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Chemical Reactions and Equations – CBSE NCERT Study Resources

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Chemical Reactions and Equations

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10th

10th - Science

Chemical Reactions and Equations

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Chapter Overview: Chemical Reactions and Equations

This chapter introduces students to the fundamental concepts of chemical reactions and their representation through chemical equations. It covers various types of reactions, balancing equations, and the effects of chemical changes in everyday life.

Chemical Reaction: A process in which one or more substances (reactants) are transformed into one or more different substances (products).

1. Chemical Reactions

A chemical reaction involves the breaking and forming of bonds between atoms to create new substances. Key characteristics include:

  • Change in state, color, or temperature
  • Evolution of gas
  • Formation of a precipitate

2. Chemical Equations

A chemical equation represents a reaction using symbols and formulas of reactants and products.

Balanced Chemical Equation: An equation where the number of atoms of each element is equal on both sides.

Example:
2H2 + O2 → 2H2O

3. Types of Chemical Reactions

Major types include:

  • Combination Reaction: Two or more substances combine to form a single product.
  • Decomposition Reaction: A single compound breaks down into simpler substances.
  • Displacement Reaction: A more reactive element displaces a less reactive one.
  • Double Displacement Reaction: Exchange of ions between two reactants.
  • Oxidation-Reduction (Redox) Reaction: Reactions involving gain/loss of oxygen or electrons.

4. Effects of Chemical Reactions

Chemical reactions impact daily life through:

  • Corrosion (e.g., rusting of iron)
  • Rancidity (spoilage of food due to oxidation)

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
What is a chemical reaction?
Answer:

A process where substances transform into new substances.

Question 2:
Name the gas evolved when zinc reacts with dilute sulfuric acid.
Answer:

Hydrogen gas (H2).

Question 3:
What is the balanced equation for burning magnesium in air?
Answer:

2Mg + O2 → 2MgO.

Question 4:
Which indicator turns pink in a basic solution?
Answer:

Phenolphthalein.

Question 5:
What type of reaction is digestion of food?
Answer:

Decomposition reaction.

Question 6:
Name the white precipitate formed when CO2 is passed through lime water.
Answer:

Calcium carbonate (CaCO3).

Question 7:
What is the product when iron reacts with copper sulfate?
Answer:

Iron sulfate + Copper (FeSO4 + Cu).

Question 8:
Which gas is released during photosynthesis?
Answer:

Oxygen (O2).

Question 9:
What is the oxidation reaction in rusting of iron?
Answer:

Iron reacts with oxygen to form iron oxide.

Question 10:
Name the reaction: 2H2O → 2H2 + O2.
Answer:

Electrolysis of water.

Question 11:
What is the precipitate formed when silver nitrate reacts with sodium chloride?
Answer:

Silver chloride (AgCl).

Question 12:
Which metal displaces copper from copper sulfate solution?
Answer:

Zinc (Zn).

Question 13:
What is the common name of CaO?
Answer:

Quicklime.

Question 14:
Name the reaction: CH4 + 2O2 → CO2 + 2H2O.
Answer:

Combustion of methane.

Question 15:
Define combination reaction with an example.
Answer:

An combination reaction occurs when two or more substances combine to form a single product.
Example: 2H2 + O2 → 2H2O (Formation of water).

Question 16:
Why is respiration considered an exothermic reaction?
Answer:

Respiration releases energy in the form of heat and ATP by breaking down glucose.
Example: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy.

Question 17:
What happens when copper reacts with moist air?
Answer:

Copper forms a green layer of basic copper carbonate (CuCO3.Cu(OH)2) due to corrosion.
Reaction: 2Cu + H2O + CO2 + O2 → CuCO3.Cu(OH)2.

Question 18:
Give an example of a displacement reaction.
Answer:

In a displacement reaction, a more reactive element replaces a less reactive one.
Example: Zn + CuSO4 → ZnSO4 + Cu (Zinc displaces copper).

Question 19:
What is the role of catalyst in a chemical reaction?
Answer:

A catalyst speeds up a reaction without being consumed.
Example: MnO2 in the decomposition of KClO3 to produce oxygen.

