CG Board Class 9 Science Chapter 10 Solutions: Chemical Reactions and Equations (रासायनिक अभिक्रियाएँ एवं समीकरण) – 2026-27

Write the balanced chemical equations for the following reactions

• (a) Hydrogen + Chlorine → Hydrogen chloride
• (b) Sodium hydroxide + Sulphuric acid → Sodium sulphate + Water

Balance the given chemical equations

• (i) CH₄ + O₂ → H_2O + CO₂
• (ii) Fe + O₂ → Fe_3O₄
• (iii) KClO₃ → KCl + O₂
• (i) Methane combustion / :
• (ii) Oxidation of Iron / :
• (iii) Decomposition of Potassium Chlorate / :

Write the balanced chemical equation for each of the following reactions and also identify the type of chemical reaction taking place

• (i) Zinc carbonate → Zinc oxide + Carbon dioxide
• (ii) Sodium hydroxide + Sulphuric acid → Sodium sulphate + Water
• (iii) Potassium bromide + Barium iodide → Potassium iodide + Barium bromide
• (i) Zinc carbonate reaction:
• (ii) Sodium hydroxide + Sulphuric acid:
• (iii) Potassium bromide + Barium iodide:

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

• Iron (Fe) is chemically more reactive than copper (Cu).
• When an iron nail is immersed in a blue copper sulphate solution (CuSO₄), iron displaces copper from the solution to form light-green ferrous sulphate (FeSO₄) and reddish-brown copper deposits on the iron nail.
• Reaction Equation: Fe(s) + CuSO₄(aq) (blue) → FeSO₄(aq) (light green) + Cu(s)

Identify which substance is getting oxidized and which is being reduced in the following reactions

• (a) 4Na(s) + O₂(g) → 2Na_2O(s)
• (b) CuO(s) + H₂(g) → Cu(s) + H_2O(l)
• (c) ZnO(s) + C(s) → Zn(s) + CO(g)

How are changes in heat (release, absorption) shown in chemical equations? Explain using examples.

1. Exothermic Reactions (Heat Released): Represented by adding the heat symbol +Q or + Energy on the product side (RHS).

• Example: C(s) + O₂(g) → CO₂(g) + Q

1. Endothermic Reactions (Heat Absorbed): Represented by writing a minus sign -Q on the product side, or writing heat/energy above the reaction arrow ( → [heat] ).

• Example: N₂(g) + O₂(g) → 2NO(g) – Q
• Example: CaCO₃(s) → [heat] CaO(s) + CO₂(g)

(i) Which of the following reactions is taking place when hydrogen chloride is formed by reaction between hydrogen and chlorine:**

• (a) Decomposition
• (b) Displacement
• (c) Combination
• (d) Double displacement

(ii) Fe_2O₃ + 2Al → Al_2O₃ + 2Fe. This reaction is an example of:

• (a) Combination
• (b) Decomposition
• (c) Displacement
• (d) Double displacement

(iii) 2PbO(s) + C(s) → 2Pb(s) + CO₂}(g). Which statement is true about this reaction:

• (1) Lead is undergoing oxidation
• (2) Carbon dioxide is undergoing oxidation
• (3) Carbon is getting oxidized to carbon dioxide
• (4) Lead oxide is being reduced to lead
• (a) 1 and 2
• (b) 3 and 4
• (c) 2 and 3
• (d) All

(iv) NaCl(aq) + AgNO₃(aq) → AgCl↓ + NaNO₃(aq). The given chemical reaction is:

• (a) Displacement
• (b) Combination
• (c) Decomposition
• (d) Double displacement

The symbolic representation of a chemical reaction using chemical symbols and formulae of reactants and products is called a chemical equation.

According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass and total number of atoms of each element on the reactant side (LHS) must equal the product side (RHS).

