CBSE Class 10 Science PYQs by chapter: what you can practise

Answer: D – 1 : 8

AI Explanation:
• Electrolysis of water produces hydrogen and oxygen in a 2:1 volume ratio, but by mass, oxygen (molar mass 32) is much heavier than hydrogen (molar mass 2).
• ✓ Option (d): The mass ratio MH : MO works out to 1 : 8, since 2 volumes of H2 (mass 4) form against 1 volume of O2 (mass 32), giving 4:32 = 1:8.
• Why the others are wrong: (a) inverts the ratio; (b) and (c) confuse the volume ratio (2:1) with the mass ratio.

Answer: B – Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

AI Explanation:
• Decomposition reactions generally require energy input (heat, light or electricity) to break bonds, making Assertion true; composting is also a genuine example of an exothermic decomposition, making Reason true too.
• ✓ Option (b): Both statements are true, but the Reason does not explain the Assertion, since composting is an exception, not the general basis for Assertion.
• Why the others are wrong: (a) wrongly links R as the explanation; (c) and (d) incorrectly mark one statement as false.

Answer:
(a) X = FeSO4 . 7H2O / Ferrous Sulphate Crystals
Y = Fe2O3 / Ferric Oxide
(b) Green to white /brown

AI Explanation:
(a)
• X is FeSO₄·7H₂O (ferrous sulphate crystals); on heating it decomposes to Fe₂O₃ (Y), releasing SO₂ and SO₃ gases.
(b)
• The colour changes from green to white as water of crystallisation is lost, then to brown as Fe₂O₃ forms.

Answer:
(i)
• A single reactant (substance) breaks down to give two or more products.
• CaCO3(s) --heat--> CaO(s) + CO2(g)
• 2AgCl(s) --sunlight--> 2Ag(s) + Cl2(g)
• 2H2O(l) --electric current--> 2H2(g) + O2(g)
(any other suitable example)
(ii)
• Because energy (heat) is released.

AI Explanation:
(i)
• A decomposition reaction is one where a single reactant breaks down into two or more products.
• Heat: CaCO₃ → CaO + CO₂; Light: 2AgCl → 2Ag + Cl₂; Electricity: 2H₂O → 2H₂ + O₂.
(ii)
• Composting is exothermic because energy (heat) is released as vegetable matter decomposes.

Answer:
• A chemical reaction involves the breaking and making of bonds between atoms to produce new substances (reactants change into products).
(i)
• Colour change: add lead nitrate solution to potassium iodide solution in a test tube — the colourless solution changes to a yellow precipitate.
• Pb(NO3)2 + 2KI → PbI2↓ (yellow) + 2KNO3 (or any other example)
(ii)
• Temperature change: calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide), releasing a large amount of heat.
• CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat (or any other example)

AI Explanation:
• A chemical reaction is a process in which bonds between atoms break and new bonds form, converting reactants into new products, often with a visible sign such as colour or temperature change.
(i)
• Colour change: lead nitrate solution mixed with potassium iodide solution turns from colourless to a yellow precipitate.
• Pb(NO3)2 + 2KI → PbI2↓ (yellow) + 2KNO3.
(ii)
• Temperature change: calcium oxide reacting with water is a highly exothermic activity.
• CaO + H2O → Ca(OH)2 + Heat, releasing a large amount of heat and forming slaked lime as the product.

Answer: B – K2SO4, Na2SO4, CaSO4

AI Explanation:
• A family of salts shares the same acidic radical (anion), differing only in the metal/basic radical.
• ✓ Option (b): K2SO4, Na2SO4 and CaSO4 are all sulphates, so they belong to the sulphate family of salts.
• Why the others are wrong: options (a), (c) and (d) each mix different acidic radicals, so they are not one family.

Answer: D – Assertion (A) is false, but Reason (R) is true.

AI Explanation:
• Acid is always added to water slowly (not water to acid), because the process is highly exothermic and can cause spattering otherwise — so the Assertion's stated method is reversed and incorrect.
• ✓ Option (d): Assertion (A) is false (the correct method is acid added to water), but Reason (R) is true — concentrated nitric acid is indeed soluble in water.
• Why the others are wrong: (a), (b), (c) all treat the Assertion as true, but the stated method is actually unsafe.

Answer:
(a)
• (i) H+ / H3O+
• (ii) OH–
(b)
• Dry HCl does not dissociate into hydrogen ions in the absence of water.

