Chemical Reactions and Equations: Class 10 Important Questions with Answers

Colour, smell & sound chart (MCQ gold)

Case-based questions (4 marks each)

(i) Name the gas over P and over Q. (1) (ii) Which electrode is the cathode? Give a reason. (1) (iii) Write the balanced equation and the type of reaction. (2)

(i) Name the process that spoilt the namkeen. (1) (ii) Which gas fills the chips packet and why? (1) (iii) Name the change in the kadhai and two conditions it needs. Suggest one way to prevent it. (2)

(i) Identify X, Y and Z. (1) (ii) Write equations for both changes. (1) (iii) In the second change, name the substance oxidised, the substance reduced and the reducing agent. (2)

(i) In which tubes does a reaction take place? (1) (ii) Write the colour change in C with the equation. (1) (iii) Arrange Zn, Fe, Cu and Ag in decreasing reactivity and explain why B shows no change. (2)

20 MCQs (1 mark each)

1. Which is a chemical change?
   (a) melting of ice (b) dissolving sugar (c) rusting of iron (d) evaporation of water
2. Number of oxygen atoms on the reactant side of 2Pb(NO₃)₂ → … is
   (a) 6 (b) 9 (c) 12 (d) 18
3. Gas set free at the cathode in electrolysis of acidified water:
   (a) O₂ (b) H₂ (c) Cl₂ (d) SO₂
4. White precipitate on mixing BaCl₂ and Na₂SO₄ solutions:
   (a) NaCl (b) BaCl₂ (c) BaSO₄ (d) Na₂SO₄
5. Colour of the precipitate of lead iodide:
   (a) white (b) yellow (c) green (d) black
6. Which pair will not react?
   (a) Zn + CuSO₄ (b) Fe + CuSO₄ (c) Cu + FeSO₄ (d) Mg + ZnSO₄
7. Chemical formula of quick lime:
   (a) CaCO₃ (b) Ca(OH)₂ (c) CaO (d) CaCl₂
8. In CuO + H₂ → Cu + H₂O, the oxidising agent is
   (a) CuO (b) H₂ (c) Cu (d) H₂O
9. Which is endothermic?
   (a) respiration (b) burning of coal (c) CaO + water (d) decomposition of CaCO₃
10. Residue left on heating ferrous sulphate is
    (a) green FeSO₄ (b) reddish-brown Fe₂O₃ (c) black FeO (d) white SO₃
11. Chips packets are flushed with
    (a) O₂ (b) H₂ (c) N₂ (d) CO₂ only for taste
12. The black coating on silver articles is
    (a) Ag₂O (b) Ag₂S (c) AgCl (d) AgNO₃
13. The green coating on copper is
    (a) CuO (b) CuSO₄ (c) basic copper carbonate (d) CuCl₂
14. aFe + bO₂ → cFe₂O₃; a, b, c are
    (a) 2,3,1 (b) 4,3,2 (c) 4,2,3 (d) 2,1,2
15. A reaction that is both double displacement and precipitation:
    (a) Zn + CuSO₄ (b) AgNO₃ + NaCl (c) CaO + H₂O (d) 2H₂O → 2H₂ + O₂
16. 2H₂ + O₂ → 2H₂O is a
    (a) combination (b) decomposition (c) displacement (d) double displacement reaction
17. Mg ribbon is rubbed with sandpaper before burning to remove
    (a) dust (b) a layer of MgO (c) moisture (d) grease
18. Volume ratio of H₂ : O₂ in electrolysis of water
    (a) 1:1 (b) 1:2 (c) 2:1 (d) 8:1
19. “Heat” written above the arrow shows
    (a) a product (b) a reactant (c) a condition (d) a catalyst only
20. In ZnO + C → Zn + CO, the substance oxidised is
    (a) ZnO (b) C (c) Zn (d) CO

Answer key

Why: 2 → 2 × (3 × 2) = 12 · 6 → Cu is below Fe · 8 → CuO gives O to H₂ · 14 → Fe 4 = 4, O 6 = 6 · 20 → C gains O.

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: A chemical equation must be balanced. R: Mass can neither be created nor destroyed in a chemical reaction. → (a)
2. A: Copper displaces silver from silver nitrate solution. R: Copper is more reactive than silver. → (a)
3. A: An iron nail kept in zinc sulphate solution shows no change. R: Zinc is more reactive than iron. → (a)
4. A: Electrolysis of water is a decomposition reaction. R: A few drops of dilute sulphuric acid are added to water before electrolysis. → (b) acid only helps conduction
5. A: Burning of magnesium is an oxidation reaction. R: Magnesium gains oxygen to form MgO. → (a)
6. A: Respiration is an endothermic reaction. R: Glucose is broken down in our cells. → (d) respiration releases energy
7. A: Whitewashed walls become shiny after 2–3 days. R: Calcium hydroxide reacts with CO₂ in air to form a thin layer of calcium carbonate. → (a)
8. A: Silver bromide is used in black and white photography. R: Silver chloride turns grey in sunlight. → (b) R is true but is about AgCl
9. A: Precipitation reactions produce insoluble salts. R: In the reaction of BaCl₂ with Na₂SO₄, NaCl is the precipitate. → (c) BaSO₄ is the ppt
10. A: Fats and oils become rancid when kept open for long. R: Rancidity is the reduction of fats and oils. → (c) it is oxidation

Give reasons (2 marks each)

1. A magnesium ribbon is cleaned before burning.

It carries a layer of magnesium oxide that stops it burning; sandpaper removes it so Mg can combine with oxygen. 2

2. Silver chloride is stored in dark-coloured bottles.

In light it decomposes: 2AgCl → 2Ag + Cl₂. Dark bottles cut off light and prevent this photolytic decomposition. 2

3. The blue colour of copper sulphate fades when an iron nail is kept in it.

Iron is more reactive than copper and displaces it: Fe + CuSO₄ → FeSO₄ (light green) + Cu. Fewer Cu²⁺ ions remain, so blue fades. 2

4. In electrolysis of water, the gas at one electrode is double the other.

Water (H₂O) has 2 H for every O; 2H₂O → 2H₂ + O₂ gives 2 volumes of H₂ (cathode) per 1 volume of O₂ (anode). 2

5. Decomposition reactions are generally endothermic.

Bonds in the single reactant must be broken, which needs energy in the form of heat, light or electricity — this energy is absorbed. 2