CBSE Class 10 Science 2025 Solved Question Paper

CBSE Class 10 Science 2025 Solved Question Paper

Complete Official Board Exam Questions with Step-by-Step Solutions | Delhi & Outside Delhi Sets

SECTION A (Multiple Choice Questions - 1 Mark Each)

Q1. In which one of the following situations a chemical reaction does not occur?

(a) Milk is left open at room temperature during summer (b) Grapes get fermented (c) An iron nail is left exposed to humid atmosphere (d) Melting of glaciers
Answer: (d) Melting of glaciers Explanation: The melting of glaciers is a physical change in which ice melts into liquid water; only the state changes without forming any new chemical substance.

Q2. In order to prepare dry hydrogen chloride gas in humid atmosphere the gas produced is passed through a guard tube (drying tube) which contains:

(a) Calcium chloride     (b) Calcium oxide     (c) Calcium hydroxide     (d) Calcium carbonate
Answer: (a) Calcium chloride Explanation: Anhydrous calcium chloride (CaCl_2) acts as a drying agent in the guard tube to absorb moisture present in the gas.

Q3. The property by virtue of which a solid material can be drawn into thin wires is called:

(a) malleability     (b) ductility     (c) rigidity     (d) resistivity
Answer: (b) ductility Explanation: Ductility is the mechanical property that allows metals to be drawn into thin wires without breaking.

Q4. Select from the following a hydrocarbon having one C-C bond and one C≡C bond:

(a) Benzene     (b) Cyclohexane     (c) Butyne     (d) Propyne
Answer: (d) Propyne Explanation: Propyne (CH_3-C \equiv CH) consists of 3 carbon atoms containing one single C-C bond and one triple C \equiv C bond.

Q5. The essential element taken up from the soil by the plants to synthesise proteins is:

(a) Phosphorus     (b) Nitrogen     (c) Iron     (d) Magnesium
Answer: (b) Nitrogen Explanation: Nitrogen is absorbed by plant roots as nitrates or ammonium ions and is essential for synthesizing amino acids and proteins.

Q6. Select TRUE statements about lymph from the following: A. Lymph vessels carry lymph through the body and finally open into larger arteries. B. Lymph contains some amount of plasma, proteins and blood cells. C. Lymph contains some amount of plasma, proteins and red blood cells. D. Lymph vessels carry lymph through the body and finally open into larger veins.

(a) A and B     (b) B and D     (c) A and C     (d) C and D
Answer: (b) B and D Explanation: Lymph carries plasma, proteins, and white blood cells (lacks RBCs) and drains into larger veins.

Q7. Plants like rose and banana have lost the capacity to produce:

(a) flowers     (b) buds     (c) seeds     (d) fruits
Answer: (c) seeds Explanation: Cultivated varieties of rose and banana reproduce predominantly via vegetative propagation as they have lost the capacity to yield viable seeds.

Q8. In a bisexual flower the male gametes are present in the:

(a) anther     (b) ovary     (c) stigma     (d) filament
Answer: (a) anther Explanation: Pollen grains containing male gametes are produced in the anther of the stamen.

Q9. When a pure-tall pea plant is crossed with a pure-dwarf pea plant, the percentage of tall pea plants in F_1 and F_2 generation pea plants will be respectively:

(a) 100%; 25%     (b) 100%; 50%     (c) 100%; 75%     (d) 100%; 100%
Answer: (c) 100%; 75% Explanation: F_1 generation plants are all Tt (100% tall). In F_2 (Tt \times Tt), the ratio is 3 tall to 1 dwarf, meaning 75% are tall.

Q10. To get an image of magnification -1 on a screen using a lens of focal length 20\text{ cm}, the object distance must be:

(a) Less than 20 cm     (b) 30 cm     (c) 40 cm     (d) 80 cm
Answer: (c) 40 cm Explanation: Magnification m = -1 occurs when the object is at 2f (u = 2 \times 20\text{ cm} = 40\text{ cm}).

Q11. An optical device 'X' is placed obliquely in the path of a narrow parallel beam of light. If the emergent beam gets displaced laterally, the device 'X' is:

(a) plane mirror     (b) convex lens     (c) glass slab     (d) glass prism
Answer: (c) glass slab Explanation: Light passing obliquely through a rectangular glass slab undergoes lateral displacement while emerging parallel to the incident direction.

