CBSE Class 10 Science 2025 Question Paper (Set 31/2/1) with Solutions

1. Natural gas is methane. Count atoms: carbon $1$ each side; hydrogen $4$ each side with $2\mathrm{H_2O}$; oxygen $2 + 2 = 4$ on the right, so $2\mathrm{O_2}$ on the left.
2. The reaction is exothermic, so the energy released is shown on the product side. Only option D is both balanced and shows the energy.

Answer: (D) $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + Energy}$

1. The sign shows a liquid burning a hand: it warns of a corrosive substance.
2. Sodium hydroxide is a strong, corrosive base. Calcium, magnesium and aluminium hydroxides are weak or sparingly soluble, so they are not marked in this way.

Answer: (C) Sodium

1. Our body works in a narrow, slightly basic range of pH $7.0$ to $7.8$ (blood is about $7.4$). Living organisms are sensitive to pH changes.

Answer: (C) 7.0 to 7.8

1. $\mathrm{Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + heat}$.
2. Aluminium is more reactive than iron, and the reaction gives out so much heat that the iron is produced in the molten state. This molten iron is poured on the gap between railway tracks to weld them.

Answer: (B) When it is heated with iron (III) oxide, molten iron is obtained.

Common mistake: Choosing A. The reaction of aluminium is with iron(III) oxide, not with iron, and the point of the process is the molten iron it makes.

1. Copper + zinc = brass. Copper + tin = bronze. (Solder is lead + tin.)
2. In the order zinc, tin the alloys are brass and bronze.

Answer: (C) Brass and Bronze

1. The guard cells on both sides of a stomatal pore take in water and swell, which opens the pore. When they lose water they shrink and the pore closes.

Answer: (D) Amount of water in guard cells

1. Capillaries have walls one cell thick, so materials can pass between the blood and the tissues. Arteries and veins have thick walls, and alveoli are air sacs in the lungs, not blood vessels.

Answer: (B) Capillaries

1. The touch signal travels from cell to cell. The cells at the base of the leaf then lose water and shrink, so the leaf folds and droops. The plant has no muscles.

Answer: (C) Change in amount of water in cells

1. Bryophyllum reproduces vegetatively: small buds form in the notches along the margin of a leaf, fall to the soil and grow into new plants.

Answer: (B) Vegetative buds produced in the notches of the leaf

1. Halving the chromosome number is meiosis, which forms the gametes. Sperms form in the testis and eggs form in the ovary, so it happens in both. Other body cells divide by mitosis and keep the number.

Answer: (C) Ovary and testis both

1. $n_{XY} = \dfrac{n_Y}{n_X} = \dfrac{4/3}{6/5} = \dfrac{4}{3} \times \dfrac{5}{6} = \dfrac{20}{18} = \dfrac{10}{9}$.

Answer: (A) $\dfrac{10}{9}$

Common mistake: Inverting the ratio. "Y with respect to X" means $n_Y$ divided by $n_X$.

1. $u = -30$ cm and $v = -60$ cm (real image, in front of the mirror).
2. $\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u} = -\dfrac{1}{60} - \dfrac{1}{30} = -\dfrac{3}{60} = -\dfrac{1}{20}$, so $f = -20$ cm.

Answer: (B) −20 cm

1. The angle of incidence is measured between the incident ray and the normal at the first face: angle $1$ (angle $7$ is the angle with the surface itself).
2. The angle of deviation is between the direction of the original (extended) incident ray and the emergent ray: angle $6$.

Answer: (D) 1 and 6

Common mistake: Measuring the angle from the surface instead of from the normal.

1. $I = 500$ mA $= 0.5$ A. $P = VI = 220 \times 0.5 = 110$ W.

Answer: (B) 110 W

Common mistake: Using 500 as the current and getting 110 000 W. Convert milliamperes to amperes first.

