NCERT Solutions Class 10 Science Chapter 1 Chemical Reactions and Equations

Mastering the NCERT Solutions Class 10 Science Chapter 1 Chemical Reactions and Equations is the fundamental first step toward scoring full marks in your CBSE Class 10 Science board examinations. This chapter forms the foundation of senior school chemistry, introducing concepts such as writing word equations, balancing chemical equations, classifying reaction types, identifying oxidation-reduction processes, and analyzing real-world phenomena like corrosion and rancidity. These comprehensive, step-by-step NCERT Science Class 10 solutions have been prepared strictly according to the latest CBSE syllabus to help you build solid conceptual clarity and master every question from the textbook.

Chapter Overview

Chapter 1 of Class 10 Science bridges the gap between basic qualitative observations and quantitative chemical equations. In CBSE board examinations, this chapter carries a consistent weightage of 5 to 6 marks, typically distributed across multiple-choice questions (MCQs), assertion-reason questions, short-answer balancing problems, and case-based or activity-based descriptive questions.

The core syllabus of this chapter is structured around four primary pillars:

• Chemical Equations: Representing chemical changes symbolically, understanding reactants and products, and balancing equations based on the Law of Conservation of Mass.
• Types of Chemical Reactions: Combination, Decomposition (Thermal, Electrolytic, and Photolytic), Displacement, Double Displacement (Precipitation), and Neutralization reactions.
• Thermal Changes in Reactions: Exothermic reactions (energy released) versus Endothermic reactions (energy absorbed), with real-life examples such as cellular respiration and photosynthesis.
• Redox Reactions and Everyday Applications: Simultaneous oxidation and reduction, identification of oxidizing and reducing agents, prevention of corrosion, and rancidity control in fatty foods.

Important Concepts, Formulas, and Reaction Mechanisms

1. Law of Conservation of Mass and Balancing Chemical Equations

According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products, which necessitates that the total number of atoms of each element remains identical on both sides of a chemical equation.

Step-by-Step Balancing Method (Hit-and-Trial / Systematic Table Method):

1. Write the skeletal chemical equation with correct chemical formulas: e.g., Fe + H₂O → Fe₃O₄ + H₂.
2. Count and list the number of atoms of each element on the Reactant side (LHS) and Product side (RHS).
3. Balance the element with the maximum number of atoms first (often oxygen or metal atoms). Here, oxygen has 4 atoms on RHS, so multiply H₂O by 4: Fe + 4H₂O → Fe₃O₄ + H₂.
4. Balance the remaining elements (Hydrogen: 8 atoms on LHS → multiply H₂ on RHS by 4; Iron: 3 atoms on RHS → multiply Fe on LHS by 3).
5. Write the final balanced equation and append state symbols (s, l, g, aq):
   3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

2. Five Fundamental Types of Chemical Reactions

• Combination Reaction: Two or more reactants combine to form a single product.
  Example: Calcium oxide (quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime):
  CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
• Decomposition Reaction: A single reactant breaks down into two or more simpler products.
  • Thermal Decomposition (Heat): 2FeSO₄(s) →[Heat] Fe₂O₃(s) + SO₂(g) + SO₃(g) (Green crystals turn reddish-brown with suffocating sulfur gas).
  • Electrolytic Decomposition (Electricity): 2H₂O(l) →[Electricity] 2H₂(g) + O₂(g) (Volume ratio of H₂ to O₂ is 2:1).
  • Photolytic Decomposition (Light): 2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g) (White AgCl turns grey; used in black-and-white photography).
• Displacement Reaction: A more reactive element displaces a less reactive element from its aqueous salt solution.
  Example: Iron nail dipped in blue copper sulphate solution turns brownish, and the blue solution turns light green:
  Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
• Double Displacement (Precipitation) Reaction: Mutual exchange of ions between two ionic compounds in aqueous solution to form an insoluble precipitate.
  Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ (White ppt) + 2NaCl(aq)
• Redox (Oxidation-Reduction) Reaction:
  • Oxidation: Gain of oxygen or loss of hydrogen / loss of electrons.
  • Reduction: Loss of oxygen or gain of hydrogen / gain of electrons.
  • Example: In CuO(s) + H₂(g) →[Heat] Cu(s) + H₂O(l), CuO is reduced to Cu (acting as the oxidizing agent), while H₂ is oxidised to H₂O (acting as the reducing agent).