Q12. A piece of wire of resistance 'R' is cut lengthwise into three identical parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R', then the value of R/R' is:

(a) 1/9     (b) 1/3     (c) 3     (d) 9
Answer: (d) 9 Explanation: Each cut part has resistance R/3. In parallel connection, \frac{1}{R'} = \frac{3}{R} + \frac{3}{R} + \frac{3}{R} = \frac{9}{R} \implies \frac{R}{R'} = 9.

Q13. An electric bulb is rated 220\text{ V}; 11\text{W}. The resistance of its filament when it glows with a power supply of 220\text{ V} is:

(a) 4400\ \Omega     (b) 440\ \Omega     (c) 400\ \Omega     (d) 20\ \Omega
Answer: (a) 4400\ \Omega Explanation: R = \frac{V^2}{P} = \frac{220 \times 220}{11} = 4400\ \Omega.

Q14. The minimum number of identical bulbs of rating 4\text{V}; 6\text{W}, that can work safely with desired brightness, when connected in series with a 240\text{ V} mains supply is:

(a) 20     (b) 40     (c) 60     (d) 80
Answer: (c) 60 Explanation: Minimum number of bulbs n = \frac{V_{\text{total}}}{V_{\text{bulb}}} = \frac{240}{4} = 60.

Q15. In the food chains given below. Select the most efficient food chain in terms of energy:

(a) Grass → Grasshopper → Frog → Snake (b) Plants → Deer → Lion (c) Plants → Man (d) Phytoplankton → Zooplankton → Small Fish → Big Fish
Answer: (c) Plants → Man Explanation: Shorter food chains involve fewer trophic steps, minimizing energy loss at each transfer.

Q16. Which one of the following gets biomagnified at different levels in a food chain?

(a) Carbon monoxide     (b) CFC's     (c) DDT     (d) Manure
Answer: (c) DDT Explanation: Non-biodegradable toxic chemicals like DDT accumulate progressively at successive higher trophic levels.

Q17. Assertion (A): In large animals, oxygen can reach different parts of the animal's body easily. Reason (R): Respiratory pigments take up oxygen from the air and carry it to body tissues.

Answer: (a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A). Explanation: Respiratory pigments like hemoglobin absorb oxygen in the respiratory organs and efficiently transport it through blood to distant body tissues.

Q18. Assertion (A): Concentrated nitric acid is diluted by adding water slowly to acid with constant stirring. Reason (R): Concentrated nitric acid is easily soluble in water.

Answer: (d) Assertion (A) is false, but Reason (R) is true. Explanation: Dilution must always be carried out by adding acid slowly into water with constant stirring because the reaction is highly exothermic.

Q19. Assertion (A): In reptiles, the temperature at which the fertilised eggs are kept decides the sex of the offsprings. Reason (R): Sex is not genetically determined in some animals.

Answer: (a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A). Explanation: In reptiles like turtles and crocodiles, sex determination depends on incubation temperature rather than sex chromosomes.

Q20. Assertion (A): When ciliary muscles contract, eye lens becomes thin. Reason (R): Ciliary muscles control the power of the eye lens.

Answer: (d) Assertion (A) is false, but Reason (R) is true. Explanation: Contraction of ciliary muscles makes the eye lens thicker (more convex) to increase its focal power for viewing nearby objects.

SECTION B (Very Short Answer Type - 2 Marks Each)

Q21. Define oxidation. Identify and name the substance oxidised in the following reaction:

Solution: Oxidation: It is defined as a chemical process involving the gain of oxygen, loss of hydrogen, or loss of electrons by a substance. Substance Oxidised: Hydrogen (H_2) is oxidised to water (H_2O) because it gains oxygen.

Q22. (A) Show the formation of magnesium chloride by electron transfer. Write the name of the cation and anion present in the compound formed. (Atomic Number: Mg = 12, Cl = 17)

Solution: Electronic configuration: Mg (2,8,2), Cl (2,8,7). Mg loses 2e^- to form Mg^{2+} (Cation). Each Cl atom gains 1e^- to form Cl^- (Anion). Cation: Magnesium ion (Mg^{2+}) | Anion: Chloride ion (Cl^-).

OR

(B) How is zinc extracted from its ore? Name the processes involved in the extraction and write chemical equations for the reactions that occur during these processes.

