CBSE Class 10 Science Sample Paper 2026-27

Answer Keyincluded with every paper

1. Q1. double displacement reaction

   A double displacement reaction involves exchange of ions between two compounds, forming two new compounds.

2. Q2. double displacement

   In this reaction, the ions of the reactants exchange partners: Ba2+ pairs with SO42- and H+ pairs with Cl-.

3. Q3. 4:1

   Balanced decomposition: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. Thus, 4 moles of NO2 are produced for every 1 mole of O2.

4. Q4. Sodium hydrogencarbonate

   Baking soda is sodium hydrogencarbonate with the chemical formula NaHCO₃. It is a mild, edible base used in cooking and as an antacid.

5. Q5. Baking soda

   Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Gypsum (CaSO₄·2H₂O), washing soda (Na₂CO₃·10H₂O), and blue vitriol (CuSO₄·5H₂O) all contain water of crystallization. Baking soda (NaHCO₃) has no such water molecules.

6. Q6. Its chemical formula is CaSO₄·2H₂O.

   Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). The formula CaSO₄·2H₂O corresponds to gypsum. When heated at 373 K, gypsum loses water of crystallisation and forms Plaster of Paris. It absorbs water to set into a hard mass and is widely used in moulding.

7. Q7. Basic

   Most metal oxides are basic; they dissolve in water to produce hydroxide ions, giving a solution with a pH greater than 7.

8. Q8. A non-metal

   Non‑metal oxides are acidic when dissolved in water, turning blue litmus red. Metal oxides are basic, turning red litmus blue.

9. Q9. Calcium and sulphur

   XO must have a +2 metal: CaO is basic and soluble enough to turn litmus. YO₂ with +4 non‑metal: SO₂ is acidic. CaO + SO₂ → CaSO₃ (white). Other pairs either do not form XO (Na, K give M₂O) or have insoluble/poorly basic oxides (MgO).

10. Q10. Ethanal

    CH₃CHO has two carbon atoms with an aldehyde group; the IUPAC name is ethanal.

11. Q11. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

    Sodium replaces the hydrogen of the hydroxyl group, forming sodium ethoxide (C₂H₅ONa) and liberating hydrogen gas. The balanced equation is as shown.

12. Q12. Ethanol

    C₂H₆O can be ethanol (functional isomer dimethyl ether does not oxidise). Ethanol on oxidation with alkaline KMnO₄ yields ethanoic acid, which is acidic and turns blue litmus red.

13. Q13. Transport of water and minerals

    Xylem is a vascular tissue that conducts water and dissolved minerals from the roots upwards to the rest of the plant.

14. Q14. Nasal cavity → pharynx → larynx → trachea → bronchi → lungs

    Air enters through the nasal cavity, passes through the pharynx, larynx, trachea, then into the bronchi and finally the lungs.

15. Q15. Arteries

    Arteries are exposed to high-pressure blood directly from the heart. To withstand this pressure, they naturally have thick muscular walls; chronic hypertension can cause further thickening (hypertrophy) to counteract the increased stress.

16. Q16. Sensory neuron → relay neuron → motor neuron

    The reflex arc follows the pathway: receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → effector. Thus, the impulse travels from sensory to relay to motor neuron.

17. Q17. Both A and R are true and R is the correct explanation of A.

    Burning magnesium forms magnesium oxide, a new substance, indicating a chemical change. The reason correctly explains the assertion.

18. Q18. A is true but R is false.

    Assertion is true as per definition of a balanced equation. Reason is false because mass is conserved, not created or destroyed, in a chemical reaction.

19. Q19. Both A and R are true but R is not the correct explanation of A.

    Assertion is true because of the law of conservation of mass in chemical reactions. Reason is a true statement from physics, but it does not explain the assertion; mass-energy conversion is negligible in chemical reactions.

20. Q20. Both A and R are true and R is the correct explanation of A.

    pH = -log[H+]. In pure water or a neutral solution, [H+] = [OH-] = 10^-7 M, so pH = 7. The reason correctly explains why pH 7 indicates neutrality.

21. Q21. A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different chemical properties. Example: 2H₂ + O₂ → 2H₂O.

    A chemical reaction involves rearrangement of atoms leading to new substances. The example shows hydrogen combining with oxygen to form water, with two molecules of hydrogen reacting with one molecule of oxygen to give two molecules of water, satisfying atom conservation.

22. Q22. The white ash is magnesium oxide (MgO) formed by the combination of magnesium with oxygen. Equation: 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants combine to form a single product.

    Magnesium burns in oxygen to produce magnesium oxide. Since two elements combine to give one compound, it is classified as a combination reaction. The white ash is pure MgO.

23. Q23. Substance oxidized: HCl (Cl⁻ ions). Substance reduced: MnO₂ (Mn⁴⁺ ions). Oxidizing agent: MnO₂. Reducing agent: HCl.

