MP Board Class 10 Science Chapter 1: Chemical Reactions a…

MP Board Class 10 Science Chapter 1: Chemical Reactions and Equations Important Questions — Chemical Reactions and Equations is the foundational chapter of Class 10 Science, carrying 6–8 marks in the MP Board board exam. This chapter covers types of chemical reactions, balancing equations, oxidation-reduction, and corrosion. Mastering these important questions ensures strong preparation for the 2027 board exams.

🧪 1. Types of Chemical Reactions

Q1. What are the five main types of chemical reactions? Explain each with an example.

Answer: The five main types of chemical reactions are:

Q2. Differentiate between combination and decomposition reactions with examples.

Answer:

Q3. What is a precipitation reaction? Give an example.

Answer: A precipitation reaction is a double displacement reaction where two aqueous salt solutions react to form an insoluble solid called precipitate. Example: When sodium sulphate (Na₂SO₄) solution is mixed with barium chloride (BaCl₂) solution, a white precipitate of barium sulphate (BaSO₄) is formed.

Equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓ (white precipitate) + 2NaCl(aq)

⚖️ 2. Balancing Chemical Equations

Q4. Explain the law of conservation of mass. How does it apply to balancing chemical equations?

Answer: The law of conservation of mass, proposed by Antoine Lavoisier, states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products. This law is applied while balancing chemical equations — the number of atoms of each element must be equal on both sides of the equation.

Steps to balance a chemical equation:
1. Write the unbalanced equation (skeleton equation)
2. Count the number of atoms of each element on both sides
3. Add coefficients (numbers placed before formulas) to balance atoms
4. Start with the element that appears in the least number of compounds
5. Balance hydrogen and oxygen atoms at the end
6. Verify that all atoms are balanced

Q5. Balance the following chemical equations:

Answer:

(a) Fe + H₂O → Fe₃O₄ + H₂
3Fe + 4H₂O → Fe₃O₄ + 4H₂

(b) Zn + HCl → ZnCl₂ + H₂
Zn + 2HCl → ZnCl₂ + H₂

(c) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(d) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(e) Al + O₂ → Al₂O₃
4Al + 3O₂ → 2Al₂O₃

Q6. Why should a chemical equation be balanced? What information does a balanced equation provide?

Answer: A chemical equation must be balanced to satisfy the law of conservation of mass. An unbalanced equation would imply that atoms are created or destroyed, which is impossible. A balanced chemical equation provides:

• The identities of reactants and products
• The relative quantities (moles) of each substance involved
• The physical states of substances (solid, liquid, gas, aqueous)
• The heat changes (exothermic or endothermic nature)
• The direction of the reaction (irreversible arrows → or reversible ⇌)

⚡ 3. Oxidation and Reduction Reactions

Q7. Define oxidation and reduction. Give one example of each.

Answer:

Oxidation: It is a process that involves:
(a) Addition of oxygen to a substance, OR
(b) Removal of hydrogen from a substance
Example: 2Cu + O₂ → 2CuO (Copper gains oxygen → copper is oxidised)

Reduction: It is a process that involves:
(a) Removal of oxygen from a substance, OR
(b) Addition of hydrogen to a substance
Example: CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen → copper is reduced)

Mnemonic: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Q8. In the reaction ZnO + C → Zn + CO, identify the substance oxidised and the substance reduced.

Answer: ZnO + C → Zn + CO

Oxidised substance: Carbon (C) — it gains oxygen to form CO.
Reduced substance: Zinc oxide (ZnO) — it loses oxygen to form Zn.

Explanation: Carbon acts as a reducing agent because it causes reduction of ZnO to Zn. Zinc oxide acts as an oxidising agent because it causes oxidation of C to CO. In every redox reaction, oxidation and reduction occur simultaneously.

Q9. What is a redox reaction? Explain with the help of an example.