Question 20:
Why is hydrogen peroxide stored in colored bottles?
Answer:

Hydrogen peroxide decomposes in light, so colored bottles prevent exposure to sunlight, slowing the reaction:
2H2O2 → 2H2O + O2.

Question 21:
What is the chemical name of rust?
Answer:

Rust is chemically known as hydrated iron(III) oxide (Fe2O3.xH2O).

Question 22:
Balance the equation: Fe + H2O → Fe3O4 + H2.
Answer:

Balanced equation:
3Fe + 4H2O → Fe3O4 + 4H2.

Question 23:
What gas is evolved when zinc reacts with dilute hydrochloric acid?
Answer:

Hydrogen gas (H2) is evolved.
Reaction: Zn + 2HCl → ZnCl2 + H2↑.

Question 24:
Why is the reaction between quicklime and water called slaking of lime?
Answer:

It releases a large amount of heat and forms slaked lime (Ca(OH)2).
Reaction: CaO + H2O → Ca(OH)2 + Heat.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Define a chemical reaction and give one example.
Answer:

A chemical reaction is a process where substances (reactants) transform into new substances (products) with different properties.
Example: Burning of magnesium ribbon in air to form magnesium oxide:
2Mg + O2 → 2MgO

Question 2:
What is a balanced chemical equation? Why is it important?
Answer:

A balanced chemical equation has equal numbers of each type of atom on both sides of the reaction.
It is important because it follows the Law of Conservation of Mass, ensuring no atoms are created or destroyed during the reaction.

Question 3:
Name the gas evolved when zinc reacts with dilute hydrochloric acid. Write the balanced equation.
Answer:

Hydrogen gas (H2) is evolved.
Balanced equation:
Zn + 2HCl → ZnCl2 + H2

Question 4:
What is the difference between combination and decomposition reactions? Give one example of each.
Answer:

Combination reactions involve two or more substances combining to form one product (e.g., 2H2 + O2 → 2H2O).
Decomposition reactions involve one compound breaking into simpler substances (e.g., 2HgO → 2Hg + O2).

Question 5:
What happens when silver chloride is exposed to sunlight? Write the reaction.
Answer:

Silver chloride decomposes into silver and chlorine gas when exposed to sunlight.
Reaction:
2AgCl (s) → 2Ag (s) + Cl2 (g)

Question 6:
What is a precipitation reaction? Give an example.
Answer:

A precipitation reaction occurs when two solutions react to form an insoluble solid (precipitate).
Example:
AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

Question 7:
Why is respiration considered an exothermic reaction?
Answer:

Respiration releases energy in the form of heat and ATP by breaking down glucose.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Question 8:
What is the role of a catalyst in a chemical reaction?
Answer:

A catalyst speeds up a reaction without being consumed.
Example: Manganese dioxide (MnO2) in the decomposition of hydrogen peroxide.

Question 9:
Write the chemical equation for the reaction between sodium hydroxide and hydrochloric acid. What type of reaction is this?
Answer:

NaOH + HCl → NaCl + H2O
This is a neutralization reaction (acid + base → salt + water).

Question 10:
What is corrosion? Give an example.
Answer:

Corrosion is the gradual destruction of metals by chemical reactions (e.g., rusting of iron).
Example: 4Fe + 3O2 + 6H2O → 4Fe(OH)3

Question 11:
Why is the reaction between quicklime and water called exothermic?
Answer:

Quicklime (CaO) reacts vigorously with water to form slaked lime (Ca(OH)2), releasing heat.
Reaction: CaO + H2O → Ca(OH)2 + Heat

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Define a chemical reaction and list two observable changes that indicate its occurrence.
Answer:

A chemical reaction is a process where reactants transform into products with new properties.
Two observable changes are:

  • Formation of a precipitate (solid particles in a solution)
  • Evolution of gas (bubbles observed)

Question 2:
Explain why combustion reactions are considered exothermic with an example.
Answer:

Combustion reactions release heat energy, making them exothermic.
Example: Burning of methane:
CH4 + 2O2 → CO2 + 2H2O + Heat
The reaction releases energy as light and heat, confirming its exothermic nature.