1. Between points $A$ and $B$ there are two paths: the top resistor directly ($12\ \Omega$), and the other three resistors in series, $A \to C \to D \to B$ ($12 + 12 + 12 = 36\ \Omega$).
2. These two paths are in parallel: $\dfrac{1}{R} = \dfrac{1}{12} + \dfrac{1}{36} = \dfrac{3 + 1}{36} = \dfrac{1}{9}$, so $R = 9\ \Omega$.

Answer: (C) $9\ \Omega$

1. Leather, cotton, wood, silk and wool come from living things and are broken down by decomposers, so they are biodegradable. Glass, plastic, nylon, polyester and DDT are not broken down by microbes.
2. Only DDT, polyester and glass are all non-biodegradable.

Answer: (C) DDT, Polyester, Glass

1. Assertion: true. Highly reactive metals hold on to oxygen more strongly than carbon does.
2. Reason: true, a more reactive metal can displace a less reactive one from its compound. But this does not explain why carbon fails for the metals at the top.

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

1. A mango seed always gives a mango tree because the traits of the parent are passed on reliably through the DNA. That is exactly what heredity means, so R explains A.

Answer: (A) Both A and R are true and R is the correct explanation of A

1. Assertion: true, nichrome has a high resistivity and does not burn at high temperatures.
2. Reason: the resistivity is high, but the resistance of an alloy like nichrome changes very little with temperature (it does not decrease). So the reason is false.

Answer: (C) A is true, but R is false

1. Animals have no enzyme that can break down coal, so they cannot digest it or get energy from it. This is because each food needs its own specific enzyme, so R explains A.

Answer: (A) Both A and R are true and R is the correct explanation of A

1. (a) It is a displacement reaction: copper is more reactive than silver, so it displaces silver from its salt solution. $\mathrm{2AgNO_3 + Cu \rightarrow 2Ag + Cu(NO_3)_2}$.
2. (b) Silver is refined by electrolytic refining (pure silver on the cathode, impure silver as the anode, and silver nitrate solution as the electrolyte).

Answer: (a) displacement reaction   (b) electrolytic refining

1. (a) A homologous series has the same functional group, and each member differs from the next by a $\mathrm{CH_2}$ unit. (1) and (4) both end in the aldehyde group $\mathrm{{-}CHO}$ and differ by one $\mathrm{CH_2}$: propanal and butanal. (2) is a ketone and (3) is a carboxylic acid.
2. (b)(i) The melting point increases as the molecular mass increases. (ii) The solubility (in water) shows a regular gradation: it decreases as the molecular mass increases.

Answer: (a) (1) and (4)   (b) melting point rises; solubility falls

1. The hind-brain has three parts: the pons, the medulla and the cerebellum.
2. The medulla controls involuntary actions such as blood pressure, salivation and vomiting. (The cerebellum controls posture and balance.)

Answer: Pons, medulla and cerebellum; the medulla

1. (a)(i) Seed coat: protects the seed. (ii) Cotyledon: stores food for the growing embryo. (iii) Radicle: grows into the root on germination. (iv) Plumule: grows into the shoot.
2. (b) Take a test tube with about $10$ g of sugar in $100$ mL of water, add a pinch of yeast granules and keep it in a warm place for $1$ to $2$ hours. The yeast cells bud and multiply, and you can see them under a microscope.

Answer: (a) protects; stores food; root; shoot   (b) sugar solution + yeast, kept warm for 1–2 hours

1. (a) The concave lens diverges a parallel beam. Its principal focus is the point on the axis from which the diverged rays appear to come (shown by the dashed lines).
2. (b)(i) The incident ray passes through the principal focus of the convex lens on the side of the object, bends at the lens and emerges parallel to the principal axis. (ii) The point is the principal focus $F$.

Answer: (a) concave lens, see diagram   (b) the principal focus F

1. $I = \dfrac{P}{V} = \dfrac{750}{220} = 3.41$ A.
2. The kettle draws $3.41$ A, which is more than the $3$ A rating of the fuse. The fuse wire would melt and break the circuit, so the kettle cannot be used with this fuse.