Answer: A redox reaction (reduction-oxidation reaction) is a chemical reaction in which oxidation and reduction occur simultaneously. One substance gets oxidised (loses electrons/gains oxygen) while another gets reduced (gains electrons/loses oxygen).

Example: CuO + H₂ → Cu + H₂O
– CuO is reduced to Cu (loses oxygen)
– H₂ is oxidised to H₂O (gains oxygen)

🔩 4. Corrosion and Rancidity

Q10. What is corrosion? Explain the corrosion of iron (rusting) and mention methods to prevent it.

Answer: Corrosion is the gradual destruction of metals due to chemical reactions with environmental substances like oxygen, moisture, acids, or gases. It is an oxidation process.

Rusting of Iron: Iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust).
Equation: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (Rust)

Conditions required for rusting: Both oxygen and water (or moisture) must be present.

Prevention methods:

• Painting/Oiling: Creates a barrier between iron and air/moisture
• Galvanisation: Coating iron with zinc (e.g., galvanised iron sheets)
• Alloying: Mixing iron with chromium and nickel to make stainless steel
• Greasing: Applying grease or oil on iron tools and machinery
• Electroplating: Coating iron with a thin layer of another metal like chromium

Q11. What is rancidity? How can it be prevented?

Answer: Rancidity is the process in which fats and oils in food materials get oxidised, resulting in a change in their smell and taste. When food containing fats and oils is exposed to air (oxygen) for a long time, it develops a foul smell and unpleasant taste.

Prevention methods:

• Vacuum sealing: Packing food in airtight containers to exclude oxygen
• Refrigeration: Low temperature slows down oxidation
• Using antioxidants: Adding substances like BHA, BHT, or Vitamin E (tocopherols)
• Nitrogen flushing: Replacing air in food packets with nitrogen gas
• Storing in dark containers: Light accelerates oxidation of fats

🏠 5. Effects of Oxidation in Daily Life

Q12. Why do we apply paint/ oil on iron articles? Explain the science behind it.

Answer: We apply paint or oil on iron articles to prevent rusting (corrosion of iron). The science behind this is that paint or oil creates a physical barrier between the iron surface and the environment. This barrier prevents oxygen and moisture from coming into direct contact with iron, which are the two essential requirements for rust formation. Without oxygen and water, the oxidation reaction (Fe → Fe₂O₃·xH₂O) cannot take place, thus protecting the iron article from rusting.

Q13. Explain why chips manufacturers flush packets with nitrogen gas?

Answer: Chips manufacturers flush the packets with nitrogen gas to prevent rancidity. Potato chips contain fats and oils that can get oxidised when exposed to oxygen in air, leading to a foul smell and unpleasant taste. Nitrogen is an inert gas that does not react with the food. By replacing the air inside the packet with nitrogen, manufacturers create an oxygen-free environment that prevents the oxidation of fats and oils, keeping the chips fresh for a longer time.

Q14. Silver ornaments turn black and copper vessels develop a green coating. Explain the chemical reactions involved.

Answer:

Silver turning black: Silver reacts with hydrogen sulphide (H₂S) present in air to form a black layer of silver sulphide (Ag₂S).
Reaction: 2Ag + H₂S → Ag₂S (black) + H₂

Copper turning green: Copper reacts with carbon dioxide (CO₂) and moisture in air to form a green layer of basic copper carbonate [CuCO₃·Cu(OH)₂].
Reaction: 2Cu + H₂O + CO₂ + O₂ → CuCO₃·Cu(OH)₂ (green)

Both these processes are examples of corrosion (oxidation of metals by environmental agents).

📝 6. Multiple Choice Questions (1 Mark Each)

MCQs are frequently asked in the MP Board Class 10 Science paper. Practice these 10 important MCQs from Chapter 1.