Question 3:
Differentiate between displacement and double displacement reactions with one example each.
Answer:

  • Displacement reaction: A more reactive element replaces a less reactive one.
    Example: Zn + CuSO4 → ZnSO4 + Cu
  • Double displacement reaction: Ions exchange between two compounds.
    Example: AgNO3 + NaCl → AgCl + NaNO3

Question 4:
Describe the corrosion process of iron and name one method to prevent it.
Answer:

Corrosion of iron (rusting) occurs when iron reacts with oxygen and water:
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Prevention method: Galvanization (coating iron with zinc).

Question 5:
Balance the chemical equation: Fe + H2O → Fe3O4 + H2
Answer:

Balanced equation:
3Fe + 4H2O → Fe3O4 + 4H2
Steps:
1. Balance Fe atoms (3 on left).
2. Balance O atoms (4 H2O).
3. Balance H atoms (4 H2).

Question 6:
What is a decomposition reaction? Give an example with a balanced equation.
Answer:

A decomposition reaction breaks a compound into simpler substances.
Example: Thermal decomposition of calcium carbonate:
CaCO3 → CaO + CO2
Heat energy splits CaCO3 into calcium oxide and carbon dioxide.

Question 7:
What is a chemical equation? Explain with an example.
Answer:

A chemical equation is a symbolic representation of a chemical reaction using formulas and symbols. It shows the reactants on the left and the products on the right, separated by an arrow (→).


Example:
2H2 + O2 → 2H2O
Here, hydrogen (H2) and oxygen (O2) are reactants, and water (H2O) is the product.
Question 8:
Why is it necessary to balance a chemical equation?
Answer:

Balancing a chemical equation ensures the Law of Conservation of Mass is followed, which states that matter cannot be created or destroyed.

Steps to balance:
1. Count atoms of each element on both sides.
2. Adjust coefficients to balance atoms.
3. Ensure the simplest whole-number ratio.


Example: 2Mg + O2 → 2MgO (balanced equation).
Question 9:
Differentiate between combination and decomposition reactions with examples.
Answer:

Combination Reaction: Two or more substances combine to form a single product.
Example: C + O2 → CO2

Decomposition Reaction: A single compound breaks down into simpler substances.
Example: 2H2O → 2H2 + O2 (electrolysis of water).

Question 10:
What happens when zinc reacts with dilute sulphuric acid? Write the balanced equation.
Answer:

Zinc reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas.

Balanced equation:
Zn + H2SO4 → ZnSO4 + H2

Observation: Bubbles of hydrogen gas are released, and the solution becomes colorless (zinc sulphate solution).

Question 11:
Explain the term corrosion with an example. How can it be prevented?
Answer:

Corrosion is the gradual destruction of metals due to chemical reactions with environmental substances like oxygen and moisture.

Example: Iron rusting (4Fe + 3O2 + 6H2O → 4Fe(OH)3).

Prevention methods:
1. Galvanization (coating with zinc).
2. Painting or oiling.
3. Using anti-rust solutions.

Question 12:
What is a precipitation reaction? Give an example with a balanced equation.
Answer:

A precipitation reaction occurs when two soluble salts react to form an insoluble solid (precipitate) and a soluble salt.

Example:
AgNO3 + NaCl → AgCl↓ + NaNO3
Here, silver chloride (AgCl) is the white precipitate formed.

Question 13:
Explain why combustion reactions are always exothermic with an example.
Answer:

Combustion reactions involve burning a substance in oxygen, releasing heat and light energy, making them exothermic.

Example: Burning methane (CH4):
CH4 + 2O2 → CO2 + 2H2O + Heat.
The energy released exceeds the energy absorbed, resulting in a net heat release.

Question 14:
Differentiate between displacement and double displacement reactions with one example each.
Answer:

  • Displacement Reaction: A more reactive element replaces a less reactive one.
    Example: Zn + CuSO4 → ZnSO4 + Cu.
  • Double Displacement Reaction: Ions exchange between two compounds.
    Example: AgNO3 + NaCl → AgCl + NaNO3.

Question 15:
What is a balanced chemical equation? Why is it essential to balance equations?
Answer:

A balanced chemical equation has equal numbers of each atom on both sides, obeying the Law of Conservation of Mass.

