Gyanyaan हिंदी

Chapter 1: Chemical Reactions and Equations

Class 10 Science · MCQs with answers

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  1. In the reaction Zn + H₂SO₄ → ZnSO₄ + H₂, which are the reactants?

    1. (A) ZnSO₄ and H₂
    2. (B) Zn and H₂SO₄
    3. (C) Zn and H₂
    4. (D) H₂SO₄ and ZnSO₄
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    Correct answer: (B) Zn and H₂SO₄

    Explanation: Substances on the left of the arrow are reactants and those on the right are products. Here Zn and H₂SO₄ are the reactants.

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  2. To balance Fe + H₂O → Fe₃O₄ + H₂, what are the coefficients of Fe, H₂O, Fe₃O₄ and H₂ respectively?

    1. (A) 3, 4, 1, 4
    2. (B) 3, 2, 1, 2
    3. (C) 1, 4, 1, 4
    4. (D) 3, 4, 1, 2
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    Correct answer: (A) 3, 4, 1, 4

    Explanation: In 3Fe + 4H₂O → Fe₃O₄ + 4H₂, both sides have 3 Fe, 8 H and 4 O atoms, so it is balanced.

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  3. What does the state symbol (aq) mean in a chemical equation?

    1. (A) solid
    2. (B) liquid
    3. (C) aqueous solution (dissolved in water)
    4. (D) gas
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    Correct answer: (C) aqueous solution (dissolved in water)

    Explanation: (aq) means the substance is dissolved in water. (s) stands for solid, (l) for liquid and (g) for gas.

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  4. Mixing barium chloride (BaCl₂) solution with sodium sulphate (Na₂SO₄) solution gives a white precipitate. What is the precipitate?

    1. (A) NaCl
    2. (B) BaSO₄
    3. (C) Na₂SO₄
    4. (D) BaCl₂
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    Correct answer: (B) BaSO₄

    Explanation: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. Barium sulphate is insoluble in water, so it forms the white precipitate, while NaCl stays dissolved.

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  5. What type of reaction is Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl?

    1. (A) Combination reaction
    2. (B) Decomposition reaction
    3. (C) Displacement reaction
    4. (D) Double displacement reaction
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    Correct answer: (D) Double displacement reaction

    Explanation: The two compounds exchange ions (Ba and Na swap partners), so it is a double displacement reaction. Because a precipitate forms, it is also called a precipitation reaction.

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  6. In the electrolysis of water, what is the ratio of the volumes of hydrogen and oxygen produced?

    1. (A) 1 : 1
    2. (B) 1 : 2
    3. (C) 2 : 1
    4. (D) 4 : 1
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    Correct answer: (C) 2 : 1

    Explanation: 2H₂O → 2H₂ + O₂. Two molecules of hydrogen form for every one molecule of oxygen, so the volume ratio is 2 : 1.

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  7. Which gases are given off when ferrous sulphate crystals are heated strongly?

    1. (A) SO₂ and SO₃
    2. (B) O₂ and SO₂
    3. (C) H₂ and SO₂
    4. (D) CO₂ and SO₃
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    Correct answer: (A) SO₂ and SO₃

    Explanation: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. The green crystals first lose water, then break down into ferric oxide, giving off gases that smell of burning sulphur.

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  8. Heating lead nitrate gives off brown fumes. Which gas are these brown fumes?

    1. (A) Oxygen (O₂)
    2. (B) Nitrogen (N₂)
    3. (C) Lead oxide (PbO)
    4. (D) Nitrogen dioxide (NO₂)
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    Correct answer: (D) Nitrogen dioxide (NO₂)

    Explanation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. The nitrogen dioxide given off is a brown gas.

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  9. Which of the following is a decomposition reaction?

    1. (A) C + O₂ → CO₂
    2. (B) CaCO₃ → CaO + CO₂
    3. (C) Zn + CuSO₄ → ZnSO₄ + Cu
    4. (D) NaOH + HCl → NaCl + H₂O
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    Correct answer: (B) CaCO₃ → CaO + CO₂

    Explanation: In a decomposition reaction, a single reactant breaks down into two or more products. On heating, CaCO₃ breaks down into CaO and CO₂.

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  10. What kind of reaction is respiration?

    1. (A) Exothermic
    2. (B) Endothermic
    3. (C) Displacement
    4. (D) Double displacement
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    Correct answer: (A) Exothermic

    Explanation: In respiration, glucose combines with oxygen to form CO₂ and water, releasing energy. A reaction that releases energy is exothermic.

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  11. Which of these processes is endothermic?

    1. (A) Burning of natural gas
    2. (B) Respiration
    3. (C) Photosynthesis
    4. (D) Decomposition of vegetable waste into compost
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    Correct answer: (C) Photosynthesis

    Explanation: In photosynthesis, plants take in energy from sunlight to make food, so it is endothermic. The other three release energy.

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  12. In the reaction CuO + H₂ → Cu + H₂O, which substance is oxidised?

    1. (A) CuO
    2. (B) H₂
    3. (C) Cu
    4. (D) H₂O
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    Correct answer: (B) H₂

    Explanation: Hydrogen gains oxygen to form H₂O, so H₂ is oxidised. CuO loses oxygen, so it is reduced.