AI Explanation:
(a)
• Acids generate H+/H3O+ ions in water; bases generate OH– ions in water.
(b)
• Dry HCl gas has no free H+ ions since it does not ionise without water, so it cannot turn dry litmus paper red.

Answer:
Na2CO3.10H2O
When baking soda is heated sodium carbonate is obtained and recrystallisation of sodium carbonate gives washing soda. /
2 NaHCO3 --heat--> Na2CO3 + H2O + CO2
Na2CO3 +10 H2O --> Na2CO3.10 H2O
Uses:
(i)In glass / soap / paper industry
(ii)In manufacture of borax
(iii)As cleansing agent for domestic purpose
(iv)Removing permanent hardness of water (Any Two)

AI Explanation:
• Washing soda's chemical formula is Na₂CO₃·10H₂O.
• It is obtained by heating baking soda (NaHCO₃) to form sodium carbonate, which is then recrystallised with water to yield washing soda crystals.
• Two common uses: manufacturing glass, soap, and paper; and removing permanent hardness of water as a domestic cleansing agent.

Answer:
• Concentrated solution of sodium chloride reacts with ammonia and carbon dioxide to obtain sodium hydrogen carbonate and ammonium chloride.
• NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
• When sodium hydrogen carbonate is heated strongly, sodium carbonate is obtained.
• 2NaHCO3 --Heat--> Na2CO3 + CO2 + H2O
• Sodium carbonate is dissolved in water to obtain washing soda.
• Na2CO3 + 10H2O → Na2CO3·10H2O
Uses:
• In glass, soap and paper industries.
• Manufacture of borax.
• As a cleaning agent for domestic purposes.
• For removing permanent hardness of water.

AI Explanation:
Given: Sodium chloride (NaCl) is the raw material for washing soda.
• Step 1: NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl (Solvay process forms sodium hydrogen carbonate).
• Step 2: 2NaHCO3 --heat--> Na2CO3 + CO2 + H2O (heating gives sodium carbonate).
• Step 3: Na2CO3 + 10H2O → Na2CO3·10H2O (crystallisation gives washing soda).
∴ Uses of washing soda: (1) glass, soap and paper industries; (2) manufacture of borax; (3) as a domestic cleaning agent; (4) for removing permanent hardness of water.

Answer: B – Al2O3 and MgO

AI Explanation:
• Metals burn in air (oxygen) to form their respective metal oxides, following standard valency-based formulas.
• ✓ Option (b): Aluminium (valency 3) forms Al2O3, and Magnesium (valency 2) forms MgO.
• Why the others are wrong: (a), (c) and (d) show incorrect formulas that do not balance the charges of Al3+/Mg2+ with O2-.

Answer: B – Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

AI Explanation:
• Metals high in the reactivity series (like Na, K, Ca) have too great an affinity for oxygen to be reduced by carbon — this makes Assertion true.
• ✓ Option (b): Both statements are true, but displacement reactions (Reason) don't explain why carbon reduction fails for reactive metals — they're unrelated facts here.
• Why the others are wrong: (a) wrongly claims a causal link; (c) and (d) incorrectly mark one statement false when both are actually true.

Answer:
'X' is Mercury.
Reaction involved in its extraction are:
2HgS + 3O2 --Δ--> 2HgO + 2SO2
2HgO --Δ--> 2Hg + O2

AI Explanation:
• Metal 'X' is Mercury (Hg), extracted from cinnabar (mercury sulphide) by roasting in air.
• 2HgS + 3O2 → 2HgO + 2SO2 (roasting forms mercury oxide)
• 2HgO → 2Hg + O2 (further heating gives free mercury metal)

Answer:
(a)
• A series of metals arranged in order of their decreasing reactivity/activity, determined by performing displacement reactions.
• Calcium, Aluminium, Lead, Copper
(b)
• Fe2O3 + 2Al → Al2O3 + 2Fe + Heat

AI Explanation:
(a)
• A reactivity series arranges metals in order of decreasing reactivity, developed by performing displacement reactions between metals and salt solutions.
• Order: Calcium, Aluminium, Lead, Copper (most to least reactive).
(b)
• Fe2O3 + 2Al → Al2O3 + 2Fe + Heat, an exothermic thermite reaction in which aluminium displaces iron from its oxide.

