Class 10 Science Chapter 1: Chemical Reactions and Equations Solutions, Notes & MCQs | HSLC Guide

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Class 10 Science: Chapter 1 - Chemical Reactions and Equations

The Ultimate Master Guide for ASSEB, NCERT & CBSE | HSLC Exam Special

📌 Chapter Overview: This chapter is the foundation of chemistry. We will learn how to identify a chemical reaction, how to write and balance chemical equations, the five main types of chemical reactions (Combination, Decomposition, Displacement, Double Displacement, and Redox), and finally, the effects of oxidation in our daily lives (Corrosion and Rancidity).

1. Chemical Reactions & Equations (Concept Notes)

1.1 What is a Chemical Reaction?

In a chemical reaction, the original substances (reactants) undergo a chemical change to form new substances (products) with entirely different properties.

How do we know a chemical reaction has taken place? (Observations)

• Change in state: e.g., liquid water turning into steam.
• Change in colour: e.g., Iron nail turning brownish in copper sulphate solution.
• Evolution of a gas: e.g., Bubbles forming when zinc reacts with acid.
• Change in temperature: e.g., The beaker becomes hot when quicklime is added to water.

1.2 Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction using chemical formulae.

• Word Equation: Magnesium + Oxygen → Magnesium oxide
• Skeletal Equation (Unbalanced): Mg + O₂ → MgO
• Balanced Equation: 2Mg + O₂ → 2MgO

⚖️ Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element must be equal on both sides (Reactants and Products). This is why we balance equations.

1.3 Balancing Chemical Equations (Hit-and-Trial Method)

Steps to balance: Fe + H₂O → Fe₃O₄ + H₂

1. Draw boxes around each formula. Do not change the subscripts inside the boxes.
2. Start with the compound containing the maximum number of atoms (Fe₃O₄). Balance Oxygen: Put 4 in front of H₂O.
   Fe + 4H₂O → Fe₃O₄ + H₂
3. Balance Hydrogen: 8 H on left, so put 4 in front of H₂ on right.
   Fe + 4H₂O → Fe₃O₄ + 4H₂
4. Balance Iron: 3 Fe on right, so put 3 in front of Fe on left.
   3Fe + 4H₂O → Fe₃O₄ + 4H₂

1.4 Writing State Symbols and Conditions

• (s) - Solid, (l) - Liquid, (g) - Gas, (aq) - Aqueous (dissolved in water)
• Conditions like Heat, Pressure, Catalyst, Sunlight are written above/below the arrow.

Example: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

2. Types of Chemical Reactions (Detailed)

Reaction Type	Definition	General Equation	Detailed Example from Textbook
Combination	Two or more reactants combine to form a single product.	A + B → AB	CaO(s) + H2O(l) → Ca(OH)2(aq) (Exothermic, used for whitewashing)
Decomposition (Thermal)	Single reactant breaks down using heat.	AB + Heat → A + B	CaCO3(s) + Heat → CaO(s) + CO2(g) (Used in cement industry)
Decomposition (Thermal)	Single reactant breaks down using heat.	AB + Heat → A + B	2FeSO4(s) + Heat → Fe2O3(s) + SO2(g) + SO3(g) (Green crystals turn white then brown)
Decomposition (Thermal)	Single reactant breaks down using heat.	AB + Heat → A + B	2Pb(NO3)2(s) + Heat → 2PbO(s) + 4NO2(g) + O2(g) (Brown fumes of NO2 evolve)
Decomposition (Electrolytic)	Single reactant breaks down using electricity.	AB + Electricity → A + B	2H2O(l) + Electricity → 2H2(g) + O2(g) (Electrolysis of water)
Decomposition (Photolytic)	Single reactant breaks down using light.	AB + Light → A + B	2AgCl(s) + Sunlight → 2Ag(s) + Cl2(g) (White turns grey; used in photography)
Displacement	A more reactive element displaces a less reactive element.	A + BC → AC + B	Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) (Blue colour fades, iron turns brown)
Double Displacement	Two compounds exchange ions. Often produces a precipitate.	AB + CD → AD + CB	Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) (White precipitate)
Redox	One reactant gets oxidised, the other gets reduced.	Redox	CuO + H2 → Cu + H2O (CuO reduced, H2 oxidised)

💡 Key Definitions:

• Exothermic Reaction: Heat is released along with products. (e.g., Respiration, Burning of natural gas).
• Endothermic Reaction: Energy is absorbed. (e.g., Decomposition of CaCO₃).
• Precipitation Reaction: Any reaction that produces an insoluble solid (precipitate).
• Oxidation: Gain of Oxygen or loss of Hydrogen.
• Reduction: Loss of Oxygen or gain of Hydrogen.