1. Which of the following is a combination reaction?
   (a) CaCO₃ → CaO + CO₂ ✓   (b) 2Mg + O₂ → 2MgO   (c) Fe + CuSO₄ → FeSO₄ + Cu   (d) AgNO₃ + NaCl → AgCl + NaNO₃
2. The process of reduction involves:
   (a) Addition of oxygen   (b) Removal of hydrogen   (c) Addition of hydrogen ✓   (d) Loss of electrons
3. Which gas is produced when zinc reacts with dilute hydrochloric acid?
   (a) Oxygen   (b) Chlorine   (c) Hydrogen ✓   (d) Carbon dioxide
4. In the reaction PbO + H₂ → Pb + H₂O, the reducing agent is:
   (a) PbO   (b) H₂ ✓   (c) Pb   (d) H₂O
5. The colour of copper sulphate solution changes when an iron nail is dipped in it because:
   (a) Iron is less reactive than copper   (b) Iron displaces copper ✓   (c) Copper displaces iron   (d) Both react to form a new compound
6. Which of the following is a double displacement reaction?
   (a) 2H₂ + O₂ → 2H₂O   (b) Zn + H₂SO₄ → ZnSO₄ + H₂   (c) NaCl + AgNO₃ → AgCl↓ + NaNO₃ ✓   (d) 2KClO₃ → 2KCl + 3O₂
7. When silver chloride is exposed to sunlight, it decomposes to form:
   (a) Ag + Cl₂ ✓   (b) Ag₂O + Cl₂   (c) AgCl₂   (d) Ag + ClO₂
8. The formula of rust is:
   (a) Fe₃O₄   (b) Fe₂O₃·xH₂O ✓   (c) FeO   (d) Fe(OH)₃
9. Which of the following is an endothermic reaction?
   (a) Burning of magnesium   (b) Decomposition of ferrous sulphate ✓   (c) Dilution of sulphuric acid   (d) Neutralisation of acid and base
10. Nitrogen gas is used in food packaging to prevent:
    (a) Corrosion   (b) Combustion   (c) Rancidity ✓   (d) Evaporation

✏️ 7. Very Short & Short Answer Questions (2–4 Marks)

1. What happens when quicklime (CaO) is added to water? Write the chemical equation and the type of reaction. (2 marks)
   Calcium oxide (quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime) with the release of a large amount of heat. This is an exothermic combination reaction.
   Equation: CaO + H₂O → Ca(OH)₂ + Heat (Combination reaction)
2. What is the colour of ferrous sulphate crystals? What change in colour is observed on heating? (2 marks)
   Fresh ferrous sulphate crystals are light green in colour. On heating, they lose water of crystallisation and the green colour fades, eventually turning into a white powder. On further heating, it decomposes to produce reddish-brown ferric oxide. The green colour is due to Fe²⁺ ions which oxidise to Fe³⁺ on heating.
3. Why is respiration considered an exothermic reaction? (2 marks)
   Respiration is considered an exothermic reaction because during respiration, glucose (C₆H₁₂O₆) reacts with oxygen to produce carbon dioxide, water, and energy. The energy released during this process is used by living organisms for various life processes. Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
4. What is a thermal decomposition reaction? Give two examples. (3 marks)
   A thermal decomposition reaction is a type of decomposition reaction where a single compound breaks down into simpler substances when heated. Heat energy is required to break the chemical bonds.
   Examples:
   (a) 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) [Ferrous sulphate → Ferric oxide + Sulphur dioxide + Sulphur trioxide]
   (b) CaCO₃(s) → CaO(s) + CO₂(g) [Limestone → Quicklime + Carbon dioxide]
5. Differentiate between an exothermic and endothermic reaction with examples. (3 marks)
   Exothermic reaction: A reaction that releases heat energy. The surroundings become hotter. Example: Burning of natural gas — CH₄ + 2O₂ → CO₂ + 2H₂O + Heat.
   Endothermic reaction: A reaction that absorbs heat energy. The surroundings become cooler. Example: Electrolysis of water — 2H₂O → 2H₂ + O₂ (requires electrical energy).
6. Write the balanced chemical equation for the reaction between sodium sulphate and barium chloride. Name the type of reaction. (3 marks)
   Equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓(s) + 2NaCl(aq)
   This is a double displacement reaction (specifically a precipitation reaction). A white precipitate of barium sulphate is formed because BaSO₄ is insoluble in water.
7. Explain the term ‘photochemical decomposition’ with an example. (3 marks)
   Photochemical decomposition is a type of decomposition reaction where a compound breaks down into simpler substances in the presence of light (sunlight). Light energy provides the activation energy needed to break chemical bonds.
   Example: Silver chloride (AgCl) decomposes to silver and chlorine in the presence of sunlight.
   Equation: 2AgCl(s) → 2Ag(s) + Cl₂(g) (in sunlight)
   This reaction is used in black and white photography.
8. What is meant by a ‘balanced chemical equation’? Why is it important? (4 marks)
   A balanced chemical equation has an equal number of atoms of each element on both the reactant and product sides. It is important because:
   (a) It follows the law of conservation of mass — mass cannot be created or destroyed
   (b) It gives the correct quantitative relationship between reactants and products
   (c) It helps in calculating the amount of reactants needed and products formed
   (d) It provides accurate information for stoichiometric calculations in chemical industries and laboratories

