating. Their relation to phototropism is this: when light falls on a shoot from one side, auxin moves away from the illuminated side and collects on the shaded side. Auxin makes cells grow longer, so the cells on the shaded side elongate more than those on the lit side; that side of the shoot becomes longer and the shoot therefore bends **towards** the light. That directional growth in response to light is phototropism.
    Step-by-step solution →
    OR
    Q23(b) 2 marks
    There is a hormone which regulates carbohydrates, proteins and fat metabolism in the human body. Name the hormone and the gland which secretes it. State the importance of iodised salt in our diet.
    Show answer

    Thyroxine is secreted by the thyroid gland, which needs iodine from iodised salt to make it; lack of iodine causes goitre.

    The hormone is **thyroxine**, and it is secreted by the **thyroid gland**, which lies in the neck. Iodised salt matters because the thyroid gland needs iodine in order to make thyroxine, and the body cannot make iodine for itself — it has to come from the diet. A trace of iodine added to table salt supplies it, so that enough thyroxine is made and the metabolism of carbohydrates, proteins and fats stays properly regulated. If the diet is short of iodine the gland cannot make enough thyroxine and the person may develop goitre, in which the thyroid enlarges and the neck swells.
    Step-by-step solution →
  4. Q24 2 marks
    List four characteristics of the images formed by convex mirrors.
    Show answer

    The image is virtual, erect, diminished and formed between the pole and the principal focus, behind the mirror.

    For every position of the object, the image formed by a convex mirror is: (i) virtual — it lies behind the mirror and cannot be caught on a screen; (ii) erect — the same way up as the object, since is positive; (iii) diminished — always smaller than the object, since lies between and ; and (iv) formed between the pole and the principal focus of the mirror, since lies between and .
    Step-by-step solution →
  5. Q25 2 marks
    State Ohm’s law. Give reason why the shape of voltage versus current graph for a conductor should be a straight line passing through the origin.
    Show answer

    At constant temperature (Ohm's law); the graph is a straight line through the origin because is constant and when .

    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 and hence , where the constant is the resistance of the conductor. The graph is a straight line because is constant, so has the form : equal increases in current give equal increases in potential difference and every reading has the same ratio . It passes through the origin because putting gives — with no current there is no potential difference across the conductor — so is itself a reading. The slope of the line is the resistance .
    Step-by-step solution →
  6. Q26 2 marks
    In a straight conductor, the current is flowing vertically downwards. State the pattern of the magnetic field lines around the conductor. Name the rule which helps us to determine the direction of the magnetic field lines in this case.
    Show answer

    Concentric circles in horizontal planes round the wire, clockwise as seen from above for a downward current; the right-hand thumb rule.

    The magnetic field lines around the conductor are concentric circles centred on the wire, lying in planes at right angles to it — so, the wire being vertical, in horizontal planes. The circles are crowded together close to the wire and become farther apart with distance, showing that the field is strongest near the conductor and weakens away from it. With the current flowing vertically downwards, the lines are directed clockwise as seen from above (anticlockwise as seen from below). The rule used to find this direction is the **right-hand thumb rule** (Maxwell’s corkscrew rule): hold the conductor in the right hand with the thumb pointing along the current, and the curl of the fingers gives the direction of the field lines.
    Step-by-step solution →

Questions 27–33 · 3 marks each

  1. Q27 3 marks
    Give one chemical equation each for the chemical reaction in which the following changes occur (Mention the change in temperature/colour and/or the compound precipated with the equation) :
    1. (a)Change in temperature
    2. (b)Change in colour
    3. (c)Formation of precipitate
    Show answer

    Quicklime in water gets hot; iron turns blue copper sulphate pale green; sodium sulphate with barium chloride gives white barium sulphate.

    (a) Change in temperature — quicklime reacts with water in an exothermic reaction, so the mixture becomes hot: (temperature rises). (b) Change in colour — an iron nail placed in blue copper sulphate solution displaces copper, and the blue changes to the pale green of iron sulphate: (blue to pale green, with red-brown copper deposited). (c) Formation of precipitate — sodium sulphate solution mixed with barium chloride solution gives a white precipitate of barium sulphate: (white precipitate of barium sulphate).
    Step-by-step solution →
  2. Q28 3 marks
    What is chlor-alkali process ? Write chemical equation for the reaction involved in this process. Name the electrode at which
    1. (i)chlorine gas, and
    2. (ii)hydrogen gas is given off.
    Show answer

    Brine electrolysis gives sodium hydroxide, chlorine and hydrogen; chlorine is given off at the anode and hydrogen at the cathode.