Answer: No: the kettle draws $\approx 3.4$ A, more than $3$ A, so the fuse will melt

1. (a)(i) In a decomposition reaction a single reactant breaks down to give two or more products.
2. Heat: $\mathrm{CaCO_3 \xrightarrow{heat} CaO + CO_2}$. Light: $\mathrm{2AgCl \xrightarrow{sunlight} 2Ag + Cl_2}$. Electricity: $\mathrm{2H_2O \xrightarrow{electric\ current} 2H_2 + O_2}$.
3. (ii) Because heat is given out when vegetable matter is broken down by microbes, the reaction is exothermic.
4. (b) A combination reaction joins two or more reactants into a single product, while a decomposition reaction splits a single reactant into two or more products, so the two are opposite.
5. Combination: carbon + oxygen $\rightarrow$ carbon dioxide, $\mathrm{C + O_2 \rightarrow CO_2 + heat}$. Decomposition: calcium carbonate $\rightarrow$ calcium oxide + carbon dioxide, $\mathrm{CaCO_3 \xrightarrow{heat} CaO + CO_2}$.

Answer: (a) one reactant breaks into two or more products; heat is released in composting   (b) opposite processes, examples above

1. Sodium (atomic number 11) has the electron arrangement 2, 8, 1: one dot (electron) in its outer shell. Oxygen (atomic number 8) has 2, 6: six dots in its outer shell, and it needs two more to complete an octet.
2. Each sodium atom gives its single outer electron to oxygen. Two sodium atoms give two electrons to one oxygen atom: $\mathrm{2Na \rightarrow 2Na^+ + 2e^-}$ and $\mathrm{O + 2e^- \rightarrow O^{2-}}$.
3. The oppositely charged ions attract: $\mathrm{(Na^+)_2\,O^{2-}}$, that is $\mathrm{Na_2O}$. The cation is $\mathrm{Na^+}$ and the anion is $\mathrm{O^{2-}}$.

Answer: $\mathrm{Na_2O}$: cation $\mathrm{Na^+}$, anion $\mathrm{O^{2-}}$

1. (a) Hormones are chemical messengers made in one part of the body that travel in the blood to a target organ or tissue and regulate a body function.
2. (b) Example: when the blood sugar level rises, the cells of the pancreas detect it and make more insulin, which lowers the blood sugar. When the level falls, the pancreas cells sense this and reduce insulin secretion. So the amount of the hormone is adjusted by the effect it has: this is feedback regulation.

Answer: Hormones are chemical messengers; blood sugar and insulin form a feedback loop

1. Let $F$ be the dominant gene for free earlobes and $f$ the recessive gene for attached earlobes. Attached earlobes can only be $ff$.
2. (b) The father has attached earlobes: $ff$. Half the children have attached earlobes ($ff$), so each of them gets an $f$ from the mother as well as from the father. Half have free earlobes, so the mother must carry both $F$ and $f$: she is $Ff$.
3. (a) Cross $ff \times Ff$. The father gives only $f$. The mother gives $F$ or $f$ in equal numbers. The children are $Ff$ (free earlobes) and $ff$ (attached earlobes) in the ratio $1 : 1$, that is $50\%$ each.

Answer: (a) $Ff$ : $ff$ = 1 : 1 (free : attached)   (b) father $ff$, mother $Ff$

1. The object and image are on the same side of a convex lens, so the image is virtual (and erect, since the object is within the focal length).
2. (i) The image is virtual and erect.
3. (ii) $h = +2.0$ cm, $h' = +8.0$ cm, $u = -6.0$ cm. $m = \dfrac{h'}{h} = \dfrac{v}{u}$: $\dfrac{8.0}{2.0} = \dfrac{v}{-6.0}$, so $v = -24$ cm. The image is $24$ cm from the lens on the same side as the object.
4. (iii) $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = -\dfrac{1}{24} + \dfrac{1}{6} = \dfrac{-1 + 4}{24} = \dfrac{1}{8}$, so $f = +8$ cm.