Answer:
(i)
• (I) 2HgS(s) + 3O2(g) --[Heat]--> 2HgO(s) + 2SO2(s) (cinnabar); 2HgO(s) --[Heat]--> 2Hg(l) + O2(g)
• (II) 2CuS + 3O2(g) --[Heat]--> 2Cu2O(s) + 2SO2(g); 2Cu2O + Cu2S --[Heat]--> 6Cu(s) + SO2(g) (Deduct half mark each for unbalanced equation)
(ii)
• (I) Silver sulphide / Ag2S
• (II) Basic Copper carbonate / Cu(OH)2.CuCO3

AI Explanation:
(i)
• (I) Cinnabar (HgS) is roasted: 2HgS + 3O2 → 2HgO + 2SO2, then further heated: 2HgO → 2Hg + O2.
• (II) Copper sulphide ore is roasted: 2CuS + 3O2 → 2Cu2O + 2SO2, followed by self-reduction: 2Cu2O + Cu2S → 6Cu + SO2.
(ii)
• (I) Silver tarnishes to form a black coating of silver sulphide (Ag2S) on reacting with sulphur compounds in air.
• (II) Copper develops a green coating of basic copper carbonate, Cu(OH)2·CuCO3, on prolonged exposure to moist air.

Answer: D – Propyne

AI Explanation:
• A molecule with one C-C single bond and one C≡C triple bond needs exactly 3 carbon atoms in a chain.
• ✓ Option (d): Propyne (CH3-C≡CH) has exactly one C-C bond and one C≡C bond.
• Why the others are wrong: (a) benzene is a ring with alternating bonds; (b) cyclohexane has only C-C bonds; (c) butyne (4 carbons) has one C≡C bond but two C-C bonds.

Answer: A – Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).

AI Explanation:
• Propanal (C3H6O, an aldehyde) and propanone (C3H6O, a ketone) share the same molecular formula but differ in structure — making them structural isomers.
• ✓ Option (a): Both statements are true, and having the same molecular formula with different structures is exactly why they're isomers, so Reason correctly explains Assertion.
• Why the others are wrong: (b), (c), (d) incorrectly deny the true statements or the causal link between them.

Answer:
• Low melting and boiling points indicate weak intermolecular forces of attraction between molecules.
• Being non-conductors of electricity indicates that bonding in these compounds does not give rise to any ions (covalent bonds/sharing of electrons do not form charged particles).

AI Explanation:
• Low melting and boiling points show that carbon compounds are held together by weak intermolecular forces of attraction, unlike ionic compounds.
• Being poor conductors shows their covalent bonding does not produce free ions or charged particles needed to carry current.

Answer:
• Oxygen is added to ethanol to produce ethanoic acid, which is why it is an oxidation reaction.
• Oxidising agent: Alkaline potassium permanganate (or Acidified potassium dichromate).
• Equation: CH3CH2OH —[Alkaline KMnO4 + Heat, or acidified K2Cr2O7 + Heat]→ CH3COOH
• Difference: this is an oxidation reaction, whereas ethanol burning in oxygen is a combustion reaction.

AI Explanation:
• Oxidation adds oxygen to a substance — here, ethanol gains an oxygen atom to form ethanoic acid.
• Oxidising agent: alkaline KMnO4 or acidified K2Cr2O7, with heat.
• Equation: CH3CH2OH → CH3COOH using the oxidising agent and heat.
• Difference: this controlled oxidation differs from combustion, where ethanol burns directly in oxygen releasing heat and light, producing CO2 and H2O.

Answer:
(a)
• Functional group: The element (or group) replacing hydrogen in a carbon compound is called a heteroatom; these heteroatoms confer special properties to the compound and are known as functional groups.
• (i) Functional group in propanol: –OH (Alcohol)
• (ii) Functional group in propanone: >C=O (Ketone)
(b)
• (i) Dehydration: C₂H₅OH →(443 K, Conc. H₂SO₄, excess) C₂H₄ + H₂O (ethanol → ethene)
• (ii) Oxidation: C₂H₅OH →(Alkaline KMnO₄/heat) CH₃COOH (ethanol → ethanoic acid)

AI Explanation:
(a)
• A functional group is a heteroatom or group of atoms replacing hydrogen in a carbon chain, giving the compound its characteristic chemical properties. Propanol contains the –OH (alcohol) group; propanone contains the >C=O (ketone) group.
(b)
• (i) Ethanol converts to ethene by dehydration: heating with excess concentrated sulphuric acid at 443 K removes water, C₂H₅OH → C₂H₄ + H₂O.
• (ii) Ethanol converts to ethanoic acid by oxidation: heating with alkaline potassium permanganate oxidises the alcohol group to a carboxylic acid, C₂H₅OH → CH₃COOH.