    The chlor-alkali process is the process in which electricity is passed through a concentrated aqueous solution of sodium chloride (brine); the brine decomposes to give sodium hydroxide solution, chlorine gas and hydrogen gas. It is called 'chlor-alkali' because of two of its products — 'chlor' for chlorine and 'alkali' for sodium hydroxide. The reaction is . (i) Chlorine gas is given off at the **anode**, the positive electrode. (ii) Hydrogen gas is given off at the **cathode**, the negative electrode (and sodium hydroxide is formed in the solution near the cathode).
    Step-by-step solution →
  3. Q29(a) 3 marks
    1. (i)Name the movement that occurs all along the gut in the human digestive system. How do these movements help in the digestion of food ?
    2. (ii)Where is bile juice stored in the human body ? List its two roles.
    Show answer

    Peristalsis pushes food along and mixes it with digestive juices; bile is stored in the gall bladder and neutralises acid and emulsifies fats.

    (i) The movement is **peristalsis** (peristaltic movement) — rhythmic waves of contraction and relaxation of the smooth muscles in the wall of the alimentary canal, which occur all along the gut. It helps digestion in two ways: it pushes the food steadily forward through the whole canal so that each region gets the food for the right time, and it churns the food and mixes it thoroughly with the digestive juices, so that the enzymes act on every part of it and the digested food is brought into contact with the absorbing surface. (ii) Bile juice is made in the liver but is **stored in the gall bladder**. Its two roles are: (1) it is alkaline, so it neutralises the acidic food coming from the stomach and makes the medium alkaline, which the pancreatic enzymes need in order to act; and (2) its bile salts **emulsify** fats — they break large fat globules into many small globules, greatly increasing the surface area so that the enzyme lipase can digest the fat efficiently.
    Step-by-step solution →
    OR
    Q29(b) 3 marks
    Give reason for each of 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 gas.
    3. (iii)During anaerobic respiration, a 3-carbon molecule is formed as an end product instead of in human beings.
    Show answer

    Alveoli need dense capillaries for fast diffusion; haemoglobin carries poorly soluble oxygen; with too little oxygen, pyruvate becomes 3-carbon lactic acid.

    (i) Because the alveoli are the site of gas exchange, and exchange happens only by diffusion. A dense capillary network gives a very large surface of blood in contact with the air, brings the blood within one cell's thickness of the alveolar air so the diffusion path is extremely short, and — because the blood keeps flowing — keeps carrying oxygen away and bringing carbon dioxide in, so the concentration difference across the wall stays steep. All three make the exchange fast enough for the body's needs. (ii) Because oxygen is only very slightly soluble in blood plasma, so it must be carried by a pigment: haemoglobin, whose iron-containing haem groups have a high affinity for oxygen and bind it reversibly, picking it up in the lungs and releasing it in the tissues. Carbon dioxide, on the other hand, is about twenty times more soluble in plasma, so enough of it can be carried simply dissolved (mainly as bicarbonate) and it needs no pigment. (iii) Because glucose (6 carbon) is first broken down in the cytoplasm by glycolysis into two molecules of pyruvate (3 carbon), a step that needs no oxygen; the complete breakdown of pyruvate to carbon dioxide occurs only in the mitochondria and **requires** oxygen. In human muscles during vigorous exercise oxygen is in short supply, so pyruvate cannot take that route and is instead converted into lactic acid, which is also a 3-carbon molecule. Hence a 3-carbon end product is formed instead of .
    Step-by-step solution →
  4. Q30 3 marks
    1. (a)List two constituents of Central Nervous System (CNS). How are these components protected ?
    2. (b)Write two limitations of the use of electrical impulses.
    Show answer

    The CNS (brain, spinal cord) is protected by bone, meninges and fluid; electrical impulses reach only nerve-connected cells and need reset time.