Balancing is essential because:

  • It ensures mass is neither created nor destroyed.
  • Provides correct stoichiometric ratios for reactions.

Question 16:
Describe the corrosion of iron and a method to prevent it.
Answer:

Corrosion of iron (rusting) occurs when iron reacts with oxygen and water:
4Fe + 3O2 + 6H2O → 4Fe(OH)3.

Prevention method: Galvanization (coating iron with zinc) to block contact with air and moisture.

Question 17:
How does a decomposition reaction differ from a combination reaction? Give one example of each.
Answer:

  • Decomposition Reaction: A single compound breaks into simpler substances.
    Example: 2H2O → 2H2 + O2.
  • Combination Reaction: Two or more substances combine to form one product.
    Example: C + O2 → CO2.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain combination reaction with an example from NCERT and a real-world application.
Answer:
Concept Overview

A combination reaction occurs when two or more substances combine to form a single product.

Process Explanation

Our textbook shows the reaction between calcium oxide (CaO) and water (H2O) forming calcium hydroxide (Ca(OH)2).

Real-world Application

In industries, combination reactions are used to produce slaked lime for whitewashing walls.

Question 2:
Describe decomposition reaction using electrolysis of water as an NCERT example and its importance.
Answer:
Concept Overview

Decomposition reactions involve breaking down a compound into simpler substances.

Process Explanation

We studied the electrolysis of water (H2O) into hydrogen (H2) and oxygen (O2) using electricity.

Real-world Application

This reaction is crucial in producing hydrogen fuel for clean energy solutions.

Question 3:
What is a displacement reaction? Illustrate with NCERT example of iron and copper sulphate and its industrial use.
Answer:
Concept Overview

Displacement reactions occur when a more reactive element replaces a less reactive one in a compound.

Process Explanation

Our textbook shows iron (Fe) displacing copper (Cu) from copper sulphate (CuSO4) solution.

Real-world Application

This principle is used in metallurgy to extract metals from their ores.

Question 4:
Explain double displacement reaction with NCERT example of sodium sulphate and barium chloride. Mention its laboratory use.
Answer:
Concept Overview

Double displacement reactions involve exchange of ions between two compounds.

Process Explanation

We studied the reaction between sodium sulphate (Na2SO4) and barium chloride (BaCl2) forming white precipitate of barium sulphate.

Real-world Application

This reaction is used in laboratories to test for sulphate ions.

Question 5:
Define redox reaction using NCERT example of copper oxide and hydrogen. Explain its significance in metal extraction.
Answer:
Concept Overview

Redox reactions involve both oxidation and reduction occurring simultaneously.

Process Explanation

Our textbook shows hydrogen (H2) reducing copper oxide (CuO) to copper (Cu) while getting oxidized to water.

Real-world Application

Such reactions are fundamental in extracting metals from their oxides in industries.

Question 6:
What is corrosion? Explain with NCERT example of iron rusting and methods to prevent it.
Answer:
Concept Overview

Corrosion is the gradual destruction of metals by chemical reactions with their environment.

Process Explanation

We studied how iron (Fe) reacts with oxygen (O2) and moisture to form rust (Fe2O3.xH2O).

Real-world Application

Prevention methods include galvanization and painting, widely used in protecting iron structures.

Question 7:
Explain the process of corrosion with an example. How can it be prevented? Discuss its economic impact.
Answer:

Corrosion is the gradual destruction of metals by chemical or electrochemical reactions with their environment, such as oxygen and moisture. A common example is the rusting of iron, where iron reacts with oxygen and water to form hydrated iron(III) oxide (rust). The chemical equation for this reaction is:


4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s)

Over time, this weakens the metal, leading to structural damage.


Prevention methods include:

  • Galvanization: Coating iron with a layer of zinc to prevent contact with air and moisture.
  • Painting/Oiling: Creating a barrier to shield the metal from environmental factors.
  • Alloying: Mixing iron with other metals (e.g., chromium in stainless steel) to enhance resistance.

Economic impact: Corrosion leads to huge financial losses due to the replacement of damaged structures (e.g., bridges, vehicles) and maintenance costs. Preventive measures, though initially expensive, save money in the long run by increasing the lifespan of metal objects.