Answer: B – Nitrogen

AI Explanation:
• Proteins are made of amino acids, which contain nitrogen as a key structural component.
• ✓ Option (b): Nitrogen, absorbed from soil as nitrates, is essential for protein synthesis.
• Why the others are wrong: (a), (c), (d) are important plant nutrients but are not the defining element of amino acids/proteins.

Answer: D – Assertion (A) is false, but Reason (R) is true.

AI Explanation:
• In large animals, simple diffusion is too slow to supply oxygen to all body parts, so a circulatory system with respiratory pigments is required.
• ✓ Option (d): Assertion (A) is false — oxygen does not reach all parts easily by diffusion alone in large animals — but Reason (R) is true, since respiratory pigments do carry oxygen to tissues.
• Why the others are wrong: (a), (b), (c) all treat the Assertion as true, contradicting why large animals evolved a circulatory system.

Answer:
• Through the pores present in the walls of capillaries some amount of plasma, proteins and blood cells escape into intercellular spaces in the tissue to form the tissue fluid called lymph.
• Lymph carries digested and absorbed fat from intestine / drains excess fluid from extracellular space back into the blood.

AI Explanation:
• Lymph forms when plasma, proteins, and some blood cells leak through capillary pore walls into intercellular spaces, becoming tissue fluid.
• Its key function is transporting digested fat from the intestine and draining excess fluid back into the bloodstream.

Answer:
(i) Salivary amylase - converts Starch to sugar
(ii) Bile salts – changing the acidic food alkaline/ emulsifies fats.
(iii) Trypsin – Helps in digestion of proteins / Lipase – Breaking down emulsified fats

AI Explanation:
• (i) Saliva contains salivary amylase, which breaks down starch into simpler sugars.
• (ii) Bile juice contains bile salts, which neutralise the acidic food coming from the stomach and emulsify fats into smaller droplets for easier digestion.
• (iii) Pancreatic juice contains trypsin, which digests proteins, and lipase, which breaks down the emulsified fats into fatty acids and glycerol.

Answer:
(i)
• Herbivores eating grass need a longer small intestine to digest cellulose.
• Meat is easier to digest, so meat-eating animals (carnivores) have a shorter small intestine.
(ii)
• Role of Pancreas: secretes pancreatic juice containing trypsin (for digesting proteins) and lipase (for breaking down emulsified fats).
• Role of Bile: emulsifies fats and makes the medium alkaline in the small intestine so that pancreatic enzymes can act.
(iii)
• The inner lining of the small intestine has numerous finger-like projections called villi, which increase the surface area for absorption of food.
• The villi are richly supplied with blood vessels which carry the absorbed food to every cell of the body.

AI Explanation:
(i)
• Herbivores need a longer small intestine to digest cellulose from grass, while carnivores, eating easily digestible meat, have a shorter small intestine.
(ii)
• Pancreas: secretes pancreatic juice with trypsin (digests proteins) and lipase (breaks down emulsified fats).
• Bile: emulsifies fats and makes the intestinal medium alkaline so pancreatic enzymes can act effectively.
(iii)
• The small intestine's lining has finger-like villi that increase surface area and are richly supplied with blood vessels to absorb and transport digested food to body cells.

Answer: D – Medulla

AI Explanation:
• Different hindbrain regions control distinct involuntary functions such as breathing, blood pressure, salivation and vomiting.
• ✓ Option (d): The medulla controls involuntary actions such as salivation, vomiting, blood pressure and heart rate.
• Why the others are wrong: (a) cerebellum governs posture/balance, (b) cerebrum controls voluntary actions and thinking, (c) pons mainly relays signals — none controls salivation/vomiting.

Answer: B – Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

AI Explanation:
• Voluntary actions like writing/talking need signals from the nervous system to reach the relevant muscles.
• ✓ Option (b): Both statements are true, but the Reason (definition of peripheral nervous system) does not explain why the nervous system communicates with muscles during writing/talking.
• Why the others are wrong: (a) wrongly claims R explains A; (c) and (d) wrongly mark one true statement as false.

Answer:
(a) Brain is protected in bony box / skull / cranium / fluid filled balloon.
(b) Region of brain: Hind brain and its part is cerebellum.