    (a) The two constituents of the central nervous system are the **brain** and the **spinal cord**. Both are very delicate and both are protected in the same three ways: the brain is enclosed in a bony box, the cranium (skull), and the spinal cord runs inside the vertebral column (backbone); inside the bone each is wrapped in three membranes called the meninges; and between the membranes there is cerebrospinal fluid, which cushions them and acts as a shock absorber. (b) Two limitations of the use of electrical impulses are: (1) an electrical impulse travels only along nerve fibres, so it can reach only those cells that are connected by nervous tissue and not each and every cell of the body; and (2) after a cell has generated and transmitted an impulse, it needs time to reset its ion distribution before it can generate and transmit a new one, so impulses cannot be created and sent continuously. These are the reasons animals also use chemical communication through hormones.
    Step-by-step solution →
  5. Q31 3 marks
    Define the term 'power of accommodation' of human eye. Write the name of the part of eye which plays a major role in this process and explain what happens when the human eye focuses
    1. (i)on nearby objects, and
    2. (ii)on distant objects.
    Show answer

    Power of accommodation is the eye lens's ability to change its focal length; ciliary muscles contract for near objects and relax for distant ones.

    The power of accommodation of the human eye is the ability of the eye lens to adjust its own focal length, by changing its curvature, so that the images of objects lying at different distances are always formed sharply on the retina. The part of the eye that plays the major role is the **ciliary muscle**, which acts on the flexible eye lens through the suspensory ligaments. (i) On a **nearby** object: the ciliary muscles contract, the tension in the suspensory ligaments is released, the eye lens becomes thicker and more curved, its focal length decreases (its power increases), and the strongly diverging rays from the near object are converged on to the retina. Taking the retina at cm and the near point at cm, , so cm ( D). (ii) On a **distant** object: the ciliary muscles relax, the suspensory ligaments become taut, the eye lens becomes thinner and less curved, its focal length increases (its power decreases), and the nearly parallel rays from the distant object are focused on the retina. With at infinity, cm ( D).
    Step-by-step solution →
  6. Q32 3 marks
    1. (a)What is an electric fuse ? Explain its role in a domestic circuit.
    2. (b)An electric heater of rating 2 kW; 220 V is to be operated in a room having fuse rating of 5 A. What is likely to happen when the heater is switched on ? Justify your answer with necessary calculations.
    Show answer

    A fuse melts and breaks the circuit on excess current; the heater draws 9.09 A, more than 5 A, so the fuse will blow.

    (a) An electric fuse is a safety device made of a short piece of thin wire of a metal or alloy of high resistance and low melting point (such as a tin-lead alloy), connected in series in the **live** wire of a circuit. Its role in a domestic circuit is to protect the circuit and the appliances from damage due to a short circuit or overloading: so long as the current is below the rated value the fuse simply carries it, but if the current exceeds that value the heat produced () melts the fuse wire, the circuit is broken and the current stops, preventing damage to the appliances and the wiring and preventing fire. (b) Current drawn by the heater A. This is much greater than the A rating of the fuse (the excess is about A). Therefore the fuse wire will melt — the fuse will blow — as soon as the heater is switched on; the circuit will break and the heater will not run. Equivalently, a A fuse at V can pass at most W kW, whereas the heater needs kW. A fuse of at least A would be required.
    Step-by-step solution →
  7. Q33 3 marks
    Consider the following food chain : Plants → Deer → Lion In this food chain, state the impact of removing all the organisms of the second trophic level (i.e., Deer) on the organisms of the first and the third trophic levels ? Will the impact be the same for the organisms in the third trophic level if they were part of the food web of the ecosystem ? Justify your answer.
    Show answer

    Plants increase and become overcrowded; lions starve; in a food web lions switch to other prey, so the impact is smaller.

    Impact on the **first** trophic level (plants): with no deer to graze on them the plants are no longer checked, so their population increases rapidly and becomes overcrowded; they then compete with one another for sunlight, water, minerals and space, many are stunted or die, and the habitat becomes unbalanced and degraded. Impact on the **third** trophic level (lions): in this chain the deer are the lions' only food, so the lions lose their entire food supply — they starve, their numbers fall sharply, and they either die or migrate away in search of prey. **No**, the impact on the third trophic level would **not** be the same if the lions were part of a food web. In a food web an organism has several sources of food, so a lion feeds not only on deer but also on other herbivores such as rabbits, wild boar or antelope. If the deer were removed, only one of its several food routes would be lost; the lion would switch to the alternative prey, its food supply would be reduced but not cut off, and its population would survive with only a small fall. This is exactly why a food web makes an ecosystem far more stable than a single food chain.
    Step-by-step solution →

Questions 34–36 · 5 marks each

  1. Q34(a) 5 marks
    Write the chemical name and the formula of a carbon compound having two carbon atoms in its molecule and belongs to a family of functional group – OH. What happens when this compound reacts with the following ? (Give chemical equation and the name of main product formed in each case)
    1. (i)Excess conc. sulphuric acid () at 443 K
    2. (ii)Ethanoic acid in the presence of an acid
    3. (iii)Acidified potassium dichromate
    4. (iv)Sodium metal
    Show answer