NCERT In-Text Questions and Detailed Step-by-Step Solutions

In-Text Questions (Page 6)

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

Answer: Magnesium is a reactive metal. When exposed to moist air, it slowly reacts with atmospheric oxygen and carbon dioxide to form a stable, protective coating of basic magnesium oxide / magnesium carbonate [MgO • MgCO₃] on its surface. This layer prevents further direct contact between magnesium and atmospheric oxygen, hindering ignition. Cleaning the ribbon with sandpaper removes this oxide film, exposing the pure magnesium metal so it can burn smoothly with a dazzling white flame.

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

Answer:

1. H₂(g) + Cl₂(g) → 2HCl(g)
2. 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
3. 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Question 3: 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.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer:

1. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
2. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

In-Text Questions (Page 10)

Question 1: A solution of a substance 'X' is used for white washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' named in (i) above with water.

Answer:

1. The substance 'X' is Calcium oxide (commonly called quicklime). Its chemical formula is CaO.
2. When calcium oxide reacts vigorously with water, it forms slaked lime (calcium hydroxide) with the evolution of a large amount of heat:
   CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

Question 2: Why is the amount of gas collected in one of the test tubes in Activity 1.7 (electrolysis of water) double of the amount collected in the other? Name this gas.

Answer: Water molecule (H₂O) consists of two atoms of hydrogen chemically combined with one atom of oxygen in a 2:1 ratio by volume. During the electrolysis of water, water decomposes according to the balanced chemical equation:
2H₂O(l) →[Electric Current] 2H₂(g) + O₂(g)
Since 2 moles of hydrogen gas are liberated at the cathode for every 1 mole of oxygen gas liberated at the anode, the volume of hydrogen gas collected is exactly double the volume of oxygen gas. The gas collected in double quantity is Hydrogen (H₂).

In-Text Questions (Page 13)

Question 1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: Iron is chemically more reactive than copper in the electrochemical reactivity series. When an iron nail is placed in blue copper sulphate (CuSO₄) solution, iron displaces copper from its solution to form light-green iron(II) sulphate (FeSO₄), while metallic copper deposits as a reddish-brown layer on the iron nail:
Fe(s) + CuSO₄(aq) [Blue] → FeSO₄(aq) [Pale Green] + Cu(s) [Reddish-brown]

Question 2: Give an example of a double displacement reaction other than the reaction between sodium sulphate and barium chloride.

Answer: Reaction between lead(II) nitrate and potassium iodide solutions in water forms a bright yellow precipitate of lead(II) iodide and aqueous potassium nitrate:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ [Yellow Precipitate] + 2KNO₃(aq)

Question 3: Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Answer:

1. 4Na(s) + O₂(g) → 2Na₂O(s):
   • Substance Oxidised: Sodium (Na), as it gains oxygen to form Na₂O.
   • Substance Reduced: Oxygen (O₂), as it gains electrons from sodium.
2. CuO(s) + H₂(g) → Cu(s) + H₂O(l):
   • Substance Reduced: Copper(II) oxide (CuO), as it loses oxygen to form metallic copper (Cu).
   • Substance Oxidised: Hydrogen (H₂), as it gains oxygen to form water (H₂O).

NCERT Textbook Exercise Solutions (End-of-Chapter)

Question 1: 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.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Answer: (i) (a) and (b)
Explanation: Lead oxide (PbO) loses oxygen to become lead (Pb), so PbO is reduced (not lead). Carbon (C) gains oxygen to form carbon dioxide (CO₂), so carbon is oxidised (not carbon dioxide). Statements (a) and (b) are incorrect.

Question 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
Explanation: Aluminium is more reactive than iron; hence, it displaces iron from its oxide (Fe₂O₃) to form aluminium oxide (Al₂O₃).

Question 3: What happens when dilute hydrochloric acid is added to iron filings? 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.

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

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

Answer: A balanced chemical equation is a symbolic representation of a chemical reaction in which the total number of atoms of each participating element is equal on both the reactant side (LHS) and the product side (RHS).
Chemical equations must be balanced to satisfy the fundamental Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. Thus, the total mass of the reactants before the reaction must equal the total mass of the products formed after the reaction.

Question 5: 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.

