class 10 science NCERT Solutions

 Here is a comprehensive, exam-oriented answer key for the NCERT Class 10 Science – Chapter 1: Chemical Reactions and Equations – Exercise Questions 1 to 20, explained clearly and concisely for both teaching and revision:

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1. Identify incorrect statements:

Reaction:

$$2\text{PbO}(s) + \text{C}(s) \rightarrow 2\text{Pb}(s) + \text{CO}_2(g)$$

✅ Correct analysis:

• Lead oxide (PbO) is reduced to lead.

• Carbon (C) is oxidised to CO₂.

• CO₂ is not getting oxidised, it is a product.

✅ Incorrect statements: (a) and (b)

Answer: (i) (a) and (b)

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2. Type of reaction:

$$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$$

✅ Explanation: Aluminium displaces iron from its oxide → Displacement reaction

Answer: (d) displacement reaction

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3. Reaction of HCl with iron filings:

$$\text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \uparrow$$

✅ Hydrogen gas and iron chloride are produced.

Answer: (a)

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4. Balanced chemical equation:

A balanced chemical equation has equal number of atoms of each element on both sides.

✅ Why balance?
To obey the Law of Conservation of Mass.

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5. Translate and balance:

(a) $3\text{H}_2 + \text{N}_2 \rightarrow 2\text{NH}_3$

(b) $2\text{H}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{H}_2\text{O} + 2\text{SO}_2$

(c) $3\text{BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 \rightarrow 2\text{AlCl}_3 + 3\text{BaSO}_4$

(d) $2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2 \uparrow$

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6. Balance these equations:

(a)

$$2\text{HNO}_3 + \text{Ca(OH)}_2 \rightarrow \text{Ca(NO}_3)_2 + 2\text{H}_2\text{O}$$

(b)

$$2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$

(c)

$$\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3$$

(d)

$$\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 \downarrow + 2\text{HCl}$$

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7. Write and balance reactions:

(a)

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

(b)

$$\text{Zn} + 2\text{AgNO}_3 \rightarrow \text{Zn(NO}_3)_2 + 2\text{Ag}$$

(c)

$$2\text{Al} + 3\text{CuCl}_2 \rightarrow 2\text{AlCl}_3 + 3\text{Cu}$$

(d)

$$\text{BaCl}_2 + \text{K}_2\text{SO}_4 \rightarrow \text{BaSO}_4 \downarrow + 2\text{KCl}$$

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8. Identify reaction types:

(a) Double Displacement (precipitation)

$$2\text{KBr} + \text{BaI}_2 \rightarrow 2\text{KI} + \text{BaBr}_2$$

(b) Thermal Decomposition

$$\text{ZnCO}_3 \xrightarrow{\text{Heat}} \text{ZnO} + \text{CO}_2$$

(c) Combination Reaction

$$\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$$

(d) Displacement Reaction

$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$$

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9. Exothermic vs Endothermic:

• Exothermic Reaction: Heat is released
  Example: Combustion of fuel

• Endothermic Reaction: Heat is absorbed
  Example: Decomposition of calcium carbonate

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10. Respiration as Exothermic:

• Glucose is oxidised in cells → Releases energy

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$$

✅ Hence, exothermic reaction

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11. Decomposition vs Combination:

• Combination: Two/more substances → One product

  $$\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$$

• Decomposition: One compound → Simpler substances

  $$\text{CaCO}_3 \xrightarrow{\text{Heat}} \text{CaO} + \text{CO}_2$$

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12. Decomposition Reactions (Energy-based):

• Heat:

  $$2\text{Pb(NO}_3)_2 \xrightarrow{\text{Heat}} 2\text{PbO} + 4\text{NO}_2 + \text{O}_2$$

• Light:

  $$2\text{AgCl} \xrightarrow{\text{Sunlight}} 2\text{Ag} + \text{Cl}_2$$

• Electricity:

  $$2\text{H}_2\text{O} \xrightarrow{\text{Electricity}} 2\text{H}_2 + \text{O}_2$$

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13. Displacement vs Double Displacement:

• Displacement: One element replaces another

  $$\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$$

• Double Displacement: Ions are exchanged

  $$\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 \downarrow + 2\text{NaCl}$$

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14. Silver recovery using copper:

$$2\text{AgNO}_3 + \text{Cu} \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag}$$

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15. Precipitation Reaction:

A reaction in which insoluble solid (precipitate) forms when two solutions are mixed.
Example:

$$\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 \downarrow + 2\text{NaCl}$$

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16. Oxidation and Reduction (Oxygen-based):

• Oxidation (Gain of oxygen):

  $$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

• Reduction (Loss of oxygen):

  $$\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$$

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17. Shiny metal ‘X’ turns black:

• X = Copper (Cu)

• Black substance = Copper(II) oxide (CuO)

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18. Why paint iron articles?

✅ To prevent rusting (corrosion) by blocking contact with air and moisture.

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19. Why flush chips with nitrogen?

✅ To prevent rancidity by removing oxygen which causes oxidation of fats/oils.

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20. Explain:

(a) Corrosion:
Slow degradation of metals due to environmental factors.
Example: Rusting of iron

(b) Rancidity:
Spoilage of food containing fats/oils due to oxidation.
Example: Spoiled ghee or oily snacks

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