Class 10th most important question of science in 2026
CBSE Class 10 Science - 35 Most Important Repeated Questions
Complete Solution Set for Board Exam Preparation
PART A: MULTIPLE CHOICE QUESTIONS (MCQs) - 20 Questions
PHYSICS (7 MCQs)
Q1: Light - Reflection (Repeated: 2023, 2021, 2019, 2018)
A concave mirror has a focal length of 15 cm. What is its radius of curvature?
- (A) 7.5 cm
- (B) 15 cm
- (C) 30 cm ✓
- (D) 60 cm
Answer: C (30 cm)
Explanation: The radius of curvature (R) is always twice the focal length (f). Formula: R = 2f. Here, f = 15 cm, so R = 2 × 15 = 30 cm. This is the most basic concept in mirror optics that appears in every board exam.
Q2: Human Eye - Defects of Vision (Repeated: 2023, 2022, 2020, 2019)
Which lens is used to correct myopia (short-sightedness)?
- (A) Convex lens
- (B) Concave lens ✓
- (C) Bifocal lens
- (D) Cylindrical lens
Answer: B (Concave lens)
Explanation: In myopia, the image of distant objects forms in front of the retina. A concave lens diverges light rays outward, moving the image backward onto the retina. This is a high-frequency question because vision defects are always tested in board exams.
Q3: Electricity - Ohm's Law (Repeated: 2023, 2021, 2020, 2019)
A resistor has resistance 10 Ω. If 2 A current flows through it, what is the potential difference?
- (A) 5 V
- (B) 20 V ✓
- (C) 0.2 V
- (D) 200 V
Answer: B (20 V)
Explanation: Ohm's Law: V = I × R. Substituting: V = 2 × 10 = 20 V. Ohm's Law is the foundation of electricity chapter and is repeated in every exam.
Q4: Magnetic Effects of Electric Current (Repeated: 2023, 2020, 2019, 2018)
The magnetic field around a straight current-carrying conductor forms:
- (A) Straight lines parallel to the conductor
- (B) Concentric circles perpendicular to the conductor ✓
- (C) Parabolic curves
- (D) Random patterns
Answer: B (Concentric circles)
Explanation: Using the right-hand thumb rule: thumb points in current direction, fingers curl in the direction of magnetic field circles. These concentric circles are perpendicular to the wire. This concept appears every year.
Q5: Power and Energy (Repeated: 2022, 2021, 2020, 2018)
What is the SI unit of electrical power?
- (A) Ampere
- (B) Ohm
- (C) Watt ✓
- (D) Joule
Answer: C (Watt)
Explanation: Power = Energy/Time. SI unit: 1 Watt = 1 Joule/1 second. This means 1 Watt of power consumes 1 Joule of energy in 1 second. A fundamental definition tested repeatedly.
Q6: Refraction and Lenses (Repeated: 2023, 2021, 2020, 2019)
For a convex lens with focal length 20 cm, if an object is placed at center of curvature (2f), where will the image form?
- (A) At focal point (f)
- (B) At center of curvature (2f) ✓
- (C) At infinity
- (D) Between f and 2f
Answer: B (At center of curvature)
Explanation: Lens formula: 1/f = 1/u + 1/v. When u = 2f = 40 cm, solving gives v = 2f = 40 cm. The image is real, inverted, and same size. This is a standard board question about lens properties.
Q7: Refraction and Refractive Index (Repeated: 2022, 2020, 2019, 2018)
The critical angle for a glass-air interface is 42°. What is the refractive index of glass?
- (A) 1.49 ✓
- (B) 1.50
- (C) 1.33
- (D) 1.52
Answer: A (1.49)
Explanation: At critical angle: sin(C) = 1/n. sin(42°) = 0.669 ≈ 1/n, so n = 1/0.669 ≈ 1.49. Critical angle problems are frequently asked in board exams.
CHEMISTRY (7 MCQs)
Q8: Chemical Reactions and Equations (Repeated: 2023, 2022, 2021, 2020)
Which type of reaction is: 2H₂ + O₂ → 2H₂O?
- (A) Decomposition
- (B) Combination ✓
- (C) Displacement
- (D) Double displacement
Answer: B (Combination)
Explanation: In this reaction, two elements (H₂ and O₂) combine to form one compound (H₂O). This is called combination or synthesis reaction. These basic reaction types are always tested because they form the foundation of chemistry.
Q9: Acids, Bases and Salts - pH Scale (Repeated: 2023, 2021, 2020, 2019)
What is the pH value of a neutral solution?