Question 8:
What is a displacement reaction? Give one example with a balanced chemical equation. How is it different from a double displacement reaction?
Answer:

A displacement reaction occurs when a more reactive element replaces a less reactive one in a compound. For example, zinc displaces copper from copper sulfate solution:
Zn + CuSO4 → ZnSO4 + Cu.

In contrast, a double displacement reaction involves the exchange of ions between two compounds, forming new products. For example:
AgNO3 + NaCl → AgCl + NaNO3.

Key differences:

  • Displacement: One element replaces another in a compound.
  • Double displacement: Two compounds exchange ions.

Question 9:
Describe the thermal decomposition of lead nitrate with a balanced equation. What observations indicate the reaction? Why is this reaction important industrially?
Answer:

The thermal decomposition of lead nitrate breaks it into simpler substances on heating:
2Pb(NO3)2 → 2PbO + 4NO2 + O2.

Observations:

  • Brown fumes of nitrogen dioxide (NO2).
  • Yellow residue of lead oxide (PbO).
  • Gas bubbles due to oxygen (O2).

Industrial importance:

  • NO2 is used in nitric acid production.
  • PbO is used in glass and ceramics.
  • O2 supports combustion.

Question 10:
Explain exothermic and endothermic reactions with examples. How is the energy change represented in their equations?
Answer:

An exothermic reaction releases heat energy (e.g., combustion of methane):
CH4 + 2O2 → CO2 + 2H2O + heat.

An endothermic reaction absorbs heat energy (e.g., photosynthesis):
6CO2 + 6H2O + sunlight → C6H12O6 + 6O2.

Energy representation:

  • Exothermic: Heat is written as a product (right side).
  • Endothermic: Heat/energy is written as a reactant (left side).

Question 11:
Explain the process of corrosion with an example. How can it be prevented? Discuss the economic impact of corrosion.
Answer:

Corrosion is the gradual destruction of metals by chemical or electrochemical reactions with their environment, such as oxygen and moisture. A common example is the rusting of iron, where iron reacts with oxygen and water to form hydrated iron(III) oxide (rust). The chemical equation for this reaction is:
4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s)
This reddish-brown compound weakens the metal structure.

Prevention methods include:

  • Galvanization: Coating iron with zinc to prevent contact with air and water.
  • Painting/Oiling: Creating a barrier to moisture and oxygen.
  • Alloying: Mixing iron with other metals like chromium to make stainless steel.

Economic impact: Corrosion leads to huge financial losses due to the replacement of damaged structures (bridges, vehicles, etc.) and maintenance costs. It also affects safety, as weakened structures can collapse.

Question 12:
Describe a displacement reaction with a suitable example. How is it different from a double displacement reaction? Provide real-life applications of both.
Answer:

A displacement reaction occurs when a more reactive element replaces a less reactive one in a compound. For example, when zinc reacts with copper sulfate, zinc displaces copper:
Zn + CuSO4 → ZnSO4 + Cu.
Here, zinc is more reactive than copper, so it takes copper's place.

In contrast, a double displacement reaction involves the exchange of ions between two compounds. For example:
AgNO3 + NaCl → AgCl + NaNO3.
Here, silver and sodium swap their partners.

Differences:

  • Displacement: One element replaces another in a compound.
  • Double Displacement: Two compounds exchange ions.

Applications:

  • Displacement: Used in metallurgy to extract metals (e.g., iron from iron oxide using carbon).
  • Double Displacement: Used in water treatment (e.g., precipitation of impurities) and soap-making (saponification).

Question 13:
Explain the process of corrosion with an example. Describe how it is a redox reaction and suggest one method to prevent it. (5 marks)
Answer:

Corrosion is the gradual deterioration of metals due to their reaction with environmental substances like oxygen, moisture, or acids. A common example is the rusting of iron, where iron reacts with oxygen and water to form hydrated iron(III) oxide (rust). The chemical equation for this process is:


4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s)

This is a redox reaction because:

  • Oxidation: Iron loses electrons (Fe → Fe3+ + 3e-).
  • Reduction: Oxygen gains electrons (O2 + 4e- → 2O2-).