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

    The compound is **ethanol**, (that is, ; molecular formula ). (i) With excess conc. at K it is dehydrated: — main product **ethene**. (ii) With ethanoic acid and an acid catalyst it is esterified: — main product **ethyl ethanoate**, a sweet-smelling ester. (iii) With acidified potassium dichromate it is oxidised: — main product **ethanoic acid**, the orange dichromate turning green. (iv) With sodium metal the hydrogen of the –OH group is replaced: — main product **sodium ethoxide**, with hydrogen gas evolved.
    Step-by-step solution →
    OR
    Q34(b) 5 marks
    1. (i)Conversion of ethanol to ethanoic acid is an oxidation reaction. Why ? Name the oxidizing agent in this conversion and write the chemical equation for this oxidation reaction. Explain how this reaction is different from the reaction in which ethanol burns in the presence of oxygen.
    2. (ii)What is a homologous series of carbon compounds ? Write the name of the first-two members of a homologous series of compounds having functional group – COOH.
    Show answer

    Ethanol gains oxygen to form ethanoic acid, a partial oxidation unlike combustion; the first two carboxylic acids are methanoic and ethanoic acid.

    (i) It is an oxidation because oxygen is added to the molecule and hydrogen is removed from it: ethanol is and ethanoic acid is , so one oxygen atom is gained and two hydrogen atoms are lost, and the alcohol group becomes the acid group . The oxidising agent is **alkaline potassium permanganate** () — acidified potassium dichromate () may also be used. The equation is . It differs from the burning of ethanol, , in that the oxidation is partial and controlled — the carbon skeleton survives and a carbon compound, ethanoic acid, is obtained, the oxygen coming from a chemical oxidising agent on warming — whereas combustion is complete and rapid: the molecule is broken down entirely by atmospheric oxygen, no carbon compound remains, and a large amount of heat and light is given out. (ii) A homologous series is a family of carbon compounds having the same functional group, in which any two successive members differ by one unit (that is, by u in molecular mass) and which can all be represented by the same general formula; the members show similar chemical properties and a gradual gradation in physical properties. The first two members of the series are **methanoic acid** () and **ethanoic acid** ().
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  2. Q35(a) 5 marks
    1. (i)List two roles of each of the following in human reproductive system : (I) Seminal vesicles and Prostate glands (II) Oviduct (III) Testis
    2. (ii)What happens in a human female when (I) fertilisation takes place, and (II) an egg is not fertilised ?
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    Glands feed and carry sperms, the oviduct hosts fertilisation, the testis makes sperms and testosterone; fertilisation starts pregnancy, otherwise menstruation follows.

    (i)(I) Seminal vesicles and prostate glands: (1) their secretions are added to the sperms in the vas deferens and provide a fluid medium in which the sperms are transported — sperms plus these fluids make up the semen; (2) the secretion nourishes the sperms (it supplies fructose as an energy source) and, being alkaline, neutralises the acidity of the urethra and of the female tract so that the sperms remain alive and active. (II) Oviduct (fallopian tube): (1) it receives the ovum released by the ovary and carries it towards the uterus by ciliary action and muscular contraction; (2) it is the **site** where fertilisation takes place — the sperm meets and fuses with the egg here, and the oviduct then carries the zygote to the uterus. (III) Testis: (1) it produces the male gametes, the sperms; (2) it secretes the male sex hormone testosterone, which brings about the secondary sexual characters at puberty and regulates the formation of sperms. (ii)(I) When fertilisation takes place: the zygote formed in the oviduct divides to form a ball of cells, the embryo; the embryo travels to the uterus and is implanted in its thick, spongy lining; a placenta develops through which the embryo takes nutrition and oxygen from the mother's blood; the uterine lining is retained so menstruation stops; and the embryo develops into a foetus over about nine months, at the end of which the baby is born. (II) When an egg is not fertilised: the egg lives only about one day and then dies; the thick uterine lining prepared to receive it is no longer needed, so it breaks down and is shed through the vagina as blood and mucus — this is menstruation, which lasts about two to eight days and recurs about every twenty-eight days.
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    OR
    Q35(b) 5 marks
    1. (i)Name and state the reproductive process taking place in (I) Plasmodium, and (II) Leishmania.
    2. (ii)Name three techniques/devices used by human females to avoid pregnancy. Also mention the side effects of each.
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    Plasmodium divides by multiple fission and Leishmania by binary fission; oral pills, Copper-T and tubectomy are three female methods, each with side effects.