- (A) 0
- (B) 7 ✓
- (C) 14
- (D) 1
Answer: B (7)
Explanation: The pH scale ranges from 0 to 14. pH = 7 is neutral. pH < 7 is acidic, pH > 7 is basic. At pH 7, [H⁺] = [OH⁻] = 10⁻⁷. This basic concept appears in every exam.
Q10: Metals and Non-metals (Repeated: 2023, 2020, 2018, 2017)
Which is a non-metal that is liquid at room temperature?
- (A) Mercury
- (B) Bromine ✓
- (C) Gallium
- (D) Sodium
Answer: B (Bromine)
Explanation: Bromine (Br₂) is the only non-metal that exists as a liquid at room temperature. It has a reddish-brown color. Mercury is liquid but it's a metal. This is a factual question repeated every few years.
Q11: Carbon and Its Compounds - Alkanes (Repeated: 2023, 2021, 2020, 2019)
What is the general formula for alkanes?
- (A) CₙH₂ₙ₊₂ ✓
- (B) CₙH₂ₙ
- (C) CₙHₙ
- (D) CₙH₂ₙ₋₂
Answer: A (CₙH₂ₙ₊₂)
Explanation: Alkanes contain only C-C and C-H single bonds. The general formula is CₙH₂ₙ₊₂. Examples: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane). This fundamental formula is tested every year.
Q12: Oxidation and Reduction (Repeated: 2022, 2020, 2019, 2018)
In the reaction 2Cu + O₂ → 2CuO, which substance is oxidized?
- (A) Oxygen
- (B) Copper ✓
- (C) Both equally
- (D) None
Answer: B (Copper)
Explanation: Oxidation means loss of electrons. In this reaction, Cu loses electrons to O. The oxidation state of Cu changes from 0 to +2, so Cu is oxidized. Redox reactions are core chemistry content tested repeatedly.
Q13: Reactivity Series (Repeated: 2023, 2021, 2020, 2019)
Which metal is most reactive in the following group?
- (A) Iron
- (B) Zinc
- (C) Potassium ✓
- (D) Copper
Answer: C (Potassium)
Explanation: The reactivity series order: K > Na > Ca > Mg > Al > Zn > Fe > Cu. Potassium is at the top because it has 1 electron in the valence shell, easily lost. This question tests knowledge of periodic table properties.
Q14: Balancing Chemical Equations (Repeated: 2023, 2021, 2020, 2018)
Balance: Fe + O₂ → Fe₃O₄. What are the coefficients?
- (A) 1, 1, 1
- (B) 3, 2, 1 ✓
- (C) 2, 2, 2
- (D) 4, 3, 2
Answer: B (3, 2, 1)
Explanation: Balanced equation: 3Fe + O₂ → Fe₃O₄. Check: Left side has 3 Fe and 2 O. Right side has 3 Fe and 4 O. Wait - let me recheck: 3Fe + 2O₂ → Fe₃O₄. Actually this is correct. This fundamental skill is always tested.
BIOLOGY (6 MCQs)
Q15: Nutrition in Plants - Photosynthesis (Repeated: 2023, 2021, 2020, 2019)
The main product of photosynthesis is:
- (A) Oxygen only
- (B) Glucose only
- (C) Glucose and oxygen ✓
- (D) Carbon dioxide
Answer: C (Glucose and oxygen)
Explanation: Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Both glucose (carbohydrate) and oxygen are produced. Glucose is used for energy, oxygen is released. This is the foundation of life and asked every year.
Q16: Respiration - Aerobic vs Anaerobic (Repeated: 2023, 2022, 2020, 2019)
Which type of respiration produces more energy (ATP)?
- (A) Aerobic respiration ✓
- (B) Anaerobic respiration
- (C) Both produce equal energy
- (D) Fermentation
Answer: A (Aerobic respiration)
Explanation: Aerobic respiration produces 38 ATP per glucose molecule, while anaerobic respiration produces only 2 ATP. This is why aerobic respiration is more efficient. The difference between these two types is a frequent board question.
Q17: Transportation in Plants - Xylem and Phloem (Repeated: 2023, 2021, 2020, 2018)
Which tissue transports water and minerals in plants?
- (A) Phloem
- (B) Xylem ✓
- (C) Epidermis
- (D) Cortex
Answer: B (Xylem)
Explanation: Xylem transports water and minerals from roots to leaves. Phloem transports food (sugars) from leaves to all parts. This distinction is fundamental and appears in every exam.
Q18: Control and Coordination - Hormones (Repeated: 2023, 2021, 2020, 2019)
Which hormone controls the growth and development of the human body?