Answer: (i) virtual and erect   (ii) $v = -24$ cm   (iii) $f = +8$ cm

Common mistake: Reading 8.0 cm and 6.0 cm as 80 cm and 60 cm: the decimal point is easy to miss in the printed paper.

1. (i) In series the resistances add: $R_s = 2 + 3 + 6 = 11\ \Omega$.
2. (ii) In parallel: $\dfrac{1}{R_p} = \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{6} = \dfrac{3 + 2 + 1}{6} = 1$, so $R_p = 1\ \Omega$.

Answer: (i) $11\ \Omega$   (ii) $1\ \Omega$

1. Function: the ozone layer shields the surface of the Earth from the harmful ultraviolet (UV) radiation of the Sun, which can cause skin cancer.
2. Formation: UV radiation splits some molecular oxygen into free oxygen atoms, $\mathrm{O_2 \rightarrow O + O}$. These atoms combine with molecular oxygen to form ozone, $\mathrm{O + O_2 \rightarrow O_3}$.
3. The chemicals mainly responsible for ozone depletion are the chlorofluorocarbons (CFCs).

Answer: Absorbs UV; O + O₂ → O₃; CFCs

1. (a)(i) $X$ is ethanoic acid (acetic acid), $\mathrm{CH_3COOH}$. Its $5$–$8\%$ solution is vinegar. Structure: $\mathrm{H{-}C(H)(H){-}C(=O){-}O{-}H}$ (a methyl group joined to the carboxyl group).
2. (ii) It is a weak acid, so the pH of its solution is higher than that of a mineral acid of the same concentration.
3. (iii) Esterification: $\mathrm{CH_3COOH + CH_3CH_2OH \xrightarrow{conc.\ H_2SO_4} CH_3COOCH_2CH_3 + H_2O}$ (ethyl ethanoate).
4. (iv) $\mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + CO_2 + H_2O}$. The salt is sodium ethanoate (sodium acetate) and the gas is carbon dioxide.
5. (b)(i) Saturated hydrocarbons have only single bonds between the carbon atoms (general formula $\mathrm{C_nH_{2n+2}}$), for example propane, $\mathrm{CH_3CH_2CH_3}$. Unsaturated hydrocarbons have at least one double or triple bond (general formulas $\mathrm{C_nH_{2n}}$ and $\mathrm{C_nH_{2n-2}}$), for example propene, $\mathrm{CH_2{=}CH{-}CH_3}$.
6. (ii) Add hydrogen in the presence of a nickel or palladium catalyst (hydrogenation). This is commonly used in the vegetable oil industry to turn oils into ghee.
7. (iii) Butene, $\mathrm{CH_2{=}CH{-}CH_2{-}CH_3}$ (but-1-ene), which has four carbon atoms and a double bond.

Answer: (a) ethanoic acid; higher pH than a mineral acid; esterification; sodium ethanoate   (b) see steps; hydrogenation with Ni/Pd; butene

1. (a)(i) Herbivores such as cows eat grass, which is hard to digest because of cellulose, so they need a longer small intestine. Meat is easier to digest, so carnivores such as tigers have a shorter small intestine.
2. (ii) The pancreas secretes pancreatic juice that contains trypsin to digest proteins and lipase to break down emulsified fats. Bile juice (from the liver) emulsifies fats, breaking large globules into smaller ones, and makes the medium alkaline so that the pancreatic enzymes can act.
3. (iii) The inner wall of the small intestine has millions of finger-like projections called villi. They increase the surface area for absorption, and they are rich in blood vessels that carry the absorbed food to every cell of the body.
4. (b)(i) The rings of cartilage keep the air passage from collapsing when there is no air in it.
5. (ii) During inhalation the ribs are lifted up and outward and the diaphragm flattens (moves down). The chest cavity becomes larger, so air is sucked into the lungs (alveoli).
6. (iii) During heavy exercise the muscle cells do not get enough oxygen, so they respire anaerobically: pyruvate is turned into lactic acid. The build-up of lactic acid in the muscles causes cramps.