AI Explanation:
(a)
• The brain is enclosed within the skull (cranium) and cushioned by cerebrospinal fluid, protecting it from mechanical shock.
(b)
• Loss of posture and balance points to the hindbrain, specifically the cerebellum, which coordinates muscle movements and body balance.

Answer:
(a)
• Hormones are chemical messengers (substances) which regulate body functions / hormones are biochemical substances produced in one part of the body and move to the target organ or tissue to regulate body function.
(b)
• Example: if the sugar level in blood rises, it is detected by cells of the pancreas which respond by producing more insulin to lower the blood sugar level.
• As the blood sugar level falls, this is detected by the cells of the pancreas and insulin secretion is reduced.

AI Explanation:
(a)
• Hormones are chemical messengers produced in one part of the body that travel to a target organ to regulate its function.
(b)
• Example: rising blood sugar is detected by pancreas cells, which secrete more insulin to lower it; as sugar falls, insulin secretion reduces — a feedback loop keeping hormone levels precise.

Answer:
(i) In plants, hormones move by simple diffusion and no specialised glands are involved; in animals, hormones are transported through the blood to target organs and are released by endocrine glands.
(ii) (1) Cerebrum/forebrain (2) Cerebellum/hindbrain (3) Medulla/hindbrain (4) Hypothalamus/forebrain.
(iii) The brain is protected by the skull (a bony box/fluid-filled balloon), and the spinal cord is protected by the backbone/vertebral column.

AI Explanation:
(i)
• In plants, hormones travel by simple diffusion from the cells that make them, with no specialised glands involved.
• In animals, hormones are secreted by specific endocrine glands and carried through the bloodstream to distant target organs.
(ii)
• (1) Cerebrum — intelligence, (2) cerebellum — riding a bicycle (balance/coordination), (3) medulla — vomiting, (4) hypothalamus — controlling hunger.
(iii)
• The brain is enclosed and protected within the bony skull, cushioned further by cerebrospinal fluid, while the spinal cord runs safely through the vertebral column (backbone).

Answer: C – seeds

AI Explanation:
• Some cultivated plant varieties, propagated vegetatively (via cuttings, suckers etc.), have lost the ability to produce viable seeds through sexual reproduction.
• ✓ Option (c): Rose and banana varieties are usually propagated vegetatively and have lost the capacity to produce seeds.
• Why the others are wrong: these plants can still produce flowers, buds and (in banana's case) fruits, just not viable seeds.

Answer: B – Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).

AI Explanation:
• The placenta enables nutrient transfer from mother to embryo, but simply describing it as a disc embedded in the uterine wall does not explain why nutrition transfer occurs.
• ✓ Option (b): Both statements are individually true, but the Reason describes placental structure rather than explaining the nutrition function.
• Why the others are wrong: Since both statements are true, (c) and (d) are ruled out; since R does not explain A, (a) is also incorrect.

Answer:
(a) Provides a lower temperature than the normal body temperature for sperm formation.
(b) The secretion of the glands helps in the transport of sperms and provides nutrition.

AI Explanation:
(a)
• Testes are located in the scrotum, outside the abdominal cavity, to keep a temperature lower than body temperature, essential for healthy sperm formation.
(b)
• Glands along the vas deferens (seminal vesicles, prostate) secrete fluids that nourish sperms and aid their smooth transport during ejaculation.

Answer:
Self-Pollination:
• Pollen grains are transferred from stamen to the stigma of the same flower.
Cross Pollination:
• Transfer of pollen grains from stamen of one flower to the stigma of another flower of same species.
Better for survival:
• Cross pollination is better as it leads to more variations which is better for survival of the species over time.

AI Explanation:
• Self-pollination transfers pollen within the same flower, producing offspring genetically similar to the parent.
• Cross-pollination transfers pollen between different flowers of the same species, mixing genetic material from two parents.
• Cross-pollination is better for species survival because it introduces greater genetic variation, improving the population's ability to adapt to changing environmental conditions and resist diseases.

Answer:
(i)
• (I) Ovary: Produces female gamete (egg) and female hormones (oestrogen).
• (II) Fallopian tube: Site of fertilization.
• (III) Uterus: Site of implantation and embryonic development.
(ii)
• Mechanical barrier – Condoms.
• Surgical method – blocking the vas deferens in males (Vasectomy).

