Chemical Reactions and Equations | Detailed Class 10 Notes

Chemical Reactions and Equations

Class 10 NCERT Science - Chapter 1

Detailed Notes with Explanations, Important Concepts, Activities, Equations & All Questions Solved

Introduction Chemical Equations Types of Reactions Oxidation & Reduction Everyday Effects All Questions

1. Chemical Reactions in Daily Life

When substances undergo a change in their chemical composition and properties, a chemical reaction takes place.

Examples from daily life:

Milk left at room temperature in summers → turns sour
Iron nail/tawa left in humid air → rusts
Grapes get fermented
Food is cooked / digested in our body
We respire

                    Observation that confirms a chemical reaction has occurred:

                    • Change in state

                    • Change in colour

                    • Evolution of a gas

                    • Change in temperature

Activity 1.1: Burning of Magnesium Ribbon

Magnesium ribbon burns with a dazzling white flame forming white ash (magnesium oxide).

Magnesium + Oxygen → Magnesium oxide

Observation: Dazzling white flame and white powder (MgO) is formed.

1.1 Chemical Equations

Word Equation vs Chemical Equation

Magnesium + Oxygen → Magnesium oxide

Mg + O₂ → MgO (Unbalanced / Skeletal equation)

Balancing Chemical Equations

Based on Law of Conservation of Mass (mass can neither be created nor destroyed).

Step-by-step Balancing (Example: Fe + H₂O → Fe₃O₄ + H₂)

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Element	Reactants (LHS)	Products (RHS)
Fe	3	3
H	8	8
O	4	4

                    Physical State Symbols:

                    (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution

1.2 Types of Chemical Reactions

1.2.1 Combination Reaction

Two or more reactants combine to form a single product. Generally exothermic.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime + Water → Slaked lime)

Other examples: Burning of coal, respiration (glucose + oxygen → CO₂ + H₂O + energy).

1.2.2 Decomposition Reaction

Single reactant breaks down into two or more products. Generally endothermic.

Thermal Decomposition: CaCO₃(s) Heat → CaO(s) + CO₂(g)

2FeSO₄(s) Heat → Fe₂O₃(s) + SO₂(g) + SO₃(g)

Photolytic: 2AgCl(s) Sunlight → 2Ag(s) + Cl₂(g)

Electrolytic: 2H₂O(l) Electricity → 2H₂(g) + O₂(g)

1.2.3 Displacement Reaction

More reactive metal displaces less reactive metal from its salt solution.

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Zinc and lead also displace copper.

1.2.4 Double Displacement Reaction

Exchange of ions between two compounds. Often forms a precipitate.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

1.2.5 Oxidation and Reduction (Redox Reactions)

Oxidation: Gain of oxygen or loss of hydrogen

Reduction: Loss of oxygen or gain of hydrogen

2Cu + O₂ Heat → 2CuO (Oxidation)

CuO + H₂ Heat → Cu + H₂O (Reduction of CuO)

Recall Activity 1.1: Magnesium is oxidised to magnesium oxide.

1.3 Effects of Oxidation Reactions in Everyday Life

1.3.1 Corrosion

Metals get attacked by moisture, acids, oxygen etc. and get corroded.

Rusting of iron: Formation of reddish-brown Fe₂O₃·xH₂O.

Corrosion damages bridges, cars, ships etc.

1.3.2 Rancidity

Oxidation of fats and oils in food items causes bad smell and taste.

Prevention: Use of antioxidants, airtight containers, flushing chips packets with nitrogen gas.

All Questions from the Chapter - Solved

In-Text Questions

1. Why should a magnesium ribbon be cleaned before burning in air?

Answer: The surface of magnesium ribbon is covered with a thin layer of magnesium oxide (MgO) which prevents it from burning. Cleaning with sandpaper removes this oxide layer so that magnesium can burn vigorously in air.

2. Write the balanced chemical equations for the following reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answers:
(i) H₂(g) + Cl₂(g) → 2HCl(g)
(ii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(iii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

3. Write balanced chemical equations with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and solution of sodium chloride.
(ii) Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.

Answers:
(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ with water.

Answer:
(i) Substance ‘X’ is calcium oxide (quick lime), CaO.
(ii) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer: In electrolysis of water, water decomposes as: 2H₂O → 2H₂ + O₂. Hydrogen gas is collected at cathode and oxygen at anode. Volume of hydrogen is double that of oxygen because 2 molecules of hydrogen are produced for every 1 molecule of oxygen.

Exercises (NCERT) - Fully Solved

1. Which of the statements about the reaction 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect?
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

Answer: Incorrect statements are (b) and (c).
Correct: Carbon is oxidised to CO₂ and lead oxide is reduced to lead.

2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a
(a) combination reaction (b) double displacement reaction (c) decomposition reaction (d) displacement reaction

Answer: (d) displacement reaction (also a redox reaction).

3. What happens when dilute hydrochloric acid is added to iron fillings?

Answer: (a) Hydrogen gas and iron chloride are produced.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced chemical equation has equal number of atoms of each element on both reactant and product sides. Equations must be balanced to satisfy the Law of Conservation of Mass.

5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.

Ans: N₂(g) + 3H₂(g) → 2NH₃(g)

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

Ans: 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)

Ans: 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)