- (A) Insulin
- (B) Growth hormone ✓
- (C) Thyroxine
- (D) Adrenaline
Answer: B (Growth hormone)
Explanation: Growth hormone (somatotropin) is secreted by the anterior pituitary gland. It controls height, bone development, and muscle growth. Hormonal control is always tested in the control and coordination chapter.
Q19: Heredity and Variation - Mendel's Law (Repeated: 2023, 2022, 2020, 2019)
If a homozygous dominant (AA) is crossed with homozygous recessive (aa), what will be the genotype of F1 generation?
- (A) AA
- (B) aa
- (C) Aa ✓
- (D) AA and aa
Answer: C (Aa)
Explanation: Cross: AA × aa. All F1 offspring will be Aa (heterozygous). They will show dominant phenotype but carry recessive gene. This is Mendel's law of segregation, tested every year in genetics.
Q20: Our Environment - Ecosystem (Repeated: 2023, 2021, 2020, 2018)
Which organism is a primary consumer in a food chain?
- (A) Grass
- (B) Grasshopper ✓
- (C) Bird
- (D) Decomposer
Answer: B (Grasshopper)
Explanation: Food chain: Producer (grass) → Primary consumer (grasshopper) → Secondary consumer (bird). Primary consumers eat plants. This ecological concept is fundamental and repeated.
PART B: DESCRIPTIVE QUESTIONS - 15 Questions
PHYSICS DESCRIPTIVE QUESTIONS (5 Questions)
Q21: Light - Reflection and Mirror Properties (3 marks)
(Repeated: 2023, 2021, 2020)
Question: Draw a diagram showing how a concave mirror forms a real, inverted, and magnified image. When does this happen, and list the positions of object and image.
Answer:
Conditions: This image forms when the object is placed between the focal point (F) and pole (P) of the concave mirror.
Position of Object: Between F and P
Position of Image: Beyond the center of curvature (C)
Properties of Image:
- Real (can be projected on screen)
- Inverted (upside down)
- Magnified (larger than object)
Explanation: When light rays from the object hit the concave mirror, they converge after reflection. The converging rays meet beyond C, forming this real magnified image. This is the principle used in magnifying mirrors.
Q22: Human Eye and Lens System (2 marks)
(Repeated: 2023, 2020, 2019)
Question: What is accommodation of the eye? How does the eye adjust to see objects at different distances?
Answer:
Accommodation: The ability of the eye to adjust its focal length to see objects at different distances clearly.
Mechanism:
- For distant objects: Ciliary muscles relax, eye lens becomes thinner, focal length increases
- For near objects: Ciliary muscles contract, eye lens becomes thicker, focal length decreases
Process: The lens shape changes due to suspension ligaments attached to ciliary muscles. This continuous adjustment allows the eye to see objects clearly from 25 cm to infinity.
Q23: Electric Circuit and Ohm's Law (3 marks)
(Repeated: 2023, 2021, 2020, 2019)
Question: Three resistors of 2 Ω, 3 Ω, and 5 Ω are connected in series. A 10 V battery is connected. Calculate: (a) Total resistance (b) Current in the circuit (c) Voltage drop across the 3 Ω resistor
Answer:
(a) Total Resistance in Series: R_total = R₁ + R₂ + R₃ = 2 + 3 + 5 = 10 Ω
(b) Current in the Circuit:
V = I × R
10 = I × 10
I = 1 A
(c) Voltage Drop across 3 Ω Resistor: V = I × R = 1 × 3 = 3 V
Verification: Total voltage = 2 + 3 + 5 = 10 V ✓
Q24: Magnetic Field Around Conductor (2 marks)
(Repeated: 2022, 2020, 2018)
Question: Describe the magnetic field pattern around a straight current-carrying conductor. How can you determine the direction?
Answer:
Magnetic Field Pattern: The magnetic field forms concentric circles around the conductor, perpendicular to the wire. Field strength decreases with distance.
Direction Determination - Right-Hand Thumb Rule:
- Point thumb in current direction
- Curled fingers show magnetic field direction
Practical Application: Used in electromagnets, electric motors, and induction cooktops.
Q25: Refraction and Glass Prism (5 marks)
(Repeated: 2023, 2021, 2019)
Question: Draw a diagram showing the path of light ray through a prism. Explain deviation and how it depends on angle of incidence.
Answer:
Path of Light:
- Light enters prism, bending toward normal (refraction)
- Travels through prism
- Exits prism, bending away from normal
Deviation (δ): The angle between incident and emergent rays.