📄 8. Long Answer Questions (5–6 Marks)

Q1. Describe with an activity to show that a chemical reaction has taken place when iron nails are dipped in copper sulphate solution. Write the balanced equation and identify the type of reaction.

Answer:

Activity:
1. Take about 10 mL of blue copper sulphate (CuSO₄) solution in a beaker.
2. Clean three iron nails with sandpaper and dip them in the copper sulphate solution.
3. Observe after about 15-20 minutes.

Observations:
(a) The blue colour of copper sulphate solution fades gradually and finally becomes greenish (colour of iron sulphate solution).
(b) A reddish-brown coating of copper metal is deposited on the iron nails.

Conclusion: A chemical reaction has taken place. Iron, being more reactive than copper, displaces copper from copper sulphate solution. This is a displacement reaction (also a redox reaction).

Balanced equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Here, Fe is oxidised (Fe⁰ → Fe²⁺) and Cu²⁺ is reduced (Cu²⁺ → Cu⁰).

Q2. Explain the different types of decomposition reactions with suitable examples. Why is decomposition of silver chloride used in photography?

Answer: Decomposition reactions are of three types based on the energy source:

1. Thermal Decomposition (Heat energy):
– Decomposition of a compound using heat
– Example: 2KClO₃ → 2KCl + 3O₂ (Potassium chlorate decomposes to potassium chloride and oxygen)

2. Electrolytic Decomposition (Electrical energy):
– Decomposition of a compound using electricity (electrolysis)
– Example: 2H₂O → 2H₂ + O₂ (Electrolysis of water produces hydrogen and oxygen gases)
– At cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻ (Reduction — hydrogen gas)
– At anode: 2H₂O → O₂ + 4H⁺ + 4e⁻ (Oxidation — oxygen gas)

3. Photochemical Decomposition (Light energy):
– Decomposition of a compound using light energy
– Example: 2AgBr → 2Ag + Br₂ (Silver bromide decomposes to silver and bromine in sunlight)

Why used in photography: Silver chloride (AgCl) and silver bromide (AgBr) are light-sensitive compounds. When exposed to light, they decompose to form metallic silver which appears black on photographic film. The regions where light falls turn dark, creating a negative image. The unexposed AgBr is removed during developing, and the remaining silver particles form the image. This photosensitivity makes silver halides ideal for traditional black-and-white photography.

Q3. What is corrosion? Explain the factors affecting rusting of iron and describe various methods for prevention of corrosion with suitable examples.

Answer:

Corrosion is the slow destruction of metals by the action of air, moisture, acids, or other chemicals on their surface. It is a natural process of oxidation that converts a refined metal into a more stable form like oxide, hydroxide, or sulphide.

