CBSE Class 10 Science Board Exam 2026 Solutions & Answer Key

Complete solutions and answer key for the CBSE Class 10 Science Board Exam Paper 2026

Post Date: February 27, 2026

If you have just stepped out of the examination hall and are looking to evaluate your performance, you are in the right place. We have provided the complete and accurate CBSE Class 10 Science Board Exam Paper 2026 Solutions to help you verify your answers.

The Class 10 Science board exam tests core concepts across Physics, Chemistry and Biology with analyzing your paper is the best way to estimate your final score. Below, you will find detailed, step-by-step explanations and the official-style marking scheme for all question paper sets (Set 1, 2 and 3). Scroll down to review the comprehensive answer key or download the solved question paper PDF for your reference.

CBSE Class 10 Science Question Paper 2026 PDF Download

Section A Biology

1. Choose the equation of reaction that correctly represents anaerobic respiration in muscles:
(A) Glucose –> Pyruvate –[Absence of O₂]–> Ethanol + CO₂ + Energy
(B) Glucose –[O₂]–> Pyruvate –[O₂]–> Energy
(C) Glucose –[O₂]–> Pyruvate –[Lack of oxygen]–> Ethanol + Energy
(D) Glucose –> Pyruvate –[Lack of oxygen]–> Lactic acid + Energy
Answer:
(D) Glucose –> Pyruvate –[Lack of oxygen]–> Lactic acid + Energy
Explanation:
In muscles, during anaerobic respiration (no oxygen), glucose breaks down to pyruvate, then pyruvate converts to LACTIC ACID + Energy. Ethanol + CO₂ is produced in YEAST, not muscles.

2. Which of the following is a non-biodegradable pollutant?
(A) Paper
(B) DDT
(C) Wood
(D) Vegetable peel
Answer:
(B) DDT
Explanation:
DDT cannot be broken down by microorganisms. It accumulates in food chain (biomagnification). Paper, Wood, Vegetable peel are all biodegradable.

3. An ecosystem contains maximum number of:
(A) Producers
(B) Herbivores
(C) Carnivores
(D) Omnivores
Answer:
(A) Producers
Explanation:
According to 10% energy law, energy decreases at each trophic level. Producers (plants) form the base of food pyramid and are maximum in number and biomass.

4. Identify the part that controls the closing and opening of the stomatal pore in leaves of plants.
Select the correct option
(A) Stomata
(B) Epidermal cells
(C) Guard cells
(D) Chloroplasts
Answer:
(C) Guard cells
Explanation:
Guard cells are bean-shaped cells surrounding stomatal pore.
– Turgid guard cells –> pore opens
– Flaccid guard cells –> pore closes

5. Select the INCORRECT statement from the following:
(A) Ozone is a molecule formed by three atoms of oxygen.
(B) Ozone shields earth’s surface from harmful infrared radiations.
(C) CFC’s are linked to decline of O3 layer in our atmosphere.
(D) Ozone at higher levels is a product of UV radiations acting on oxygen molecule.
Answer:
(B) Ozone shields earth’s surface from harmful infrared radiations.
Explanation:
Statement (B) is INCORRECT.
Ozone shields earth from UV radiations, NOT infrared radiations.
All other statements are correct.

6. In human beings, the implantation of fertilised egg takes place in which part of female reproductive system?
(A) Oviduct
(B) Cervix
(C) Uterus
(D) Vagina
Answer:
(C) Uterus
Explanation:
After fertilization in the fallopian tube, zygote travels and implants in the thick spongy wall of the UTERUS (endometrium).