3. Effects of Oxidation in Everyday Life

• Corrosion: When a metal is attacked by substances around it (moisture, acids, etc.). E.g., Rusting of iron (reddish-brown), black coating on silver, green coating on copper.
• Rancidity: When fats and oils in food are oxidised, they become rancid and their smell and taste change. Prevention: Adding antioxidants, keeping in airtight containers, flushing bags with nitrogen gas.

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4. Complete Textbook Questions & Answers (NCERT/ASSEB)

Covering all In-text and Back-Exercise Questions.

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Ans: Magnesium ribbon is cleaned by rubbing with sandpaper to remove the layer of magnesium oxide (MgO) formed on its surface. This layer prevents the magnesium from burning properly. Cleaning exposes the fresh metal surface for the reaction.

Q2. Write the balanced equation for the following chemical reactions:

(i) Hydrogen + Chlorine → Hydrogen chloride
H₂(g) + Cl₂(g) → 2HCl(g)

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)

(iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Q3. Write a balanced chemical equation with state symbols for the following reactions:

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Q4. 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' named in (i) above with water.

(i) The substance 'X' is Calcium Oxide (Quick Lime). Formula: CaO.
(ii) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

Q5. 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.

The reaction is the electrolysis of water. Water (H₂O) contains Hydrogen and Oxygen in a 2:1 ratio by volume. Therefore, the gas collected at the cathode (Hydrogen) is double the volume of the gas collected at the anode (Oxygen). The gas collected in the double amount is Hydrogen.

Q6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron is more reactive than copper. When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution. The blue colour of copper sulphate fades and the solution turns light green due to the formation of ferrous sulphate.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Q7. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) (Yellow precipitate of Lead iodide is formed).

Q8. Identify the substances that are oxidised and the substances that are reduced in the following reactions:

(i) 4Na(s) + O₂(g) → 2Na₂O(s)
Sodium (Na) is oxidised (gains oxygen). Oxygen (O₂) is reduced.

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Hydrogen (H₂) is oxidised (gains oxygen). Copper Oxide (CuO) is reduced (loses oxygen).

Q9. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.

Ans: (i) (a) and (b) are incorrect. Lead is getting reduced (correct), but carbon dioxide is not getting oxidised; carbon is getting oxidised.

Q10. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a: (a) combination reaction (b) double displacement reaction (c) displacement reaction (d) decomposition reaction.

Ans: (c) Displacement reaction. Aluminium is more reactive than iron, so it displaces iron from its oxide.

Q11. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.

Ans: (a) Hydrogen gas and iron chloride are produced.

Q12. What is a balanced chemical equation? Why should chemical equations be balanced?

A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides. It should be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction.

Q13. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Q14. Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃ (Already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Q15. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

(a) Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)

Q16. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double Displacement Reaction
(b) ZnCO₃(s) → ZnO(s) + CO₂(g) — Decomposition Reaction (Thermal)
(c) H₂(g) + Cl₂(g) → 2HCl(g) — Combination Reaction
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement Reaction

Q17. What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic: Reactions in which heat is released. E.g., CaO + H₂O → Ca(OH)₂ + Heat.
Endothermic: Reactions in which heat is absorbed. E.g., CaCO₃ + Heat → CaO + CO₂.

Q18. Why is respiration considered an exothermic reaction? Explain.