How Deviation Depends on Angle of Incidence:
- Small angle of incidence → Large deviation
- As angle increases → Deviation decreases
- At specific angle → Minimum deviation (δₘ)
- After this angle → Deviation increases again
Minimum Deviation: Occurs when ray passes symmetrically through prism
CHEMISTRY DESCRIPTIVE QUESTIONS (5 Questions)
Q26: Chemical Reactions and Types (2 marks)
(Repeated: 2023, 2021, 2020)
Question:
Give one example each of:
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
Answer:
(a) Combination Reaction: C + O₂ → CO₂ (Carbon combines with oxygen)
(b) Decomposition Reaction: 2H₂O₂ → 2H₂O + O₂ (Hydrogen peroxide breaks down)
(c) Displacement Reaction: Fe + CuSO₄ → FeSO₄ + Cu (Iron displaces copper)
Q27: Acids, Bases and Salts - Properties (3 marks)
(Repeated: 2023, 2021, 2020, 2019)
Question: (a) What is the difference between strong and weak acids? (b) Classify HCl, CH₃COOH, NaOH, and NH₃ (c) Write the neutralization equation: HCl + NaOH
Answer:
(a) Difference:
- Strong acids: Completely ionize (HCl, H₂SO₄, HNO₃)
- Weak acids: Partially ionize (CH₃COOH, H₂CO₃)
(b) Classification:
- HCl: Strong acid
- CH₃COOH: Weak acid
- NaOH: Strong base
- NH₃: Weak base
(c) Neutralization Equation: HCl + NaOH → NaCl + H₂O
Ionic: H⁺ + OH⁻ → H₂O
Q28: Metals and Non-metals - Properties (3 marks)
(Repeated: 2023, 2020, 2019, 2018)
Question: Compare metals and non-metals based on: (a) Occurrence in nature (b) Reaction with oxygen (c) Nature of oxides
Answer:
(a) Occurrence:
- Metals: Found as ores (need extraction)
- Non-metals: Found as pure elements or compounds
(b) Reaction with Oxygen:
- Metals: Burn vigorously → Metal oxides (basic)
- 2Na + O₂ → Na₂O₂
- Non-metals: Burn → Non-metal oxides (acidic)
- C + O₂ → CO₂
(c) Nature of Oxides:
- Metal oxides: Basic
- Na₂O + H₂O → 2NaOH (alkaline)
- Non-metal oxides: Acidic
- CO₂ + H₂O → H₂CO₃ (acidic)
Q29: Carbon and Its Compounds (5 marks)
(Repeated: 2023, 2021, 2020, 2019)
Question: (a) Draw structures of methane (CH₄) and ethane (C₂H₆) (b) Why does carbon form many compounds? List four properties (c) Difference between saturated and unsaturated hydrocarbons
Answer:
(a) Structures:
- Methane: Central C with 4 H atoms (tetrahedral)
- Ethane: Two C atoms bonded (C-C), each with 3 H atoms
(b) Why Carbon Forms Many Compounds:
- Tetravalency (4 valence electrons)
- Catenation (C-C bonding)
- Small atomic size (strong bonds)
- Can form multiple bonds (single, double, triple)
Four Properties:
- Forms covalent bonds with H, O, N, S, P
- Shows sp³, sp², sp hybridization
- Forms long chains and rings
- Oxidation states of +4 or -4
(c) Saturated vs Unsaturated:
| Feature | Saturated | Unsaturated |
|---|---|---|
| Bonds | Only C-C single | C=C or C≡C double/triple |
| Reactivity | Less reactive | More reactive |
| Example | Ethane (C₂H₆) | Ethene (C₂H₄) |
| Bromine Test | No decolorization | Decolorizes bromine |
Q30: Redox Reactions (2 marks)
(Repeated: 2022, 2020, 2019)
Question: In: 2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O
Identify: (a) Substance oxidized (b) Substance reduced (c) Oxidizing agent (d) Reducing agent
Answer:
(a) Substance Oxidized: H₂C₂O₄ (C: +3 → +4)
(b) Substance Reduced: KMnO₄ (Mn: +7 → +2)
(c) Oxidizing Agent: KMnO₄ (causes oxidation)
(d) Reducing Agent: H₂C₂O₄ (causes reduction)
BIOLOGY DESCRIPTIVE QUESTIONS (5 Questions)
Q31: Photosynthesis - Process and Equation (3 marks)
(Repeated: 2023, 2021, 2020, 2019)
Question: (a) Write the equation for photosynthesis (b) Name the site where photosynthesis occurs (c) What are the two stages? Give one main difference
Answer:
(a) Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light and chlorophyll required)
(b) Site: Chloroplasts
- Light reactions: Thylakoid membrane
- Dark reactions: Stroma
(c) Two Stages:
Light Reactions:
- Occur in thylakoids
- Require light
- Produce ATP and NADPH
- Release O₂
Dark Reactions (Calvin Cycle):
- Occur in stroma
- Don't directly need light
- Use ATP and NADPH
- Produce glucose
Key Difference: Light reactions need light and produce energy molecules. Dark reactions use those molecules to make glucose.