7. When a human egg is fertilized by a sperm having ‘Y’ chromosome, the zygote has the following combination of chromosomes:
(A) 44 + XX
(B) 22 + XX
(C) 44 + XY
(D) 22 + XY
Answer:
(C) 44 + XY
Explanation:
Egg = 22 + X chromosomes
Sperm with Y = 22 + Y chromosomes
Zygote = (22 + X) + (22 + Y) = 44 + XY –> Male child

Questions No. 8 & 9 consist of two statements – Assertion (A) and Reason (R).
Answer these questions selecting the appropriate option from (A), (B), (C) and (D) given below:
(A) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of the Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason R is true.
8. Assertion (A): Bacteria that can withstand heat have better chances of survival in a heat wave.
Reason (R): Accumulation of variations in a species increases chances of survival in changing environment.
Answer:
(A)
Explanation:
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of the Assertion (A).
Heat-resistant bacteria survive heat waves due to accumulated variations — this is the basis of natural selection.

9. Assertion (A): In case of iodine deficiency in our diet, there is a possibility that we might suffer from goitre.
Reason (R): One of the symptoms of goitre is swelling of neck.
Answer:
(B)
Explanation:
Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
R only states a symptom, not the cause-effect relationship.

10. Briefly mention the steps in double-circulation through human heart.
Answer:
Double circulation = blood passes through heart TWICE per cycle.
1. PULMONARY CIRCULATION (Right side):
Deoxygenated blood
–> Right Atrium
–> Right Ventricle
–> Lungs (via pulmonary artery)
–> Gets oxygenated
–> Left Atrium (via pulmonary vein)

2. SYSTEMIC CIRCULATION (Left side):
Oxygenated blood
–> Left Atrium
–> Left Ventricle
–> All body parts (via Aorta)
–> Deoxygenated blood returns to Right Atrium (via Vena cava)
Result: Oxygenated and deoxygenated blood never mix.

11. Why the lungs always contain a residual volume of air?
Answer:
Lungs always contain residual volume (~1000-1200 mL) because:
1. Lungs are in a CLOSED THORACIC CAVITY with elastic walls
that cannot be compressed completely.
2. ALVEOLI never collapse completely due to their elastic
nature and surfactant lining.
Importance:
– Ensures sufficient time for oxygen absorption into blood.
– Allows CONTINUOUS GAS EXCHANGE between breaths.
– Prevents lungs from collapsing entirely.

12. (A). Observe the given figure and identify the labelled parts P, Q, R and S:

Class 10 Biology CBSE Question 12 showing Reflex Action

Answer:
P = Receptor (Sensory nerve ending in skin/finger)
Q = Sensory (Afferent) nerve / Relay neuron
R = Effector (Muscle)
S = Motor (Efferent) nerve

The figure shows a REFLEX ARC:
Stimulus (hot object) –> Receptor(P) –> Sensory nerve(Q) –> Spinal cord –> Motor nerve(S) –> Effector/Muscle(R) –> Hand withdraws (Message also sent to brain simultaneously)

12. (B). Which of the plant hormones are responsible for the following processes?
(i) Promote cell division
(ii) Inhibition of growth
(iii) Detection of light
(iv) Wilting of leaves
Answer:
(i) Promote cell division –> Cytokinin
(ii) Inhibition of growth –> Abscisic Acid (ABA)
(iii) Detection of light –> Auxin (causes phototropism)
(iv) Wilting of leaves –> Abscisic Acid (ABA)

13. (a). Mention any one harmful effect of using plastic bags on the environment. Suggest better alternatives.
Answer:
Harmful effect:
Plastic bags are NON-BIODEGRADABLE. They persist for hundreds
of years, clog drains, pollute soil and water, harm animals,
and release toxic chemicals when burned.

Better alternatives:
– Cloth / Jute bags (reusable, biodegradable)
– Paper bags (biodegradable, recyclable)
– Bamboo baskets (eco-friendly)
– Reusable bags made of natural fibres

(b). Paddy fields require a large amount of water. The pesticides and chemical fertilizers used are washed down into the soil or waterbodies. How do these chemicals reach our bodies? What is this phenomenon known as?
Answer:
Path of chemicals reaching our body:
Pesticides/Fertilizers –> washed into water bodies
–> absorbed by aquatic plants and small organisms (plankton)
–> small fish eat plankton
–> larger fish eat smaller fish
–> humans eat larger fish / crops from contaminated soil
At each step, concentration of chemicals INCREASES because they are stored in fatty tissues and not broken down easily.
Phenomenon: BIOMAGNIFICATION (Biological Magnification)
The concentration of harmful chemicals increases at each successive trophic level, reaching HIGHEST concentration in top consumers (humans).