Solution: 1. Roasting (for sulphide ore ZnS): Heated strongly in presence of excess air. 2. Calcination (for carbonate ore ZnCO_3): Heated strongly in limited air. 3. Reduction: Reduced to zinc metal using carbon (coke).

Q23. "Plants use a variety of techniques to get rid of waste material." Justify this statement giving any four ways.

Solution: 1. Leaf Abscission: Storing metabolic wastes in old leaves that fall off. 2. Transpiration: Releasing excess water through stomata. 3. Cellular Vacuoles: Accumulating resins, gums, and tannins in old xylem/bark. 4. Root Excretion: Excreting waste substances directly into the surrounding soil.

Q24. Explain with the help of a flow chart that in human beings father is responsible for the sex (male or female) of the child.

Solution: Mothers contribute only X chromosomes in eggs (XX). Fathers produce two types of sperms: 50% carry X and 50% carry Y (XY).
  • If an X-bearing egg fuses with an X-bearing sperm \rightarrow XX (Female child).
  • If an X-bearing egg fuses with a Y-bearing sperm \rightarrow XY (Male child).
Hence, the father's gametes determine the sex of the offspring.

Q25. (A) Draw a ray diagram to show the refraction of a ray of light passing through an equilateral glass prism. Mark the angle through which the emergent ray bends from the direction of the incident ray and also name it.

Solution: The angle between the direction of the incident ray and the emergent ray is called the Angle of Deviation (\angle D).

OR

(B) Name the type of lenses required by the persons for the correction of their defect of vision called presbyopia. Write the structure of the lenses commonly used for the correction of this defect giving reason for such designs.

Solution: Lens Type: Bifocal lenses. Structure & Design Reason: The upper portion consists of a concave lens to facilitate distant vision. The lower portion consists of a convex lens to enable near vision.

Q26. What are magnetic field lines. List two important properties of magnetic field lines.

Solution: Magnetic field lines are continuous imaginary curves depicting the direction and relative strength of the magnetic field around a magnet. Properties:
  1. They form closed continuous loops (emerge from North pole and enter South pole externally, and travel South to North internally).
  2. Two magnetic field lines never intersect each other at any point.

SECTION C (Short Answer Type - 3 Marks Each)

Q27. (A) Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical equation:

Solution: We balance chemical equations to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction. Balanced Equation:

Q28. Design an activity to show that metals are good conductors of heat and have high melting points.

Solution: Clamp an iron/copper wire on a stand and fix a wax pin near its free end. Heat the opposite end with a burner. The wax melts after some time and the pin falls off, showing heat conducts through metal. Since the wire does not melt upon heating, it demonstrates high melting points.

Q29. State the enzyme/salt present in the following and mention their function in digestion: (i) Saliva     (ii) Bile Juice     (iii) Pancreatic Juice

Solution: (i) Saliva: Contains Salivary Amylase; breaks down complex starch into simple sugar (maltose). (ii) Bile Juice: Contains Bile Salts; emulsifies large fat globules into smaller droplets and creates an alkaline medium. (iii) Pancreatic Juice: Contains Trypsin (digests proteins) and Lipase (digests emulsified fats).

Q30. State two limitations of electrical impulses in multicellular organisms. Why is chemical communication better than electrical impulses?

Solution: Limitations of Electrical Impulses:
  1. Reach only cells connected by nervous tissue, not all body cells.
  2. Cells take time to reset mechanisms before generating new impulses (non-continuous).
Why Chemical Communication is Better: Hormones diffuse throughout all body fluids, reaching target cells across the organism continuously without requiring direct nerve connectivity.

Q31. If we want to obtain a virtual and magnified image of an object by using a concave mirror of focal length 18\text{ cm}, where should the object be placed? Use mirror formula to determine the object distance for an image of magnification +2.

Solution: Object must be placed between pole P and principal focus F (distance < 18\text{ cm}). Given f = -18\text{ cm} and m = +2: Using mirror formula \frac{1}{v} + \frac{1}{u} = \frac{1}{f}: Object distance is 9\text{ cm} in front of the mirror.

Q32. The electrical resistivity of three materials A, B and C at 20^\circ\text{C} is: A (10^{17}\ \Omega\text{m}), B (44 \times 10^{-6}\ \Omega\text{m}), C (1.62 \times 10^{-8}\ \Omega\text{m}). (i) Classify materials as conductor, alloy, insulator. (ii) Give one example and use of each in electrical heating appliances.