    (i)(I) Plasmodium reproduces by **multiple fission**. The nucleus of the single-celled parasite divides repeatedly to give many nuclei inside one cell; the cytoplasm then divides around each nucleus, and the parent cell breaks up, releasing many daughter cells simultaneously. (II) Leishmania reproduces by **binary fission**. The parent cell divides into two equal daughter cells, and the division always takes place in a **definite orientation** — lengthwise, along the long axis of the cell, with respect to the whip-like flagellum at one end — so each daughter receives a flagellum. (ii) Three techniques or devices used by human females, with their side effects: (1) **Oral contraceptive pills** — hormonal tablets that change the hormonal balance so that eggs are not released; side effects: they disturb the hormonal balance and may cause nausea, headache, weight gain, irregular bleeding and mood changes, and prolonged use can lead to more serious effects. (2) **Intrauterine contraceptive devices** such as the Copper-T or loop, placed in the uterus, which prevent fertilisation and implantation; side effects: irritation of the uterus, pain and excessive bleeding during periods, and the risk of infection or inflammation of the reproductive tract. (3) **Surgical method** — **tubectomy**, in which the fallopian tubes are blocked or cut so that the egg cannot meet a sperm; side effects: being an operation it carries a risk of infection and inflammation and of post-operative pain if not done properly, and it is normally irreversible. (The female condom, a barrier device, is an acceptable alternative: it may slip or tear and so is less reliable, and it can cause irritation or an allergic reaction.)
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  3. Q36(a) 5 marks
    1. (i)If and respectively are the total resistances of the series and parallel combinations of the three resistors of resistances , and , then write the relationships among (I) , , and and (II) , , and .
    2. (ii)An electric oven operates on the mains voltage of 220 V. This oven consumes kWh of electrical energy in one hour. Calculate : (I) power rating of the oven, (II) current drawn by the oven, and (III) resistance of the oven when in operation.
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    Series: , parallel: reciprocals add; the oven is W, draws A and has resistance .

    (i)(I) For the series combination: . (II) For the parallel combination: . (ii)(I) Power rating kW W. (II) Current drawn A. (III) Resistance in operation .
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    OR
    Q36(b) 5 marks
    1. (i)Define the term electric power. Express it in terms of potential difference (V) and resistance (R).
    2. (ii)The resistance of a metal wire of length 6 m is . If the area of cross-section of the wire is , calculate the electrical resistivity of the material of the wire. How would the resistivity be affected if this wire is gradually stretched so that its length is doubled ? Justify your answer.
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    Power is ; the resistivity is and does not change when the wire is stretched.

    (i) Electric power is the rate at which electric energy is consumed or dissipated in a circuit, that is, the electrical work done per unit time; its SI unit is the watt. Since and, by Ohm's law, , the power in terms of the potential difference and the resistance is . (ii) Using , the resistivity is . If the wire is stretched so that its length is doubled, the resistivity is **not** affected — it remains . Justification: resistivity is a characteristic property of the **material** of the wire; it depends only on the nature of the material and on its temperature, and not on the length, the area of cross-section or the shape of the specimen. Stretching changes only the geometry of this particular piece of wire, not the metal it is made of. What does change is the resistance: since the volume of metal is unchanged, doubling the length halves the area of cross-section, so .
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Questions 37–39 · 4 marks each

  1. Q37 4 marks
    The metals produced by various reduction processes are generally not very pure. They contain impurities of other metals and non-metals which must be removed to obtain pure metals. The most widely used process for refining metals such as gold, silver, zinc, copper, etc. is electrolytic refining.
    1. (a)What is the cathode and anode made of in the refining of silver by this process?[1]
    2. (b)Name the electrolyte used in this process and write its chemical formula.[1]
    3. (c)(i)What happens when an electric current is passed through the electrolyte?[2]
    4. (c)(ii)OR : You have two beakers A and B containing ferrous sulphate solution. If you dip a strip of aluminium in beaker A and a strip of copper in beaker B, what will be your observations after about three hours? Give reason for each of your observations.[2]
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    Silver refining uses an impure silver anode, a pure silver cathode and silver nitrate; aluminium displaces iron from ferrous sulphate but copper does not.