To prevent corrosion, one effective method is galvanization, where iron is coated with a layer of zinc. Zinc acts as a sacrificial anode, corroding preferentially and protecting the iron underneath.

Question 14:
Explain the process of corrosion with an example of iron. Discuss its impact and suggest two methods to prevent it, giving reasons for each method.
Answer:

Corrosion is the gradual destruction of metals by chemical or electrochemical reactions with their environment. In the case of iron, it reacts with oxygen and moisture to form hydrated iron(III) oxide (rust). The chemical equation is:


4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s)

This reddish-brown flaky substance weakens the iron structure, leading to economic losses and safety hazards.


Prevention Methods:

  • Galvanization: Coating iron with a layer of zinc prevents exposure to moisture and oxygen, as zinc is more reactive and corrodes first.
  • Painting/Oiling: Forms a protective barrier that blocks air and water from reaching the iron surface.

Both methods are cost-effective and widely used in industries and daily life.

Question 15:
Describe a displacement reaction with an example. Write its balanced chemical equation and explain why it is considered a redox reaction.
Answer:

A displacement reaction occurs when a more reactive element displaces a less reactive one from its compound. For example, when zinc reacts with copper sulfate, zinc replaces copper:


Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

This is a redox reaction because:

  • Oxidation: Zinc loses electrons (Zn → Zn2+ + 2e-).
  • Reduction: Copper gains electrons (Cu2+ + 2e- → Cu).

The simultaneous transfer of electrons makes it a redox reaction. Such reactions are crucial in metallurgy and battery operations.

Question 16:
Describe a displacement reaction with a suitable example. How is it different from a double displacement reaction? Provide one real-life application of each.
Answer:

A displacement reaction occurs when a more reactive element replaces a less reactive one in a compound. For example, when zinc reacts with copper sulfate, zinc displaces copper:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Here, zinc is more reactive than copper, so it takes copper's place.

Difference from double displacement: In a double displacement reaction, two compounds exchange ions to form new compounds, e.g.,
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
No change in reactivity is involved here—just an exchange.

Applications:

  • Displacement: Extraction of metals (e.g., iron from iron oxide using carbon).
  • Double displacement: Precipitation reactions in water treatment to remove impurities.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A student observes that when iron nails are left in a copper sulfate solution, the solution turns green and a brown coating forms on the nails. Explain the reaction and identify the type of reaction.
Answer:
Case Summary

Iron nails react with copper sulfate, forming green iron sulfate and brown copper coating.

Scientific Principle
  • This is a displacement reaction (Fe + CuSO4 → FeSO4 + Cu).
  • Iron displaces copper as it is more reactive.
Solution Approach

Our textbook shows similar reactions like zinc displacing copper. Real-world example: Galvanization uses such reactions to prevent rust.

Question 2:
A baker adds yeast to dough, causing it to rise. Identify the gas released and the type of reaction. How is this reaction useful?
Answer:
Case Summary

Yeast ferments sugar in dough, releasing gas and making it rise.

Scientific Principle
  • This is a fermentation reaction (C6H12O6 → 2C2H5OH + 2CO2).
  • CO2 gas causes dough expansion.
Solution Approach

NCERT mentions fermentation in food production. Real-world use: Baking and brewing industries rely on this.

Question 3:
When quicklime (CaO) is added to water, it releases heat and forms slaked lime. Write the balanced equation and classify the reaction.
Answer:
Case Summary

Quicklime reacts with water, producing slaked lime and heat.

Scientific Principle
  • Balanced equation: CaO + H2O → Ca(OH)2 + heat.
  • This is an exothermic combination reaction.
Solution Approach

Our textbook shows similar reactions like sulfur dioxide with water. Real-world use: Whitewashing walls uses this reaction.

Question 4:
A silver article turns black when exposed to air. Name the reaction and the black substance formed. How can it be reversed?
Answer:
Case Summary

Silver reacts with air, forming a black layer on its surface.

Scientific Principle
  • This is a corrosion reaction (2Ag + H2S → Ag2S + H2).
  • Black substance is silver sulfide (Ag2S).
Solution Approach

NCERT mentions tarnishing of silver. Real-world solution: Polishing or using aluminum foil with baking soda reverses it.