14(a). Show a cross between round & green seed pea plant with wrinkled & yellow seeds upto F₂ generation.
Answer:
Traits:
– Round (R) is dominant over Wrinkled (r)
– Yellow (Y) is dominant over Green (y)
Parent plants:
Round Green = RRyy
Wrinkled Yellow = rrYY
F1 Generation:
RRyy x rrYY –> All RrYy (Round Yellow)
[All F1 plants look Round and Yellow]
F1 x F1 Cross:
RrYy x RrYy –> F2 Generation

14 (b). Also write down results of F₂ generation.
Answer:
F2 RESULTS (Ratio 9:3:3:1):
– Round Yellow = 9 parts
– Round Green = 3 parts
– Wrinkled Yellow = 3 parts
– Wrinkled Green = 1 part
This demonstrates Mendel’s LAW OF INDEPENDENT ASSORTMENT.

15. Kidneys help to regulate the volume of fluid and various metabolites waste products in the body.
Normally, kidney filter about 180 L of fluid daily but the volume actually excreted out is only a litre or two a day.
Patients with kidney failures can be saved by dialysis and kidney transplant.
(a). Write the structure and function of Bowman’s capsule.
Answer:
Structure: Cup-shaped double-walled structure at beginning of nephron
in kidney. Surrounds a knot of capillaries called GLOMERULUS.
Together they form the MALPIGHIAN BODY (Renal Corpuscle).
Function: Acts as FILTRATION UNIT. Blood is filtered under high pressure
in glomerulus. Filtrate (water, salts, glucose, urea, etc.)
passes into Bowman’s capsule to begin urine formation.

(b). Although kidneys filter a large amount (about 180 L daily) of fluids, still the excretion from the body is only about a litre or two. Why?
Answer:
Out of 180 L filtered, about 178-179 L is REABSORBED back into blood by nephron tubules through SELECTIVE REABSORPTION.
Useful substances reabsorbed: glucose, amino acids, water, salts.
Only waste products (urea, excess salts) remain = 1-2 L urine.

(c). What is excretion? Why is it necessary for any living organism?
Answer:
Excretion: The process of REMOVAL OF HARMFUL METABOLIC WASTE PRODUCTS produced during chemical reactions in the body.
Why it is necessary:
1. Waste products like urea, CO2, excess salts are TOXIC.
If accumulated, they poison cells and disrupt functioning.
2. Maintains CHEMICAL BALANCE (homeostasis) of body fluids.
3. Prevents CELL DAMAGE and ORGAN FAILURE.
Without excretion, toxic build-up leads to death.

(c). State two similarities between lungs and kidneys.
Answer:
Similarity 1:
Both are EXCRETORY ORGANS.
– Lungs excrete: CO2 and water vapour
– Kidneys excrete: urea and excess salts

Similarity 2:
Both maintain HOMEOSTASIS (internal balance).
– Lungs regulate: CO2 / O2 levels
– Kidneys regulate: water and salt balance

16 (A). (i) Sugarcane does not produce seeds so name the process through which it will be able to reproduce.
(ii) List any two advantages of this method.
(iii) Besides sugarcane, give two more example of plants that reproduce by this method.
(iv) Why is regeneration not possible in all the types of animals?
Answer:
(i) VEGETATIVE PROPAGATION (specifically stem cuttings)
(ii) Two advantages:
– All offspring are GENETICALLY IDENTICAL to parent (useful for maintaining desirable traits)
– Plants can be grown WITHOUT SEEDS, faster reproduction (especially useful for seedless varieties like sugarcane)
(iii) Two more examples:
– Potato (reproduces through tubers / eyes)
– Rose / Money Plant (reproduces through stem cuttings)
(iv) Regeneration not possible in all animals because:
– Higher animals have HIGHLY SPECIALISED CELLS that have lost ability to divide and form a new organism.
– Only SIMPLE ORGANISMS (Planaria, Hydra) with less specialised cells can regenerate fully.
– Complex animals have cells organised into SPECIFIC TISSUES
AND ORGANS lacking totipotency (ability to develop into a complete organism).