    In HCl, chlorine has oxidation number -1; in Cl₂, it is 0. So Cl⁻ loses electrons, gets oxidized. In MnO₂, Mn has +4; in MnCl₂, Mn has +2, so Mn gains electrons, gets reduced. Hence MnO₂ oxidizes HCl (oxidizing agent) and HCl reduces MnO₂ (reducing agent).

24. Q24. NaHCO₃ + HCl → NaCl + H₂O + CO₂. The gas evolved (CO₂) is acidic in nature.

    Sodium bicarbonate reacts with hydrochloric acid to form sodium chloride, water, and carbon dioxide. Carbon dioxide turns lime water milky and is an acidic oxide.

25. Q25. The pH will increase (become greater than 5) because sodium bicarbonate is basic in nature and will neutralize some of the acid, reducing the hydrogen ion concentration.

    Sodium bicarbonate (NaHCO₃) is a mild base. It reacts with the acid in the solution (pH 5 indicates acidic nature), decreasing the concentration of H⁺ ions, thus increasing the pH towards neutral or basic.

26. Q26. A reaction in which both oxidation and reduction take place simultaneously is called a redox reaction. Example: CuO + H₂ → Cu + H₂O. In ZnO + C → Zn + CO: Zinc oxide (ZnO) is reduced to zinc because it loses oxygen. Carbon (C) is oxidised to carbon monoxide (CO) because it gains oxygen. Thus, ZnO is the oxidising agent, and C is the reducing agent.

    Oxidation is gain of oxygen/loss of hydrogen; reduction is loss of oxygen/gain of hydrogen. In the given reaction, ZnO loses oxygen (reduction) and C gains oxygen (oxidation). Hence, it's a redox reaction.

27. Q27. Balanced equation: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O In this reaction: - MnO₂ (manganese dioxide) is reduced to MnCl₂ (manganese in +4 oxidation state reduces to +2). It acts as an oxidising agent. - HCl (hydrochloric acid) is oxidised to Cl₂ (chlorine from -1 oxidation state to 0). It acts as a reducing agent. Since both oxidation and reduction occur, it is a redox reaction.

    Balancing: Count atoms on each side: Mn:1, O:2, H:1, Cl:1 on left; Mn:1, Cl:2+2=4, H:2 on right. Need 4 HCl to get 4 H and 4 Cl, then H₂O coefficient 2. Reaction: MnO₂+4HCl→MnCl₂+Cl₂+2H₂O. Oxidation: Cl⁻ → Cl₂ (loss of electrons); Reduction: Mn⁴⁺ → Mn²⁺ (gain of electrons). Hence, redox.

28. Q28. (a) Silver reacts with hydrogen sulphide (H₂S) present in the air to form silver sulphide (Ag₂S), which is black. 2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g) (b) Copper reacts with moist carbon dioxide (CO₂) and water to form basic copper carbonate, which is green. 2Cu(s) + H₂O(l) + CO₂(g) + O₂(g) → Cu(OH)₂ · CuCO₃(s) (Sometimes written as CuCO₃·Cu(OH)₂) (c) Both are oxidation reactions leading to corrosion. The compounds formed are silver sulphide and basic copper carbonate (or copper carbonate hydroxide).

    These are examples of corrosion, a slow oxidation of metals by atmospheric gases. Silver tarnish is due to Ag₂S formation. Green patina on copper is basic copper carbonate, commonly called verdigris.

29. Q29. Dry HCl gas does not produce H⁺ ions, so it does not show acidic behaviour. In water, HCl dissociates to give H⁺ and Cl⁻ ions; H⁺ ions are responsible for the colour change of blue litmus to red.

    Acids exhibit their properties only in the presence of water because they produce hydrogen ions (H⁺) upon dissociation. Dry HCl gas has no free ions. When dissolved in water, HCl(aq) → H⁺(aq) + Cl⁻(aq), and the H⁺ ions turn blue litmus red.

30. Q30. On dilution, the concentration of H⁺ ions decreases, so the pH increases and approaches 7. However, due to the self‑ionisation of water, the H⁺ ions from water (10⁻⁷ mol/L) become significant compared to those from the acid at extreme dilution, preventing the pH from reaching exactly 7.

    pH = –log[H⁺]. Diluting the acid reduces [H⁺], raising the pH. At infinite dilution, the [H⁺] from water (10⁻⁷ M) cannot be ignored. The total [H⁺] will be slightly above 10⁻⁷ M, hence the pH approaches but remains less than 7.

31. Q31. NaHCO₃ hydrolyses to give OH⁻ ions: HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻ (basic). NaHSO₄ dissociates in water to give H⁺ ions: HSO₄⁻ → H⁺ + SO₄²⁻ (acidic). Thus, the former solution is basic and the latter is acidic.