Rusting of iron (a specific type of corrosion):
Equation: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (Hydrated iron(III) oxide — Rust)

Factors affecting rusting:

1. Presence of oxygen: Essential for rust formation — iron reacts with oxygen
2. Presence of moisture/water: Rusting accelerates in humid conditions
3. Temperature: Higher temperatures speed up the rusting process
4. Electrolytes: Presence of salts (NaCl) accelerates rusting — explains why iron rusts faster near coastal areas
5. Acidic conditions: Acids in rain (acid rain) increase the rate of corrosion

Prevention methods (with examples):

📋 Previous Year Questions (2018–2026)

❓ Frequently Asked Questions

Q1. How can we identify whether a chemical reaction has taken place?

Signs of a chemical reaction include: change in colour (e.g., green to brown), change in temperature (exothermic/endothermic), evolution of gas (bubbles), formation of precipitate (insoluble solid), and change in state or odour.

Q2. What is the difference between a physical change and a chemical change?

Physical change affects only physical properties (shape, size, state) and no new substance is formed — e.g., ice melting. Chemical change results in the formation of new substances with different chemical properties — e.g., rusting of iron.

Q3. Why do we balance chemical equations?

Chemical equations are balanced to obey the law of conservation of mass, which states that mass can neither be created nor destroyed. Balancing ensures the number of atoms of each element is equal on both sides of the equation.

Q4. What is the colour of copper sulphate solution? What happens when an iron nail is placed in it?

Copper sulphate solution is blue in colour. When an iron nail is placed in it, the blue colour fades to green (formation of iron sulphate), and a reddish-brown coating of copper is deposited on the iron nail.

Q5. What are the conditions required for rusting of iron?

Two conditions are essential: presence of oxygen (air) and presence of moisture (water). Iron rusts faster in coastal areas due to the presence of salt in air, which acts as an electrolyte and speeds up the electrochemical process.

Q6. What is the difference between oxidation and reduction?

Oxidation involves gain of oxygen or loss of hydrogen, while reduction involves loss of oxygen or gain of hydrogen. In terms of electrons: Oxidation is loss (OIL), Reduction is gain (RIG) of electrons.

Q7. Why is hydrogen peroxide stored in dark bottles?

Hydrogen peroxide (H₂O₂) undergoes photochemical decomposition in the presence of light to form water and oxygen. Dark bottles protect it from light, preventing its decomposition and extending its shelf life.

Q8. What is the difference between a displacement and a double displacement reaction?

In a displacement reaction, a more reactive element displaces a less reactive element from its compound (A + BC → AC + B). In a double displacement reaction, two compounds exchange their ions (AB + CD → AD + CB), often forming a precipitate.

Q9. Why do silver articles turn black and gold articles do not corrode?

Silver reacts with hydrogen sulphide in air to form black silver sulphide. Gold is a noble metal (least reactive) and does not react with air, moisture, or common chemicals, making it corrosion-resistant. This is why gold remains shiny forever.

Q10. What is an exothermic reaction? Give examples from daily life.

An exothermic reaction releases heat energy. Examples: burning of fuels (LPG, wood), respiration (glucose + O₂ → CO₂ + H₂O + energy), neutralisation reactions (acid + base → salt + water + heat), and dilution of strong acids.

Q11. What happens when calcium carbonate is heated? Write the reaction.

When calcium carbonate (CaCO₃, limestone) is heated strongly, it undergoes thermal decomposition to form calcium oxide (CaO, quicklime) and carbon dioxide (CO₂). The reaction is: CaCO₃(s) → CaO(s) + CO₂(g). This is used in the manufacture of cement and lime.

Q12. What is the role of an oxidising agent and a reducing agent in a redox reaction?

An oxidising agent causes oxidation of another substance while itself getting reduced (gains electrons). A reducing agent causes reduction of another substance while itself getting oxidised (loses electrons). Example: In CuO + H₂ → Cu + H₂O, CuO is the oxidising agent and H₂ is the reducing agent.

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