(B). (i) In an angiospermic flower, fertilization is said to take place when male gametes carried in pollen tube fuse with the femal gamete, present in the embryo sac. This results in lot of changes in different parts of the flower. State the changes taking place in the following structures of flower post fertilization:
(a) Zygote
(b) Ovule
(c) Ovary
(d) Sepals
Answer:
Post-fertilization changes:
(a) Zygote –> Develops into EMBRYO (future plant) through repeated cell division
(b) Ovule –> Develops into SEED (embryo + food reserve + seed coat)
(c) Ovary –> Develops into FRUIT (protects the seed)
(d) Sepals –> WITHER AND FALL OFF (no longer needed after fertilization)

(ii). Define germination.
Answer:
Germination: Process by which a SEED SPROUTS and develops into a new
plant under favourable conditions:
– Water
– Warmth
– Air (oxygen)
– Sometimes light
The radicle grows into root, plumule grows into shoot.

Section-Wise Science Paper Solutions – Chemistry

17. Which of the following is an amphoteric oxide?
(A) Na₂O
(B) K₂O
(C) CO₂
(D) Al₂O₃
Answer:
(D) Al₂O₃
Explanation:
Amphoteric oxide = reacts with BOTH acids and bases.
1. Al₂O₃ + 6HCl –> 2AlCl₃ + 3H₂O (Reacts with Acid)
2. Al₂O₃ + 2NaOH –> 2NaAlO₂ + H₂O (Reacts with Base)

18. Which of the following will NOT undergo addition reaction?
(A) C₄H₈
(B) C₂H₂
(C) C₃H₈
(D) C₂H₄
Answer:
(C) C₃H₈
Explanation:
Addition reactions occur in UNSATURATED compounds (Alkenes and Alkynes).
– C₄H₈ = Butene (alkene, double bond) –> Undergoes addition
– C₂H₂ = Ethyne (alkyne, triple bond) –> Undergoes addition
– C₂H₄ = Ethene (alkene, double bond) –> Undergoes addition
– C₃H₈ = Propane (alkane, saturated) –> Does NOT undergo addition –> Undergoes SUBSTITUTION reaction.

19. Magnesium ribbon burns with dazzling white flame and changes into a white powder as shown in the diagram given below:

Class 10 Science CBSE Board Paper Question 19

The powder is dissolved in water.
Identify the chemical formula of the white powder and the correct change observed when the solution is tested with litmus paper.
(A) MgO, turns blue litmus into red.
(B) MgO, turns red litmus blue.
(C) MgO₂, turns red litmus blue.
(D) Mg₂O, turns blue litmus red.
Answer:
(B) MgO, turns red litmus blue.
Explanation:
Reaction 1: 2Mg + O₂ –> 2MgO (White powder = Magnesium oxide)
Reaction 2: MgO + H₂O –> Mg(OH)₂ (Magnesium hydroxide)
Mg(OH)₂ is a Base –> turns RED litmus BLUE

20. Which of the following set of compounds does not belong to same homologous series?
(A) CH₄ and C₄H₁₀
(B) C₂H₆ and C₃H₈
(C) C₃H₈ and C₅H₁₂
(D) C₄H₈ and C₅H₁₂
Answer:
(D) C₄H₈ and C₅H₁₂
Explanation:
Alkane formula = CₙH₂ₙ₊₂
Alkene formula = CₙH₂ₙ
C₄H₈ –> follows CₙH₂ₙ (ALKENE series)
C₅H₁₂ –> follows CₙH₂ₙ₊₂ (ALKANE series)
They belong to DIFFERENT homologous series.
Options A, B, C are all alkanes (same series).