Answer:

1. 3H₂(g) + N₂(g) → 2NH₃(g)
2. 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
3. 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓
4. 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)↑

Question 6: 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

Answer:

1. 2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
2. 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
3. NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq) (Already balanced)
4. BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HCl(aq)

Question 7: 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

Answer:

1. Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
2. Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
3. 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
4. BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)

Question 8: 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)

Answer:

1. 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double Displacement Reaction
2. ZnCO₃(s) →[Heat] ZnO(s) + CO₂(g) — Decomposition Reaction (Thermal)
3. H₂(g) + Cl₂(g) → 2HCl(g) — Combination Reaction
4. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement Reaction

Question 9: What does one mean by exothermic and endothermic reactions? Give examples.

Answer:

• Exothermic Reactions: Chemical reactions that release energy in the form of heat, light, or sound into the surroundings.
  Example: Burning of natural gas (methane):
  CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat energy
• Endothermic Reactions: Chemical reactions that absorb energy from the surroundings (in the form of heat, light, or electrical energy) to proceed.
  Example: Thermal decomposition of limestone (calcium carbonate):
  CaCO₃(s) + Heat → CaO(s) + CO₂(g)

Question 10: Why is respiration considered an exothermic reaction? Explain.

Answer: During digestion, carbohydrate-rich foods (like rice, potatoes, bread) break down into simpler molecules called glucose (C₆H₁₂O₆). In cellular respiration, this glucose combines with inhaled oxygen inside the cells of our body to produce carbon dioxide, water, and metabolic energy stored in ATP molecules. Because chemical energy is continuously released as heat and work to maintain life processes, respiration is classified as an exothermic reaction:
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Question 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: In a combination reaction, two or more simpler substances combine to form a single complex product, whereas in a decomposition reaction, a single compound breaks down into two or more simpler substances when supplied with energy. Hence, they are chemical opposites.

• Combination Equation: 2H₂(g) + O₂(g) → 2H₂O(l)
• Decomposition Equation: 2H₂O(l) →[Electric current] 2H₂(g) + O₂(g)

Question 12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.

Answer:

1. Thermal Decomposition (Heat): Decomposition of lead nitrate crystals upon heating:
   2Pb(NO₃)₂(s) →[Heat] 2PbO(s) + 4NO₂(g)↑ [Brown fumes] + O₂(g)
2. Photolytic Decomposition (Light): Photolysis of silver bromide in sunlight:
   2AgBr(s) →[Sunlight] 2Ag(s) + Br₂(g)
3. Electrolytic Decomposition (Electricity): Electrolytic splitting of molten sodium chloride:
   2NaCl(l) →[Electric Current] 2Na(s) + Cl₂(g)

Question 13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

Feature	Displacement Reaction	Double Displacement Reaction
Definition	A more reactive element displaces a less reactive element from its aqueous salt solution.	Two ionic compounds mutually exchange their constituent ions to form two new compounds.
Number of Displacements	Single exchange of atoms/ions.	Mutual (double) exchange of ions.
Example Equation	Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)	AgNO3(aq) + NaCl(aq) → AgCl(s)↓ + NaNO3(aq)

Question 14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer: Copper is placed above silver in the reactivity series and is therefore more reactive. When copper strips are placed in silver nitrate solution, copper displaces silver, forming copper(II) nitrate solution while pure crystalline silver precipitates out:
2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)

Question 15: What do you mean by a precipitation reaction? Explain by giving examples.

Answer: Any chemical reaction in aqueous solution that results in the formation of an insoluble solid substance (called a precipitate) that separates out from the solution is termed a precipitation reaction.
Example: Mixing aqueous solutions of barium chloride and sodium sulphate produces an insoluble white precipitate of barium sulphate:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ [White ppt] + 2NaCl(aq)

Question 16: Explain the following in terms of gain or loss of oxygen with two examples each: (a) Oxidation (b) Reduction.

Answer:

• (a) Oxidation: The chemical process involving the gain of oxygen by a substance.
  • Example 1: Burning magnesium in oxygen: 2Mg(s) + O₂(g) → 2MgO(s) (Mg gains oxygen).
  • Example 2: Heating copper metal in air: 2Cu(s) + O₂(g) → 2CuO(s) (Cu gains oxygen).
• (b) Reduction: The chemical process involving the loss of oxygen from a substance.
  • Example 1: Heating zinc oxide with carbon: ZnO(s) + C(s) → Zn(s) + CO(g) (ZnO loses oxygen).
  • Example 2: Reduction of copper(II) oxide by hydrogen: CuO(s) + H₂(g) → Cu(s) + H₂O(l) (CuO loses oxygen).