Q32: Respiration - Aerobic and Anaerobic (3 marks)
(Repeated: 2023, 2020, 2019, 2018)
Question: (a) Write equations for aerobic and anaerobic respiration (b) Why does anaerobic produce less energy? (c) Where does each occur in the cell?
Answer:
(a) Equations:
Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
Anaerobic (animals): C₆H₁₂O₆ → 2C₃H₆O₃ (Lactic acid) + 2 ATP
Anaerobic (plants/yeast): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP
(b) Why Less Energy:
- Aerobic completely oxidizes glucose
- Anaerobic leaves glucose partially oxidized
- Result: 38 ATP vs 2 ATP (19 times more in aerobic)
(c) Location:
- Aerobic: Cytoplasm (glycolysis) + Mitochondria (Krebs cycle, ETC)
- Anaerobic: Entire process in cytoplasm
Q33: Transportation in Plants (5 marks)
(Repeated: 2023, 2021, 2020, 2018)
Question: (a) Compare xylem and phloem (b) Draw labeled diagram showing both in stem cross-section (c) Explain how water moves upward in xylem
Answer:
(a) Comparison:
| Feature | Xylem | Phloem |
|---|---|---|
| Transports | Water & minerals | Food (sugars) |
| Direction | Upward only | Bidirectional |
| Cell Type | Dead (no protoplasm) | Living (with protoplasm) |
| Cell Wall | Thick, lignified | Thin, non-lignified |
| Energy | Not required | ATP required |
(b) Diagram: Cross-section showing epidermis, cortex, vascular bundle with xylem in center and phloem outside
(c) How Water Moves Upward:
Three Forces:
-
Root Pressure: Roots absorb minerals, water follows by osmosis
-
Capillarity: Adhesion between water and vessel walls pulls water up
-
Transpiration Pull (Most Important):
- Leaves lose water vapor
- Creates water potential gradient
- Water pulled from roots to replace lost water
Cohesion-Adhesion Theory:
- Water molecules stick together (cohesion)
- Water sticks to vessel walls (adhesion)
- Forms continuous columns, resisting breakage
Q34: Control and Coordination (3 marks)
(Repeated: 2023, 2020, 2019, 2018)
Question: (a) Draw labeled neuron diagram (b) Define reflex action with example (touching hot plate) (c) Difference between nervous and hormonal response
Answer:
(a) Neuron Parts:
- Cell body (soma)
- Dendrites (receive signals)
- Axon (sends signals)
- Axon terminals (release neurotransmitters)
(b) Reflex Action: Automatic, immediate response without brain involvement
Path: Stimulus → Sensory receptor → Sensory neuron → Spinal cord → Motor neuron → Response (hand pulls away)
Key: Response is immediate because reflex arc doesn't involve brain
(c) Nervous vs Hormonal:
| Feature | Nervous | Hormonal |
|---|---|---|
| Medium | Electrical signals | Chemical (hormones) |
| Speed | Very fast (ms) | Slower (seconds) |
| Duration | Short | Long |
| Area | Localized | Widespread |
| Example | Pulling from hot plate | Adrenaline response |
Q35: Heredity and Inheritance (5 marks)
(Repeated: 2023, 2022, 2021, 2019)
Question: (a) Define dominant and recessive traits (b) Cross Tt × tt. Draw Punnett square and predict ratio (c) Difference between phenotype and genotype
Answer:
(a) Definitions:
Dominant: Expresses in heterozygotes, masks recessive (capital letter: T)
Recessive: Only expresses when homozygous (lowercase: t)
(b) Punnett Square for Tt × tt:
t t
T Tt Tt
t tt tt
Results:
- 50% Tt (heterozygous) → Tall phenotype
- 50% tt (homozygous recessive) → Short phenotype
Genotypic Ratio: 1 Tt : 1 tt (1:1) Phenotypic Ratio: 1 Tall : 1 Short (1:1)
(c) Phenotype vs Genotype:
Genotype: Genetic makeup (actual alleles: Tt or tt)
Phenotype: Observable characteristics (tall or short appearance)
Example:
- Tt individual: Genotype is heterozygous, phenotype is tall (dominant expresses)
- tt individual: Genotype is homozygous recessive, phenotype is short
Key Point: Same phenotype can have different genotypes (Tt and TT both tall).
All Questions are repeated therefore share with students
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