21. Four solutions, namely fructose, ethanol, hydrochloric acid and sodium hydroxide filled in four separate beakers are connected one by one in an electric circuit with a bulb. The solutions in which the bulb will glow when current passed are:
(A) ethanol and hydrochloric acid
(B) fructose and ethanol
(C) fructose and sodium hydroxide
(D) hydrochloric acid and sodium hydroxide
Answer:
(D) hydrochloric acid and sodium hydroxide
Explanation:
Bulb glows only when solution CONDUCTS ELECTRICITY (has free ions).
HCl –> H+ and Cl- ions –> CONDUCTS electricity
NaOH –> Na+ and OH- ions –> CONDUCTS electricity
Fructose and Ethanol –> NON-ELECTROLYTES –> do NOT conduct

22. Study the following table and select the correct option:
IMAGE
Answer:
(D) CH₃COONa, CH₃COOH, NaOH, Basic
Explanation:
(A) NaCl (strong acid HCl + strong base NaOH) = NEUTRAL, not Basic
(B) Na₂CO₃ (weak acid H₂CO₃ + strong base NaOH) = BASIC, not Neutral
(C) Na₂SO₄ (strong acid + strong base) = NEUTRAL, not Acidic
(D) CH₃COONa (weak acid CH₃COOH + strong base NaOH) = BASIC [CORRECT]

23. Galvanization is a process of coating iron articles with a thin layer of zinc to prevent iron from rusting. The iron is protected even if the zinc coating is scratched and iron is exposed. Which of the following is/are true about how zinc prevents the rusting of iron?
(P) A galvanised iron article does not undergo oxidation.
(Q) The zinc coating prevents contact of iron with air.
(R) Zinc provides continuous protection to iron from rusting.
(A) Only (P)
(B) Only (Q)
(C) (Q) and (R)
(D) (P) and (Q)
Answer:
(C) (Q) and (R)
Explanation:
(P) INCORRECT – Zinc itself can oxidize; iron is protected
because zinc is more reactive (sacrificial anode).
(Q) CORRECT – Zinc coating physically prevents air/moisture
from reaching iron.
(R) CORRECT – Even when scratched, zinc being more reactive
protects iron through CATHODIC PROTECTION continuously.

24. In the question two statements are given – one labelled as Assertion (A) and other labelled as Reason (R). Answer this question selection the appropriate option from (A), (B), (C) and (D) given below:
Assertion (A): Reaction of quick lime with water is an exothermic reaction.
Reason (R) : A large amount of heat is evolved on the reaction of quick lime and water.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, Reason (R) is not the correct explanation of the Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.
Answer:
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
Explanation:
CaO + H₂O –> Ca(OH)₂ + Heat
Quick lime reacts vigorously with water releasing large amount of heat –> exothermic. R directly explains A.

25. Write the preparation of Baking soda. How does it help in making cakes soft and spongy?
Answer:
Chemical name: Sodium Hydrogen Carbonate
Formula: NaHCO₃
Preparation (Solvay Process):
NaCl + H₂O + CO₂ + NH₃ –> NaHCO₃ (solid) + NH₄Cl
How it makes cakes soft and spongy:
On heating during baking:
2NaHCO₃ –[heat]–> Na₂CO₃ + H₂O + CO₂ (gas)
The CO₂ gas gets trapped in the batter and expands, creating bubbles that make cake light, soft and spongy.
Baking soda = leavening agent.