During respiration, glucose (carbohydrate) combines with oxygen in our body cells to produce carbon dioxide, water, and a large amount of energy. Since energy (heat) is released during this process, it is an exothermic reaction.
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Q19. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

In a combination reaction, two or more substances combine to form a single product (A + B → AB). In a decomposition reaction, a single substance breaks down into two or more simpler substances (AB → A + B). Hence, they are opposite processes.
Example Combination: CaO + H₂O → Ca(OH)₂
Example Decomposition: CaCO₃ → CaO + CO₂

Q20. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Heat (Thermal): CaCO₃(s) + Heat → CaO(s) + CO₂(g)
Light (Photolytic): 2AgCl(s) + Sunlight → 2Ag(s) + Cl₂(g)
Electricity (Electrolytic): 2H₂O(l) + Electricity → 2H₂(g) + O₂(g)

Q21. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Displacement: A more reactive element displaces a less reactive element from its compound.
Example: Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement: Two compounds exchange their ions to form two new compounds.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Q22. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Q23. What do you mean by a precipitation reaction? Explain by giving examples.

A reaction in which an insoluble solid (precipitate) is formed is called a precipitation reaction.
Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (White precipitate of BaSO₄).

Q24. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

(a) Oxidation (Gain of Oxygen):
1. 2Cu + O₂ → 2CuO (Copper gains oxygen)
2. CH₄ + 2O₂ → CO₂ + 2H₂O (Methane gains oxygen)

(b) Reduction (Loss of Oxygen):
1. CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen)
2. ZnO + C → Zn + CO (Zinc oxide loses oxygen)

Q25. A shiny brown coloured element X on heating in air becomes black in colour. Name the element X and the black coloured compound formed.

Element X is Copper (Cu). The black coloured compound formed is Copper(II) Oxide (CuO).
2Cu + O₂ → 2CuO

Q26. Why do we apply paint on iron articles?

We apply paint on iron articles to prevent them from coming in contact with moisture and air, thereby preventing the process of rusting (corrosion).

Q27. Oil and fat containing food items are flushed with nitrogen. Why?

Nitrogen is an unreactive gas. Flushing food bags with nitrogen displaces the oxygen present inside the packet. This prevents the oxidation of fats and oils, thereby preventing rancidity and increasing the shelf life of the food.

Q28. Explain the following terms with one example each. (a) Corrosion (b) Rancidity

(a) Corrosion: The process of slow eating away of a metal surface by the action of moisture, air, or acids. Example: Rusting of iron (formation of reddish-brown Fe₂O₃·xH₂O).
(b) Rancidity: The process of oxidation of fats and oils in food, leading to a change in smell and taste. Example: Butter turning rancid after some time.

5. HSLC Exam Special & Additional Questions (Assam Board)

🎯 HSLC High-Potential Questions (Frequently Asked in Assam Board)

Q1. What is a redox reaction? Explain with an example.

A reaction in which one reactant gets oxidised and the other gets reduced is called an oxidation-reduction reaction or redox reaction.
Example: ZnO + C → Zn + CO. Here, ZnO is reduced to Zn, and C is oxidised to CO.

Q2. Why is silver chloride stored in dark coloured bottles?

Silver chloride undergoes photolytic decomposition when exposed to sunlight, turning grey. To prevent this decomposition, it is stored in dark coloured bottles.

Q3. What is meant by a skeletal chemical equation?

An unbalanced chemical equation is called a skeletal chemical equation. In it, the number of atoms of different elements are not equal on both sides of the arrow. E.g., Mg + O₂ → MgO.

Q4. Why does the iron nail become brownish in colour and the blue colour of copper sulphate solution fade?

Iron is more reactive than copper. It displaces copper from copper sulphate solution. The displaced copper deposits on the iron nail, making it brownish. The formation of ferrous sulphate (which is light green) causes the blue colour of the solution to fade.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Q5. Identify the type of reaction in the following:
(i) CaO + H₂O → Ca(OH)₂
(ii) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
(iii) Fe + CuSO₄ → FeSO₄ + Cu
(iv) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

(i) Combination Reaction and Exothermic Reaction.
(ii) Thermal Decomposition Reaction.
(iii) Displacement Reaction.
(iv) Double Displacement Reaction (Precipitation Reaction).