Question 17: A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Answer:

• The shiny brown-coloured element 'X' is Copper (Cu).
• The black-coloured compound formed upon oxidation is Copper(II) oxide (CuO).
• Chemical Equation: 2Cu(s) [Shiny brown] + O₂(g) →[Heat] 2CuO(s) [Black]

Question 18: Why do we apply paint on iron articles?

Answer: Iron rusts when exposed to moist air (oxygen and water vapour) over time, forming hydrated iron(III) oxide [Fe₂O₃ • xH₂O]. Applying a coat of paint creates an impermeable barrier on the iron surface that cuts off direct exposure to atmospheric moisture and oxygen, effectively preventing corrosion and prolonging the structural lifespan of the iron article.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer: Food items containing fats and oils undergo oxidation when exposed to atmospheric oxygen over time, resulting in bad odour, foul taste, and rancidity. Nitrogen is an unreactive (inert) gas that displaces oxygen inside packaged food packets (like potato chips), establishing an oxygen-free environment that prevents oxidative degradation and maintains food freshness.

Question 20: Explain the following terms with one example each: (a) Corrosion (b) Rancidity.

Answer:

• (a) Corrosion: The slow, gradual deterioration and destruction of metals caused by their chemical reaction with atmospheric substances such as moisture, oxygen, carbon dioxide, or acids.
  Example: Rusting of iron forms hydrated ferric oxide (Fe₂O₃ • xH₂O); tarnishing of silver articles forming black silver sulphide (Ag₂S); formation of a green basic copper carbonate coating [CuCO₃ • Cu(OH)₂] on copper vessels.
• (b) Rancidity: The slow, aerial oxidation of unsaturated fats and oils present in food materials, producing volatile, foul-smelling carboxylic acids and aldehydes that alter taste and smell.
  Example: Butter or cooking oil left open in air developing an unpleasant taste and foul smell. Rancidity is prevented by adding antioxidants (like BHA and BHT), refrigeration, vacuum packing, or flushing packaging with nitrogen gas.

Common Mistakes and Practical Exam Tips

Examiners frequently deduct marks in Class 10 Chemistry for minor representational errors. Keep the following checkpoints in mind when writing your answers:

• Missing Physical State Symbols: Always indicate physical states — (s) for solids, (l) for liquids, (g) for gases, and (aq) for aqueous solutions — particularly in precipitation reactions where the precipitate must be marked with (s) or ↓.
• Confusing Oxidizing and Reducing Agents: Remember that the substance that gets reduced acts as the oxidizing agent, and the substance that gets oxidised acts as the reducing agent. Identify agents strictly from the reactant side of the equation.
• Thermal Decomposition Observations: When writing the decomposition of lead nitrate [2Pb(NO₃)₂], state both observations clearly: emission of pungent, reddish-brown fumes of nitrogen dioxide (NO₂) and the formation of a yellow solid residue of lead(II) oxide (PbO).
• Electrolysis Volume Ratio: Remember that Hydrogen gas is collected at the cathode (negative electrode) and Oxygen gas is collected at the anode (positive electrode) in a 2:1 volume ratio.

Board Exam Relevance and Question Blueprint

In CBSE Class 10 Science Board examinations, Chapter 1 typically features the following question formats:

1. 1-Mark Objective Questions / MCQs: Identifying reaction types, identifying oxidized/reduced substances, or predicting observations when metals react with acids or salt solutions.
2. 2-Mark and 3-Mark Short Answer Questions: Balancing complex word equations, writing chemical equations with observations for thermal decomposition reactions, or explaining redox reactions with labelled oxidizing and reducing agents.
3. 4-Mark / 5-Mark Case-Based and Integrated Questions: Activity-based questions evaluating experimental setups (e.g., Activity 1.7 electrolysis of water, Activity 1.2 lead nitrate and potassium iodide reaction, or Activity 1.1 burning of magnesium ribbon).

More NCERT Solutions and Practice

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