26. Which happens when
(i) potassium iodide solution is added to lead (II) nitrate solution?
(ii) zinc is added to copper (II) chloride solution?
(iii) hydrogen gas is passed over hot copper (II) oxide?
Write balanced chemical equations of the reactions involved in support of your answer.
Answer:
(i) Potassium iodide + Lead(II) nitrate:
Yellow precipitate of Lead Iodide forms.
Type: Double displacement / Precipitation reaction
2KI + Pb(NO₃)₂ –> PbI₂ (down arrow, yellow) + 2KNO₃

(ii) Zinc + Copper(II) chloride:
Zinc is more reactive, displaces copper.
Blue colour of CuCl₂ fades. Reddish-brown copper deposits.
Type: Displacement reaction
Zn + CuCl₂ –> ZnCl₂ + Cu (down arrow)

(iii) Hydrogen gas over hot Copper(II) oxide:
Hydrogen reduces CuO to copper.
Black CuO turns reddish-brown. Water forms.
Type: Reduction / Redox reaction
CuO + H₂ –> Cu + H₂O

27. (A) (a) Give the chemical name and formula of Plaster of Paris.
(b) Write the chemical equation of its preparation.
(c) Give any two uses of it.
Answer:
(a) Chemical name: Calcium Sulphate Hemihydrate
Formula: CaSO₄ . (1/2)H₂O

(b) Preparation (heating gypsum at 373 K):
CaSO₄ . 2H₂O –[373K]–> CaSO₄ . (1/2)H₂O + (3/2)H₂O
(Gypsum –> Plaster of Paris + Water)

(c) Two uses:
1. Making CASTS FOR FRACTURED BONES in hospitals
2. Making STATUES / decorative items by sculptors;
also used in making blackboard chalk and smoothing walls.

(B) (a) Name the acid present in ant’s sting.
(b) Give reason:
(i) While diluting an acid, it is recommended that the acid should be added to the water.
(ii) Baking soda is used as an antacid.
Answer:
(a) Acid in ant’s sting = METHANOIC ACID (Formic acid)
Formula: HCOOH

(b)(i) Why acid added to water (not water to acid):
Dissolving acid in water is EXOTHERMIC (releases heat).
If water added to acid: intense heat generated suddenly can cause solution to SPLATTER or BOIL VIOLENTLY –> acid burns.
If acid added slowly to large volume of water: heat is absorbed by large mass of water –> temperature rises
GRADUALLY and SAFELY.

(b)(ii) Baking soda as antacid:
During indigestion, stomach produces excess HCl (acidity).
Baking soda (NaHCO₃) is a MILD BASE that NEUTRALISES
excess acid:
NaHCO₃ + HCl –> NaCl + H₂O + CO₂
This relieves acidity discomfort without making stomach too alkaline.

28. Alcohol forms a homologous series with general formula CₙH₂ₙ ₊ ₁ -OH and -OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is good solvent, used in medicines, cough syrups, tonics etc.
(a) Write structural formula and name of 4th member of alcohol homologous series.
(b) What happens when ethanol is heated with alkaline KMnO₄? Write chemical equation involved.
(c) Write the chemical equation of reaction of ethanol with ethanoic acid in the presence of concentrated H₂SO₄. Write the name of this reaction
Answer:
(a) 4th member of alcohol homologous series:
1st = Methanol (CH₃OH)
2nd = Ethanol (C₂H₅OH)
3rd = Propanol (C₃H₇OH)
4th = Butanol (C₄H₉OH)
Name: BUTAN-1-OL (n-Butanol)
Structural formula: CH₃ – CH₂ – CH₂ – CH₂ – OH
Short form: CH₃CH₂CH₂CH₂OH

CBSE Board Paper Class 10 Science Solution Question 28

(b) Ethanol heated with alkaline KMnO₄:
KMnO₄ is an OXIDISING AGENT. It oxidises ethanol to ethanoic acid:
CH₃CH₂OH –[Alkaline KMnO₄]–> CH₃COOH
(Ethanol) (Ethanoic acid)
KMnO₄ acts as oxidising agent.