Answer: (a) longer for cellulose; trypsin, lipase, bile; villi   (b) keep trachea open; ribs up, diaphragm flat; lactic acid

1. (a)(i) The field lines are concentric circles on the cardboard, with the conductor at the centre. For current from X to Y (downward) they run clockwise when seen from above (from the X end).
2. (ii) The right-hand thumb rule: imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current. The fingers then wrap around the conductor in the direction of the magnetic field lines.
3. (iii) Fleming’s left-hand rule: stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force on the conductor.
4. For the electron, the current is opposite to its motion. It moves up in a field pointing to the right, so the conventional current points down. The force is out of the plane of the paper (upwards towards the viewer).
5. (b)(i) A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. Its field pattern is like that of a bar magnet: inside, the lines are parallel and evenly spaced, so the field is uniform; outside, they leave the north end and enter the south end (diagram above).
6. (ii) Place a piece of soft iron inside the current-carrying solenoid. The iron is magnetised by the field and forms an electromagnet.

Answer: (a) circles; right-hand thumb rule; Fleming’s left-hand rule, force out of the plane   (b) solenoid pattern; soft iron core

1. (a) P (strong acid) lies at the low end, about pH $0$ to $4$. Q (strong base) lies at the high end, about pH $12$ to $14$.
2. (b) (i) To increase the pH add a base such as sodium hydroxide, $\mathrm{NaOH}$. (ii) To decrease the pH add an acid such as hydrochloric acid, $\mathrm{HCl}$.
3. (c)(i) The hydronium ion concentration increases (a fall of $2$ pH units makes it $100$ times larger). The pH paper changes from yellow/orange to red/pink.
4. (c)(ii) Indigestion is caused by excess acid, so the pH of the stomach fluid is low, about $1$ to $3$. People take antacids (for example milk of magnesia). The chemical name of milk of magnesia is magnesium hydroxide, $\mathrm{Mg(OH)_2}$.

Answer: (a) P: pH 0–4; Q: pH 12–14   (b) NaOH, HCl   (c) $[\mathrm{H_3O^+}]$ increases, yellow/orange to red (OR) pH 1–3, antacid, $\mathrm{Mg(OH)_2}$

1. (a) Fertilisation takes place in the oviduct (fallopian tube).
2. (b) The lining of the uterus becomes thick and spongy and is richly supplied with blood, so that it can nourish the growing embryo.
3. (c)(i) The thick uterine lining is not needed, so it slowly breaks down and comes out through the vagina as blood and mucus along with the unfertilised egg. This is menstruation.
4. (c)(ii) Through the placenta, a special disc-like tissue embedded in the uterine wall. It has villi on the embryo’s side and blood spaces on the mother’s side, so oxygen and nutrients pass from the mother’s blood to the embryo (and waste passes back to the mother).

Answer: (a) oviduct   (b) lining thickens with blood   (c) menstruation (OR) the placenta

1. (a) Dispersion of light.
2. (b) Different colours of light bend through different angles with respect to the incident ray as they pass through the prism: violet bends the most and red the least. So the white light spreads out into seven colours.
3. (c)(i) Place a second identical prism upside down next to the first one. The spectrum from the first prism falls on the second prism, which bends the colours back in the opposite way, and a beam of white light emerges from the other side of the second prism (diagram above).
4. (c)(ii) A rainbow is formed by the dispersion of sunlight by small water droplets in the air. The light is refracted and dispersed on entering a droplet, reflected internally at the back of the droplet, and refracted again as it leaves, so a band of colours reaches the eye. The observer must stand with the Sun behind them. In the diagram, draw the Sun’s rays, the droplet, the red and violet rays and the observer’s eye with arrows.

Answer: (a) dispersion   (b) different colours bend by different angles   (c) inverted second prism recombines (OR) droplet: refraction, internal reflection, refraction