Solution: (i) A: Insulator | B: Alloy | C: Conductor. (ii) Examples & Uses:
  • A (Mica/Glass): Used as electrical insulation handle/body casing.
  • B (Nichrome): Used as heating element coil due to high resistivity & non-oxidation at high heat.
  • C (Copper): Used as connecting lead wires due to low resistivity.

Q33. What are decomposers? Give two examples. State how they maintain a balance in an ecosystem.

Solution: Decomposers are microorganisms that break down dead organic matter into simpler inorganic nutrients. Examples: Bacteria and Fungi. Ecological Balance:
  1. Nutrient Recycling: Replenish essential minerals back to soil for primary producers.
  2. Environmental Sanitation: Prevent accumulation of dead organic waste.

SECTION D (Long Answer Type - 5 Marks Each)

Q34. (A) Carbon compound 'A' on heating with excess conc. H_2SO_4 forms 'B', which on addition of H_2 gas with Ni catalyst forms 'C'. 'C' on combustion forms 2\text{ moles } CO_2 and 3\text{ moles } H_2O. Identify A, B, C, write structures and chemical equations.

Solution: Identification:
  • A: Ethanol (CH_3CH_2OH)
  • B: Ethene (CH_2=CH_2)
  • C: Ethane (CH_3-CH_3)
Reactions: 1. CH_3CH_2OH \xrightarrow[\Delta]{\text{conc. } H_2SO_4} CH_2=CH_2 + H_2O (conc. H_2SO_4 acts as dehydrating agent). 2. CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3. 3. C_2H_6 + \frac{7}{2}O_2 \rightarrow 2CO_2 + 3H_2O.

Q35. (A) (i) What is regeneration? Give one example of an organism showing it and one that does not. (ii) Filamentous structures in green pond water: Name structures, method of reproduction and explain.

Solution: (i) Regeneration: Ability of fully differentiated organisms to grow complete new individuals from cut body parts. Shows regeneration: Planaria / Hydra. Does not show: Humans/mammals (due to lack of specialized stem cells in complex tissues). (ii) Structures: Spirogyra (Algae). Method: Fragmentation. On maturation, long filaments break into smaller fragments which divide by mitosis to form new filaments.

Q36. (A) (i) Magnetic field pattern for two parallel conductors carrying current in opposite directions. (ii) Domestic A.C. supply 220 V: (a) Name three insulation wire colours. (b) Potential difference between red and black wire. (c) Role of green insulation wire.

Solution: (a) Red: Live wire | Black: Neutral wire | Green: Earth wire. (b) Potential difference = 220\text{ V}. (c) Green earth wire provides a low-resistance path to send current leakage directly to Earth, protecting users from electric shocks.

SECTION E (Case Study Questions - 4 Marks Each)

Q37. Case Study: Common Salt (NaCl) (i) Name acid and base to obtain common salt. (ii) State nature (acidic/basic/neutral) of NaCl with reason. (iii) Products formed when electric current passes through brine solution.

Solution: (i) Acid: HCl | Base: NaOH. (ii) Neutral (pH = 7) because it is formed by a strong acid and strong base reaction. (iii) Chlor-alkali process:
  • Anode: Cl_2 gas
  • Cathode: H_2 gas
  • Near Cathode: NaOH solution

Q38. Case Study: Stimulus and Response (i) Name and define action of sudden hand withdrawal from flame. (ii) Role of motor and relay neurons. (iii) Components of human nervous system.

Solution: (i) Reflex Action: Involuntary, immediate automatic response to a stimulus without conscious thinking. (ii) Motor Neuron: Transmits command signals from CNS to effector muscle. Relay Neuron: Connects sensory and motor neurons inside the spinal cord. (iii) CNS: Brain & Spinal Cord | PNS: Cranial & Spinal Nerves.

Q39. Case Study: Dispersion of Light by Glass Prism (i) Natural occurrence of spectrum. (ii) What happens to white light in prism? (iii) Two necessary conditions to observe a rainbow.

Solution: (i) Rainbow formation in the sky. (ii) White light undergoes refraction and dispersion into 7 constituent colors (VIBGYOR) due to different speeds/wavelengths. (iii) Conditions: Presence of water droplets in atmosphere and Sun positioned behind the observer.