(Ethanoic acid + Ethanol –> Ethyl ethanoate + Water)

Name of reaction: ESTERIFICATION REACTION
Product CH₃COOC₂H₅ = Ethyl ethanoate (ester, sweet fruity smell)
Conc. H₂SO₄ acts as: DEHYDRATING AGENT and CATALYST

(c) What happens when ethanol is heated with excess concentrated sulphuric acid at 443 K? Write chemical equation involve. What is the role of concentrated sulphuric acid in this reaction?
Answer:
Ethanol undergoes DEHYDRATION to form ETHENE:
CH₃CH₂OH –[Excess Conc. H₂SO₄, 443K]–> CH₂=CH₂ + H₂O
(Ethanol) (Ethene Water)
Role of concentrated H₂SO₄:
1. Acts as DEHYDRATING AGENT (removes water molecule from ethanol)
2. Acts as CATALYST (speeds up the reaction)

29. Attempt either option (A) or (B) :
(A) (i) Give reasons for the following :
(I) Ionic compounds have generally high melting points and boiling points.
(II) Solder, an alloy of lead and tin, is used for welding electrical wires.
(III) Carbon cannot reduce the oxides of Na or Mg.

(ii) The reaction of compound ‘X’ with aluminium is used to join railway tracks :
(I) Identify the compound ‘X’.
(II) Name the reaction.
(III) Write the balanced chemical equation of the reaction of compound ‘X’ with aluminium.
Answer:
(i)
(I) Reason: In ionic compounds, there are strong electrostatic forces of attraction holding the oppositely charged ions (cations and anions) together in a regular crystal lattice structure. A considerable amount of energy is required to overcome these strong inter-ionic forces to melt the solid or boil the liquid. Therefore, ionic compounds have high melting and boiling points.

(II) Reason: Solder, an alloy of lead (Pb) and tin (Sn), has a much lower melting point than its constituent metals. This low melting point allows it to melt easily when heated by a soldering iron, making it ideal for creating a quick, strong, and electrically conductive joint between wires or electrical components without overheating them.

(III) Reason: Metals like Sodium (Na) and Magnesium (Mg) are high up in the reactivity series; they are highly reactive and electropositive. This means they form very strong bonds with oxygen, having a stronger affinity for oxygen than carbon does. Therefore, carbon, which is a less reactive reducing agent compared to these highly active metals, cannot pull oxygen away from their oxides to reduce them.

(ii). (I) Compound ‘X’ is Iron(III) oxide or Ferric oxide.
(II) Compound ‘X’ is Iron(III) oxide or Ferric oxide.
(III) Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + Heat

(B) (i) Write the balanced chemical equations when :
(I) A mixture of Cu₂O and Cu₂S is heated.
(II) ZnS is heated in the presence of oxygen.
(ii) Give reasons for the following :
(I) The wires carrying current in homes have a coating of PVC.
(II) To make hot water tanks, copper is used and not steel.
(iii) Show the formation of ionic compound CaO with electron dot structure.
[Atomic number : Ca = 20, O = 8]
Answer:
(i). (I) This is a self-reduction reaction during the extraction of copper.
2Cu₂O(s) + Cu₂S(s) -(heat)→ 6Cu(l) + SO₂(g)

(II) This process is called roasting.
2ZnS(s) + 3O₂(g) -(heat)→ 2ZnO(s) + 2SO₂(g)

(ii). (I) Reason: PVC (Polyvinyl chloride) is an electrical insulator, meaning it does not conduct electricity. Coating wires with PVC provides safety by preventing electric shocks to users and preventing short circuits if two wires touch accidentally.

(II) Reason: There are two main reasons:
1. Copper is a better conductor of heat than steel, which allows it to heat up the water more quickly and efficiently using less energy.
2. Copper does not react with water, hot or cold, under normal conditions and is highly resistant to corrosion. Steel (unless it’s a specialized, expensive stainless grade) is made mostly of iron, which will eventually rust when exposed to hot water and air over time, compromising the tank’s integrity.