AI Explanation:
(i)
• Ovary: produces the female gamete (egg/ovum) and secretes female hormones such as oestrogen.
• Fallopian tube: the site where fertilisation of the egg by sperm occurs.
• Uterus: the site where the fertilised egg implants and the embryo develops until birth.
(ii)
• Condoms act as a mechanical barrier, physically preventing sperm from reaching the egg.
• Vasectomy is a surgical method in males that involves blocking or cutting the vas deferens, preventing sperm from being included in the semen ejaculated during intercourse.

Answer: A – Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).

AI Explanation:
• A mango seed germinating into a mango tree (not any other species) happens because heredity ensures reliable transmission of traits from parent to offspring.
• ✓ Option (a): Both statements are true, and heredity correctly explains why the seed grows true to type.
• Why the others are wrong: (b) wrongly denies the causal link; (c) and (d) incorrectly mark a true statement as false.

Answer:
The height of plant depends upon amount of particular plant hormone. This hormone depends upon the efficiency of enzyme (protein) which depends upon a DNA sequence (gene). If the enzyme works efficiently, lot of hormones are made and the plant will be tall.

AI Explanation:
• Plant height depends on the amount of a growth hormone produced.
• Hormone production depends on an enzyme (a protein) coded by a specific gene (DNA sequence).
• An efficient enzyme produces more hormone, making the plant tall — showing how proteins control character expression.

Answer:
(a)
• 23 pairs or 46 chromosomes.
• 1 pair or 2 chromosomes.
(b)
• In sexually reproducing organisms, chromosomes occur in pairs.
• The members of the pair separate at the time of gamete formation.
• The two germ cells fuse and form a zygote, thus restoring the same number of chromosomes as the parents.

AI Explanation:
• Human beings have 23 pairs (46) chromosomes, of which 1 pair (2 chromosomes) are sex chromosomes.
• In sexually reproducing organisms, chromosomes occur in pairs; these pairs separate when gametes form.
• Fusion of the two germ cells restores the original number of chromosomes in the zygote, keeping it constant across generations.

Answer: C – 40 cm

AI Explanation:
• Magnification m = –1 (real, inverted, same-size image) occurs only when the object is placed at 2F, i.e., u = –2f.
• ✓ Option (c): With f = 20 cm, object distance = 2 × 20 cm = 40 cm.
• Why the others are wrong: (a), (b), (d) do not correspond to the object being placed at exactly twice the focal length.

Answer:
Given: u = -10 cm, f = -15 cm
1/f = 1/v + 1/u
1/(-15) = 1/v + 1/(-10)
1/v = 1/(-15) + 1/10
v = +30 cm
∴ Position of image is 30 cm from the mirror.

AI Explanation:
• u = -10 cm, f = -15 cm; using 1/f = 1/v + 1/u
• 1/(-15) = 1/v + 1/(-10) → 1/v = 1/10 - 1/15 = 1/30
• v = +30 cm, so the image forms 30 cm behind the mirror, on the virtual side.

Answer:
Position:
1/v - 1/u = 1/f
1/v - 1/(-30) = 1/(+20)
1/v + 1/30 = 1/20 => v = +60 cm
position is 60 cm from the lens
Size: m = hi/ho = v/u
= 60/(-30) = -2
hi = -2 x 5 cm = -10 cm
size of image is 10 cm (Enlarged)

AI Explanation:
• Given: ho = 5 cm, u = -30 cm, f = +20 cm
• 1/v - 1/u = 1/f
• 1/v - 1/(-30) = 1/20
• 1/v = 1/20 - 1/30 = 1/60
• v = +60 cm
• ∴ Image at 60 cm, opposite side (real).
• m = v/u = 60/(-30) = -2
• hi = m x ho = -2 x 5 = -10 cm
• ∴ Image size = 10 cm; real, inverted, enlarged.

Answer:
(i)
• The angle of incidence is equal to the angle of reflection.
• The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.
(ii)
Given: u = −15 cm, f = −10 cm (concave mirror), h = 5.0 cm
• Mirror formula: 1/f = 1/v + 1/u
• 1/v = −1/10 + 1/15
• 1/v = −1/30
• v = −30 cm — the screen must be placed at a distance of 30 cm from the mirror, in front of it.
• Magnification: m = h′/h = −v/u
• h′ = (−v/u) × h = −(−30/−15) × 5
• h′ = −10 cm

AI Explanation:
(i)
• The angle of incidence equals the angle of reflection.
• The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
(ii)
Given: u = −15 cm, f = −10 cm, h = 5.0 cm
• 1/v = 1/f − 1/u
• 1/v = −1/10 − (−1/15) = −1/30
• v = −30 cm (screen placed 30 cm in front of the mirror)
• h′ = (−v/u) × h = −(−30/−15) × 5
• h′ = −10 cm (real, inverted, magnified image)

Answer: C – When sunlight passes through the fine particles in air, they scatter the blue colour of visible light more strongly than red.