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  13. In the reaction ZnO + C → Zn + CO, which is the reducing agent?

    1. (A) ZnO
    2. (B) Zn
    3. (C) CO
    4. (D) C
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    Correct answer: (D) C

    Explanation: Carbon takes oxygen from ZnO to form CO. A substance that removes oxygen from another (and is itself oxidised) is the reducing agent.

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  14. In the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, which substance is oxidised?

    1. (A) MnO₂
    2. (B) MnCl₂
    3. (C) HCl
    4. (D) H₂O
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    Correct answer: (C) HCl

    Explanation: HCl loses hydrogen to form Cl₂. Loss of hydrogen is oxidation, so HCl is oxidised. MnO₂ loses oxygen, so it is reduced.

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  15. What does iron need in order to rust?

    1. (A) Both air (oxygen) and water
    2. (B) Water only
    3. (C) Air only
    4. (D) Neither air nor water
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    Correct answer: (A) Both air (oxygen) and water

    Explanation: Iron rusts only when it gets both oxygen and moisture. A nail kept in dry air, or in boiled (air-free) water, does not rust.

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  16. Which gas is filled in chips packets to prevent rancidity?

    1. (A) Oxygen
    2. (B) Hydrogen
    3. (C) Chlorine
    4. (D) Nitrogen
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    Correct answer: (D) Nitrogen

    Explanation: Nitrogen is an unreactive gas. It keeps oxygen out of the packet, so the oils and fats are not oxidised and the chips do not go rancid.

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  17. Silver articles turn black when left in air. What forms on them?

    1. (A) Silver oxide
    2. (B) Silver sulphide
    3. (C) Silver chloride
    4. (D) Silver nitrate
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    Correct answer: (B) Silver sulphide

    Explanation: Silver reacts with sulphur compounds in the air to form black silver sulphide (Ag₂S). This is an example of corrosion.

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  18. Why does a green layer form on copper vessels over time?

    1. (A) Basic copper carbonate forms
    2. (B) Copper oxide forms
    3. (C) Copper sulphate forms
    4. (D) Copper nitrate forms
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    Correct answer: (A) Basic copper carbonate forms

    Explanation: Copper reacts with moist carbon dioxide in the air to form green basic copper carbonate.

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  19. Why is a magnesium ribbon rubbed with sandpaper before burning it?

    1. (A) To make the ribbon thinner
    2. (B) To add oxygen to it
    3. (C) To remove the layer of magnesium oxide on its surface
    4. (D) To remove moisture from it
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    Correct answer: (C) To remove the layer of magnesium oxide on its surface

    Explanation: Magnesium kept in air gets a layer of its oxide, which stops it from burning properly. Rubbing removes this layer.

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  20. Two or three days after whitewashing a wall with slaked lime, it becomes shiny. A thin layer of which substance causes this?

    1. (A) CaO
    2. (B) CaCO₃
    3. (C) Ca(OH)₂
    4. (D) CaCl₂
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    Correct answer: (B) CaCO₃

    Explanation: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Slaked lime slowly reacts with carbon dioxide in the air to form a shiny layer of calcium carbonate.

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  21. When copper powder is heated in air, what colour does its surface turn?

    1. (A) Green
    2. (B) White
    3. (C) Blue
    4. (D) Black
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    Correct answer: (D) Black

    Explanation: 2Cu + O₂ → 2CuO. Copper combines with oxygen to form black copper(II) oxide. This is oxidation.

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  22. Which metal can displace copper from copper sulphate solution?

    1. (A) Silver
    2. (B) Gold
    3. (C) Zinc
    4. (D) Platinum
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    Correct answer: (C) Zinc

    Explanation: A more reactive metal displaces a less reactive metal from a solution of its compound. Zinc is more reactive than copper; silver, gold and platinum are less reactive.

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  23. Assertion (A): Decomposition reactions are usually endothermic. Reason (R): Energy in the form of heat, light or electricity is needed to break down the reactant.

    1. (A) Both A and R are true, and R is the correct explanation of A.
    2. (B) Both A and R are true, but R is not the correct explanation of A.
    3. (C) A is true, but R is false.
    4. (D) A is false, but R is true.
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    Correct answer: (A) Both A and R are true, and R is the correct explanation of A.

    Explanation: Both statements are true. Decomposition needs energy, which is why these reactions take in energy, so R correctly explains A.

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  24. Assertion (A): Respiration is an endothermic reaction. Reason (R): In respiration, glucose combines with oxygen and releases energy.

    1. (A) Both A and R are true, and R is the correct explanation of A.
    2. (B) Both A and R are true, but R is not the correct explanation of A.
    3. (C) A is true, but R is false.
    4. (D) A is false, but R is true.
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    Correct answer: (D) A is false, but R is true.

    Explanation: Respiration releases energy, so it is exothermic, not endothermic. So A is false, but R is true.

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  25. Balancing a chemical equation is based on which law?

    1. (A) Ohm's law
    2. (B) Law of conservation of mass
    3. (C) Newton's law of motion
    4. (D) Law of reflection
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    Correct answer: (B) Law of conservation of mass

    Explanation: In a chemical reaction, mass is neither created nor destroyed. So each element must have the same number of atoms on both sides.

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