(iii) 1. Electronic Configurations:
Calcium (Ca, At. No. 20): 2, 8, 8, 2. It has 2 valence electrons.
Oxygen (O, At. No. 8): 2, 6. It has 6 valence electrons.
2. Ion Formation:
Calcium needs to lose 2 electrons to achieve a stable octet (8) in its outermost shell. Ca → Ca²⁺ + 2e⁻ (Calcium atom becomes a Calcium cation)
Oxygen needs to gain 2 electrons to complete its outer shell with a stable octet. O + 2e⁻ → O²⁻ (Oxygen atom becomes an Oxide anion)
3. Electron Dot Formation Representation:

Class 10 Science Board Exam Answer Key 2026 (Physics Set 1, 2 and 3)

30. Rays from the sun converge at a point 25 cm behind a convex lens. The distance at which an object be placed in front of the lens to get a virtual image, is:
(A) 20 cm
(B) 40 cm
(C) 50 cm
(D) More than 50 cm
Answer:
(A) 20 cm
Explanation:
Focal length f = 25 cm (given, sun rays converge at 25 cm)
For a VIRTUAL image from convex lens, object must be placed
BETWEEN F and optical centre, i.e., within focal length.
Object distance < 25 cm –> option (A) 20 cm is correct.

31. To restore clear vision in persons whose eye ball size has reduced, he/she should use:
(A) Converging lens
(B) Diverging lens
(C) Bifocal lens
(D) Cylindrical lens
Answer:
(A) Converging lens
Explanation:
Reduced eyeball size = image forms BEHIND the retina
= FAR-SIGHTEDNESS (Hypermetropia)
Corrected by CONVERGING (Convex) lens which converges
light rays to bring image back onto the retina.

For question number 32, 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:
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason R is the correct explanation of the Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true.

32. Assertion (A): The needle of a magnetic compass kept in strong external magnetic field, always aligns itself in north-south direction on the earth.
Reason (R): Behaviour of needle of a compass is same as that of a freely suspended bar magnet.
Answer:
(D) A is false, but R is true.
Explanation:
(A) is FALSE: When compass is in a STRONG EXTERNAL magnetic field, it aligns with that EXTERNAL field, NOT necessarily in north-south direction. Earth’s field is overridden.
(R) is TRUE: A compass needle does behave like a freely suspended bar magnet in the absence of external fields.

33. When an incident ray of light enters in a medium ‘X’ from medium ‘Y’, it bend away from the normal.
Comment about the following:
(a) Speed of light in medium ‘X’ with respect to the speed of light in medium ‘Y’.
(b) Optical density of medium ‘X’ with respect to the optical density of medium of ‘Y’.
Give reason for your answer in each case.
Answer:
(a) Speed of light in medium X w.r.t. medium Y:
Speed of light in X is GREATER than speed in Y.

Reason: When light bends AWAY from normal while going from Y to X, it means light is going from DENSER to RARER medium.
In rarer medium (X), speed of light is HIGHER.
(Speed increases as medium becomes less optically dense)

(b) Optical density of X w.r.t. optical density of Y:
Optical density of X is LESS than optical density of Y.

Reason: Light bending AWAY from normal means X is OPTICALLY RARER than Y. Optically rarer = lower optical density = light travels faster.

Frequently Asked Questions on CBSE Class 10 Science Exam 2026

Was the CBSE Class 10 Science board paper 2026 tough?

Based on initial student reactions and expert reviews, the 2026 Science paper was considered very Easy. The Physics section had a few tricky numericals, while the Biology and Chemistry sections were largely straightforward and NCERT-based.

Where can I download the official CBSE Class 10 Science Answer Key 2026?

The official CBSE answer key is typically released a few weeks after the exams conclude. However, you can get the highly accurate, expert-verified unofficial answer key and PDF solutions directly from this page to evaluate your score immediately.

Is there step-marking in the CBSE Class 10 Science exam?

Yes, the CBSE board strictly follows a step-marking scheme. Even if your final answer for a physics numerical or a chemical equation is incorrect, you will still be awarded partial marks for writing the correct formula, given values and balanced intermediate steps.

Will there be any grace marks for the 2026 Science paper?

Grace marks are only awarded by CBSE if a question is proven to be factually incorrect, out of the prescribed NCERT syllabus or containing a printing error. Any official announcements regarding grace marks will be updated here.

CBSE Class 10 Science Board Exam Paper 2026: Complete Solutions and Answer Key