AI Explanation:
• Fine particles in the atmosphere scatter shorter wavelengths (blue) more strongly than longer wavelengths (red), by Rayleigh scattering.
• ✓ Option (c): Sunlight passing through fine air particles scatters blue light more strongly than red.
• Why the others are wrong: (a) air molecules are smaller than the wavelength of light; (b) red has a longer wavelength than blue; (d) red is scattered least, not most, by fog/smoke.

Answer: B – Both, Assertion (A) and Reason (R) are true, and Reason (R) is not the correct explanation of Assertion (A).

AI Explanation:
• Both statements are factually correct: white light splits into seven colours (VIBGYOR) through a prism, and red bends least while violet bends most, due to differing refractive indices.
• ✓ Option (b): Assertion and Reason are both true, but Reason (R) explains the order of the colours' bending, not the cause of dispersion itself, so it does not correctly explain Assertion (A).
• Why the others are wrong: (a) is incorrect since R does not explain A; (c) and (d) wrongly call one of the statements false, when both are true.

Answer:
(i) Scattering of light is not prominent at such heights.
(ii) The red colour is least scattered by smoke or fog. / Red colour has longer wavelength.

AI Explanation:
(i)
• At very high altitudes the atmosphere is thin, leaving little air to scatter sunlight, so the sky appears dark.
(ii)
• Red light has the longest wavelength among visible colours and is scattered the least by smoke or fog, so danger signals use red for maximum visibility.

Answer:
(a) Hypermetropia
(b) Two causes:
• Eye ball has become too small /Eye ball is shortened
• The focal length of the eye Lens is long / eye lens becomes less convergent
(c) Focal length = 1/P = 1/2 = + 0.5 m

AI Explanation:
(a)
• A positive (converging) lens correction indicates hypermetropia (far-sightedness).
(b)
• Caused by an eyeball that is too short, or by an eye lens with too long a focal length (less convergent).
(c)
Given: P = +2.0 D
f = 1/P
f = 1/2
∴ f = +0.5 m

Answer: C – Assertion (A) is true, but Reason (R) is false.

AI Explanation:
• Bending a wire into a coil changes only its shape, not its length, area of cross-section, or the material's resistivity, so its resistance stays unchanged — the Assertion is true.
• ✓ Option (c): Assertion (A) is true, but Reason (R) is false — resistance depends on length and area too (R = ρl/A), not on resistivity alone.
• Why the others are wrong: (a) and (b) wrongly treat Reason as true; (d) wrongly marks the Assertion as false.

Answer:
Given: H = 400 J, R = 4Ω, t = 1 s
H = V²/R × t
400 = V²/4 × 1
V² = 1600
∴ V = 40 V

AI Explanation:
• H = (V²/R) × t, using the given values 400 = V²/4 × 1
• Rearranging, V² = 400 × 4 = 1600
• V = √1600 = 40 V, the potential difference developed across the resistor.

Answer:
(a)
• When 1 coulomb of charge flows through any cross-section of a conductor in 1 second, the electric current flowing through it is 1 ampere.
• 1A = 1C/1s
(b)
Given: r = 0.01 cm = 0.01×10⁻² m, ρ = 44×10⁻⁸ Ωm, R = 14Ω
R = ρl/A
l = RA/ρ = (14×22×(0.01×10⁻²)²)/(7×44×10⁻⁸)
∴ l = 1.0 m

AI Explanation:
(a)
• One ampere is the current flowing when 1 coulomb of charge passes through a cross-section of a conductor in 1 second (1A = 1C/1s).
(b)
Given: r = 0.01 cm = 0.01×10⁻² m, R = 14Ω, ρ = 44×10⁻⁸ Ωm
A = πr²
l = RA/ρ = (14 × 22 × (0.01×10⁻²)²)/(7 × 44×10⁻⁸)
l = 1.0 m
∴ The length of the wire is 1.0 metre.