    When NaHCO₃ dissolves, the bicarbonate ion acts as a weak base, accepting a proton from water to form carbonic acid and hydroxide ions. In contrast, NaHSO₄ contains HSO₄⁻, which is a stronger acid and readily donates a proton to water, producing hydronium ions. Therefore, one solution turns red litmus blue and the other blue litmus red.

32. Q32. (a) 2H2 + O2 → 2H2O; Combination reaction (two reactants combine to form a single product). (b) 2KClO3 → 2KCl + 3O2; Decomposition reaction (single reactant breaks down into two or more products). (c) Zn + 2HCl → ZnCl2 + H2; Displacement reaction (zinc displaces hydrogen from hydrochloric acid because it is more reactive).

    To balance equations, ensure equal number of atoms of each element on both sides. For (a): 2 H2 and 1 O2 give 2 H2O. Type: Combination as two substances combine. For (b): 2 KClO3 → 2 KCl + 3 O2. Type: Decomposition as one compound breaks into two. For (c): Zn + 2 HCl → ZnCl2 + H2. Type: Displacement because a more reactive metal (Zn) displaces hydrogen from acid.

33. Q33. The white solid is lead nitrate (Pb(NO3)2). The balanced equation is: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. This is a thermal decomposition reaction. To confirm the gas (NO2): the brown fumes turn moist blue litmus paper red due to acidic nature, or they have a pungent irritating smell. Alternatively, pass the gas through potassium iodide solution; it will turn brown due to liberation of iodine.

    Lead nitrate is a white crystalline solid that decomposes on strong heating to give yellow lead oxide, brown nitrogen dioxide fumes, and oxygen. The reaction is decomposition, specifically thermal decomposition. NO2 can be identified by its colour, smell, and acidic reaction with litmus.

34. Q34. (a) Endothermic reaction. (b) Ba(OH)2(s) + 2NH4Cl(s) → BaCl2(s) + 2NH3(g) + 2H2O(l). (c) It is not a redox reaction. All elements retain their oxidation states: Ba: +2, O: -2, H: +1, N: -3, Cl: -1 in both reactants and products. (d) Adding water to quicklime (CaO) is exothermic, so the beaker would become hot. Quicklime reacts with water releasing heat: CaO + H2O → Ca(OH)2 + heat. The difference is due to the nature of reactions: one absorbs heat, the other releases heat.

    An endothermic reaction absorbs heat from surroundings, causing cooling. The reaction between barium hydroxide and ammonium chloride is a classic endothermic reaction. To check redox, compare oxidation states: no change, hence not redox. Quicklime slaking is exothermic. Thus, opposite temperature changes occur because of different enthalpy changes.

35. Q35. Activity: Take two test tubes, add a small amount of sodium carbonate to one and sodium hydrogen carbonate to the other. Add 2 mL of dilute HCl using a thistle funnel. Pass the evolved gas through lime water. Lime water turns milky. Equations: (i) Na2CO3 + 2HCl → 2NaCl + H2O + CO2; (ii) NaHCO3 + HCl → NaCl + H2O + CO2. Confirmatory test: When passed through lime water, CO2 turns it milky; on excess passage, milkiness disappears. Diagram should show delivery tube attached to test tube containing reactants, with gas passed into lime water.

    The activity demonstrates that carbonates and hydrogen carbonates react with acids to release carbon dioxide. The reaction is acid + metal carbonate/hydrogen carbonate → salt + water + carbon dioxide. The carbon dioxide is identified by the lime water test: Ca(OH)2 + CO2 → CaCO3 (white precipitate, milky) + H2O; excess CO2 dissolves the precipitate: CaCO3 + H2O + CO2 → Ca(HCO3)2 (soluble, clear).

36. Q36. It is a combination reaction: CaO + H2O → Ca(OH)2, which is exothermic.

    Calcium oxide combines with water to form calcium hydroxide, releasing heat (exothermic). The balanced equation shows one reactant forming a single product.

37. Q37. The brown gas is NO2. Decomposition: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. The gas can be tested by its colour and acidic reaction with moist blue litmus.

    Heating lead nitrate gives nitrogen dioxide (brown, acidic) and oxygen. The balanced equation shows thermal decomposition producing lead oxide, NO2, and O2. NO2 acidic nature turns blue litmus red.

38. Q38. Step 1: S + O2 → SO2. Step 2: 2SO2 + O2 ⇌ 2SO3 (catalyst). Step 3: SO3 + H2SO4 → H2S2O7. SO2 is the reducing agent in step 2 (oxidation state of S changes from +4 to +6). Step 3 is a combination reaction.

    Balanced equations follow standard Contact process reactions. In step 2, SO2 reduces O2, itself oxidized: oxidation number of S increases from +4 to +6, so SO2 is the reducing agent. Step 3 is a direct combination of two compounds.