    (a) The anode is a thick block (plate) of impure silver and the cathode is a thin strip (sheet) of pure silver. (b) The electrolyte is an aqueous solution of silver nitrate, (acidified with a little dilute nitric acid). (c)(i) Silver from the anode dissolves into the electrolyte as ions, , and an equal amount of pure silver is deposited on the cathode, ; soluble impurities pass into the solution while insoluble ones settle below the anode as anode mud. OR (c)(ii) In beaker A the aluminium strip becomes coated with a greyish-black deposit of iron and grows thinner, and the pale green colour of the solution fades to colourless, because aluminium is above iron in the reactivity series and displaces it: . In beaker B there is no change at all - strip and pale green colour stay as they were - because copper is below iron in the reactivity series and cannot displace it.
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  2. Q38 4 marks
    Mendel used a number of visible contrasting characters of pea plants (Tall/Short plants, Round/Wrinkled seed, Violet/White flowers, etc.) for his experiments and worked out the rules of heredity. He conducted several experiments by making a cross with one or two pairs of visible contrasting characters of pea plants. On the basis of his observations, Mendel gave some interpretations which are helpful in the study of mechanism of inheritance of traits of different organisms.
    1. (a)When Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt) to obtain progeny plants, which two characteristics did he observe in progeny plants?[1]
    2. (b)Differentiate between dominant and recessive traits.[1]
    3. (c)(i)Consider the following cross with two pairs of contrasting characters: RRYY (Round yellow) rryy (Wrinkled green). In this cross, Mendel observed four different combinations of pea seeds in generation pea plants. State the method used by Mendel to obtain progeny. Write the new traits found in progeny. State the conclusion that was drawn from this experiment.[2]
    4. (c)(ii)OR : “It is possible that a trait is inherited, but may not be expressed.” Justify this statement.[2]
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    All plants are tall; selfing gives in , so seed shape and colour are inherited independently.

    (a) All the plants were tall, so only one parental trait (tallness) appeared; and no plant was short or of intermediate height - the traits did not blend, although every plant carried both alleles (Tt). (b) A dominant trait expresses itself even when only one copy of its allele is present, so it shows in the hybrid Tt - for example tallness; a recessive trait is masked in the hybrid and appears only when both alleles are recessive, tt - for example shortness. (c)(i) Mendel obtained the by self-pollinating the plants (RrYy, round yellow). The new traits in the are round green seeds and wrinkled yellow seeds, which appear alongside the parental round yellow and wrinkled green in the ratio 9 : 3 : 3 : 1. The conclusion drawn is that seed shape and seed colour are inherited independently of each other. OR (c)(ii) A trait is controlled by a pair of alleles, one from each parent. A recessive allele is masked whenever a dominant allele is present with it, so it is inherited without being expressed. Every plant of TT tt receives t from the short parent yet is tall; the hidden allele is not lost, and on selfing the one quarter of the plants are tt and short again.
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  3. Q39 4 marks
    A highly polished surface which reflects most of the light falling on it is called a mirror. Mirrors may be plane as well as spherical. The reflecting surface of a spherical mirror may be curved inwards or outwards. A spherical mirror where reflecting surface is curved inwards is called concave mirror, whereas spherical mirrors where reflecting surfaces are curved outwards are known as convex mirrors.
    1. (a)State the laws of reflection of light.[1]
    2. (b)Define the term ‘Principal axis of a concave mirror.’[1]
    3. (c)(i)Write the path of the following light rays after reflection from a concave mirror: (I) A ray of light moving parallel to the principal axis of a concave mirror. (II) A ray incident obliquely to the principal axis towards the pole of the concave mirror making angle .[2]
    4. (c)(ii)OR : An object is placed at 18 cm in front of a concave mirror of focal length 12 cm. Use mirror formula to determine the position of the image.[2]
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    ; the principal axis joins P and C; a parallel ray reflects through F; the image is real, at cm.

    (a) The angle of incidence is equal to the angle of reflection, ; and the incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane. (b) The principal axis of a concave mirror is the straight line passing through its pole and its centre of curvature; it is normal to the mirror at the pole. (c)(i) (I) A ray parallel to the principal axis is reflected through the principal focus F of the concave mirror. (II) A ray incident obliquely at the pole, making an angle with the principal axis, is reflected obliquely at the same angle on the other side of the principal axis, the axis being the normal at the pole. OR (c)(ii) With cm and cm, , so cm. The image is formed 36 cm in front of the mirror, on the same side as the object; it is real, inverted and, since , twice the size of the object.
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