• (a) Potassium + Water:
• 2K(s) + 2H_2O(l) → 2KOH(aq) + H₂(g)↑
• (b) Nitrogen + Hydrogen:
• N₂(g) + 3H₂(g) → 2NH₃(g)
• (c) Hydrogen sulphide + Oxygen:
• 2H_2S(g) + 3O₂(g) → 2H_2O(l) + 2SO₂(g)↑
• (d) Barium chloride + Aluminium sulphate:
• 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓

(i) Combustion of Propane:**

• (ii) Fermentation of Glucose:
• (iii) Mercuric nitrate + Potassium iodide:
• (iv) Nitric acid + Calcium hydroxide:

• (a) Magnesium + Iodine:
• Balanced Equation: Mg(s) + I₂(s) → MgI₂(s)
• Type of Reaction: Combination Reaction /
• (b) Magnesium + Hydrochloric acid:
• Balanced Equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑
• Type of Reaction: Displacement Reaction /
• (c) Zinc + Copper nitrate:
• Balanced Equation: Zn(s) + Cu(NO₃)₂(aq) → Zn(NO₃)₂(aq) + Cu(s)
• Type of Reaction: Displacement Reaction /
• (d) Sodium hydrogencarbonate decomposition:
• Balanced Equation: 2NaHCO₃(s) → [heat] Na_2CO₃(s) + H_2O(l) + CO₂(g)↑
• Type of Reaction: Decomposition Reaction (Thermal Decomposition) /

In a combination reaction, two or more simple substances combine to form a single complex product.

• Equation: 2H₂(g) + O₂(g) → 2H_2O(l)
• In a decomposition reaction, a single complex reactant breaks down into two or more simpler products.
• Equation: 2H_2O(l) → [electricity] 2H₂(g) + O₂(g)
• Thus, the processes are exact inverses of each other in terms of reactants and products.

1. Thermal Decomposition:

• CaCO₃(s) → [heat] CaO(s) + CO₂(g)↑

1. Photochemical Decomposition:

• 2AgCl(s) → [sunlight] 2Ag(s) + Cl₂(g)↑

1. Electrolytic Decomposition:

• 2H_2O(l) → [electric current] 2H₂(g)↑ + O₂(g)↑

• Example 1:
• CuO(s) + H₂(g) → Cu(s) + H_2O(l)
• Oxidized: H₂ → H_2O; Reduced: CuO → Cu.
• Example 2:
• ZnO(s) + C(s) → Zn(s) + CO(g)
• Oxidized: C → CO; Reduced: ZnO → Zn.

Reactions that occur absorbing light energy are called photochemical reactions.

• 1. Photosynthesis: Plants synthesize glucose from CO₂ and water in sunlight.
• 6CO₂(g) + 6H_2O(l) → [sunlight/chlorophyll] C_6H₁₂O₆(aq) + 6O₂(g)
• 2. Decomposition of Silver Chloride: White AgCl turns grey in sunlight as it decomposes into silver metal and chlorine gas.
• 2AgCl(s) → [sunlight] 2Ag(s) + Cl₂(g)

A reaction in which two aqueous soluble salts react to produce an insoluble solid product called a precipitate (↓) is called a precipitation reaction.

• Example: Mixing aqueous solutions of barium chloride and sodium sulphate forms a white insoluble precipitate of barium sulphate.
• Na_2SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

Reactions in which heat energy is released along with the formation of products.

• Example: Combustion of natural gas: CH₄(g) + 2O₂(g) → CO₂(g) + 2H_2O(g) + Heat

Reactions in which heat energy is absorbed during product formation.

• Example: Thermal decomposition of limestone: CaCO₃(s) + Heat → CaO(s) + CO₂(g)

Yes, we completely agree with Hanif.**

• Reasons:

1. Combination Reaction: Two reactants (Mg and O₂) combine together to produce a single product (MgO).
2. Oxidation Reaction: Magnesium gains oxygen to form magnesium oxide, and loses electrons to form Mg²⁺ ions.
3. Exothermic Reaction: The burning takes place with a dazzling white flame releasing intense heat and light energy.