Answer: B – red for live wire, black for neutral wire and green for earth wire

AI Explanation:
• The standard domestic wiring convention uses red for the live wire, black for the neutral wire, and green for the earth wire.
• ✓ Option (b): Red for live, black for neutral and green for earth wire.
• Why the others are wrong: (a), (c) and (d) swap or mismatch the standard colour codes for live, neutral and earth wires.

Answer: A – Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).

AI Explanation:
• The earth wire connects an appliance's metallic body to a plate buried in the ground, giving leakage current a safe path to earth.
• ✓ Option (a): Both statements are true, and the Reason correctly explains why the earth wire prevents electric shocks.
• Why the others are wrong: (b), (c) and (d) deny the explanatory link or mark a true statement as false.

Answer:
• An electric fuse is placed in series with the device to prevent the electric circuit from possible damage by stopping the flow of unduly high electric current.
• If current larger than the specified value flows through the circuit, the fuse melts and breaks the electric circuit.

AI Explanation:
• An electric fuse is a short, thin wire of low melting point, connected in series with a circuit or appliance.
• If current larger than the specified safe value flows due to a short-circuit or overload, the fuse wire melts and breaks the circuit, protecting the appliance from damage.

Answer:
(a) Any three:
• Magnitude of current
• Strength of the magnetic field
• Orientation of the conductor in the magnetic field
• Length of the conductor
(b)
• When the current-carrying conductor is placed at right angles (perpendicular) to the direction of the magnetic field.
(c)
• Fleming's Left-Hand Rule
• Application: Electric motors / Microphones / Generators (any one)

AI Explanation:
(a)
• The force on a current-carrying conductor depends on the current's magnitude, the magnetic field's strength, and the conductor's orientation relative to the field (also its length).
(b)
• Force is maximum when the conductor is placed perpendicular to the magnetic field, since the force depends on the sine of the angle between them.
(c)
• Fleming's Left-Hand Rule gives the direction of force; it is applied in the working of electric motors.

Answer: D – 5000 kJ

AI Explanation:
Given: Energy at 3rd trophic level (Frog) = 50 kJ; only 10% of energy transfers between successive trophic levels.
Energy at 2nd level = 50 × 10 = 500 kJ
Energy at 1st (producer) level = 500 × 10 = 5000 kJ
∴ Available energy at producer level = 5000 kJ.
✓ Option (d): 5000 kJ is correct.
Why the others are wrong: The other values come from dividing instead of multiplying, or from using the wrong number of trophic-level steps.

Answer: B – Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

AI Explanation:
• Ozone depletion, caused mainly by CFCs, was first sharply noticed in the 1980s, making the Assertion true; the natural formation of ozone from oxygen atoms combining with O2 is also a true statement, but it does not explain the depletion.
• ✓ Option (b): Both statements are true, but the Reason does not explain the Assertion.
• Why the others are wrong: (a) wrongly treats R as the explanation; (c) and (d) wrongly mark one statement false.

Answer:
(a)
• Ozone (O₃)
• Excessive use of chlorofluorocarbons (CFC's) / Freons
(b)
• The higher energy ultra violet radiations split apart molecular oxygen (O₂) into free oxygen (O) atoms. These atoms then combine with the molecular oxygen to form ozone.
• O₂ →(UV) O + O
• O + O₂ → O₃ (Ozone)

AI Explanation:
(a)
• 'X' is ozone (O₃); it depletes mainly due to excessive use of chlorofluorocarbons (CFCs)/Freons.
(b)
• High-energy UV radiation splits O₂ into free oxygen atoms, which then combine with O₂ molecules to form ozone in the upper atmosphere.

Answer:
(a)
• The energy captured by plants does not revert to solar input, and the energy that passes to herbivores does not revert back to autotrophs.
• As energy moves progressively through the trophic levels, it is no longer available to the previous level.
• The energy available at each trophic level gets progressively diminished due to loss of energy at each level. (any two)
(b)
• 100 J
• Autotrophs (10000 J) → Primary consumer (1000 J) → Secondary consumer (100 J)
• Only 10% of the energy of the organic matter of the previous trophic level is transferred to the next trophic level (10% law).

AI Explanation:
(a)
• Energy captured by producers never reverts back to the sun or to the previous trophic level once it moves forward through the food chain.
• Energy available at each trophic level diminishes progressively, so it cannot flow backward.
(b)
• Only 10% of energy transfers at each trophic level (10% law): 10,000 J → 1,000 J (primary consumers) → 100 J (secondary consumers).
∴ Secondary consumers receive 100 J.