6. Interactive MCQs & Concept Check

1. Which of the following is a decomposition reaction?

A) CaO + H₂O → Ca(OH)₂

B) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

C) Fe + CuSO₄ → FeSO₄ + Cu

D) NaOH + HCl → NaCl + H₂O

Show Answer

Correct Answer: B - Ferrous sulphate breaks down into simpler substances upon heating.

2. The reaction between lead nitrate and potassium iodide is an example of:

A) Combination Reaction

B) Displacement Reaction

C) Double Displacement Reaction

D) Decomposition Reaction

Show Answer

Correct Answer: C - Ions are exchanged, producing a yellow precipitate of lead iodide.

3. Which gas is evolved when dilute hydrochloric acid is added to zinc granules?

A) Oxygen

B) Hydrogen

C) Carbon dioxide

D) Nitrogen

Show Answer

Correct Answer: B - Zn + 2HCl → ZnCl₂ + H₂

4. What happens when a piece of copper is heated in air?

A) It turns green

B) It melts

C) It turns black due to formation of CuO

D) No reaction takes place

Show Answer

Correct Answer: C - 2Cu + O₂ → 2CuO (Black copper oxide).

5. In the reaction PbO + C → Pb + CO, which substance is oxidised?

A) PbO

B) C

C) Pb

D) CO

Show Answer

Correct Answer: B - Carbon (C) gains oxygen to form CO, so it is oxidised. PbO is reduced to Pb.

7. HOTS (Higher Order Thinking Skills) Questions

🧠 Challenge Your Mind! (Critical Thinking for HSLC)

Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Explain with the help of a chemical equation.

Ans: Iron is more reactive than copper. When an iron nail is dipped in copper sulphate solution, iron displaces copper from the solution. The blue colour of copper sulphate fades and the solution turns light green due to the formation of ferrous sulphate.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Q2. A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed. Write the balanced chemical equation.

Ans: Element 'X' is Copper (Cu). The black coloured compound formed is Copper(II) Oxide (CuO).
Reaction: 2Cu + O₂ → 2CuO

Q3. Why do we apply paint on iron articles? Explain the chemical process involved.

Ans: We apply paint on iron articles to prevent them from coming in contact with moisture and air, thereby preventing the process of rusting (corrosion). The paint acts as a barrier, preventing the oxidation of iron.

Q4. Oil and fat containing food items are flushed with nitrogen. Why?

Ans: Nitrogen is an unreactive gas. Flushing food bags with nitrogen displaces the oxygen present inside the packet. This prevents the oxidation of fats and oils, thereby preventing rancidity and increasing the shelf life of the food.

Q5. Can a displacement reaction be a redox reaction? Explain with an example.

Ans: Yes, a displacement reaction can be a redox reaction.
Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Here, Iron (Fe) is oxidised (loses electrons) and Copper (Cu²⁺) is reduced (gains electrons). Since both oxidation and reduction occur, it is a redox reaction.

8. Real-World Connections

🌍 Connecting Chemistry to Life:

• Whitewashing Walls: Calcium oxide (quick lime) is mixed with water to form calcium hydroxide (slaked lime). This is applied to walls. Over time, it reacts with atmospheric carbon dioxide to form a thin, shiny layer of calcium carbonate (CaCO₃), giving a beautiful finish.
  Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
• Black and White Photography: Silver bromide (AgBr) is used. When light falls on it, it decomposes into silver metal (black) and bromine gas. This is a photolytic decomposition reaction.
  2AgBr(s) + Sunlight → 2Ag(s) + Br₂(g)
• Antacids: When you have acidity, you take antacids (like Milk of Magnesia). It's a double displacement reaction where the base neutralises the excess stomach acid (HCl).
• Chocolate Wrappers: They are often flushed with nitrogen to prevent the chocolate from becoming rancid and losing its taste.
• Rusting of Iron Gates: Iron reacts with oxygen and moisture in the air to form hydrated iron(III) oxide (rust).
  4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O
• Respiration: The process by which our body breaks down food to release energy is an exothermic combination reaction.

Prepared exclusively for SchoolNotes Axom | Class 10 Science - Chapter 1

Stay tuned for Chapter 2: Acids, Bases, and Salts.