Question 5:
A student observes that when iron nails are left in a copper sulfate solution, the solution turns green and a brown coating forms on the nails. Explain the reaction and identify the type of reaction.
Answer:
Case Summary

Iron nails react with copper sulfate, forming iron sulfate and copper.


Scientific Principle
  • This is a displacement reaction (Fe + CuSO4 → FeSO4 + Cu).
  • Iron displaces copper as it is more reactive.

Solution Approach

Our textbook shows similar reactions, like zinc displacing copper. The brown coating is copper, and the green color is due to iron sulfate.

Question 6:
A baker adds yeast to dough, causing it to rise. Identify the gas released and the type of reaction. How is this similar to NCERT’s respiration example?
Answer:
Case Summary

Yeast ferments sugar in dough, releasing gas.


Scientific Principle
  • This is a decomposition reaction (C6H12O6 → 2C2H5OH + 2CO2).
  • CO2 gas causes dough to rise.

Solution Approach

Our textbook shows respiration (glucose → CO2 + energy), a similar breakdown process.

Question 7:
When magnesium ribbon burns in air, it forms a white powder. Write the balanced equation and explain why it’s a combination reaction. Compare it to NCERT’s rusting example.
Answer:
Case Summary

Magnesium reacts with oxygen to form magnesium oxide.


Scientific Principle
  • Balanced equation: 2Mg + O2 → 2MgO.
  • Two substances combine, so it’s a combination reaction.

Solution Approach

Like rusting (Fe + O2 → Fe2O3), both involve oxygen combining with a metal.

Question 8:
A student mixes silver nitrate and sodium chloride solutions, forming a white precipitate. Name the precipitate and the reaction type. How is this used in black-and-white photography?
Answer:
Case Summary

Silver nitrate and sodium chloride react to form silver chloride.


Scientific Principle
  • Reaction: AgNO3 + NaCl → AgCl + NaNO3.
  • It’s a double displacement reaction.

Solution Approach

Our textbook shows AgCl is light-sensitive, used in photographic films to capture images.

Question 9:
A student observes that when iron nails are left in a copper sulfate solution, the solution turns green and a brown coating forms on the nails. Explain this using a chemical equation and identify the type of reaction.
Answer:
Case Summary

Iron nails react with copper sulfate, forming iron sulfate and copper.

Scientific Principle

This is a displacement reaction where iron displaces copper from its compound.

Solution Approach
  • Equation: Fe + CuSO4 → FeSO4 + Cu
  • Type: Single displacement (NCERT Example: Reaction of zinc with copper sulfate).
  • Real-world: Galvanization uses similar principles.
Question 10:
A baker adds yeast to dough, causing it to rise. Identify the gas released and the type of reaction. How is this similar to the respiration process?
Answer:
Case Summary

Yeast fermentation produces gas, making dough rise.

Scientific Principle

It’s a decomposition reaction where sugar breaks down into simpler compounds.

Solution Approach
  • Gas: Carbon dioxide (CO2).
  • Reaction: C6H12O6 → 2C2H5OH + 2CO2 (NCERT Example: Respiration in cells).
  • Similarity: Both release CO2 and energy.
Question 11:
When magnesium ribbon burns in air, it forms a white powder. Write the balanced equation and explain why it’s a combination reaction. Compare it with burning coal.
Answer:
Case Summary

Magnesium reacts with oxygen to form magnesium oxide.

Scientific Principle

Two substances combine to form one product (combination reaction).

Solution Approach
  • Equation: 2Mg + O2 → 2MgO (NCERT Example: Burning of sulfur).
  • Coal burning: C + O2 → CO2 (similar combination).
  • Both release heat and light.
Question 12:
A farmer treats soil with slaked lime to reduce acidity. Name the reaction and write its equation. How does this relate to neutralization in daily life?
Answer:
Case Summary

Slaked lime (Ca(OH)2) neutralizes acidic soil.

Scientific Principle

It’s an acid-base reaction forming water and salt.

Solution Approach
  • Equation: Ca(OH)2 + H2SO4 → CaSO4 + 2H2O (NCERT Example: Antacid tablets).
  • Daily use: Treating bee stings with baking soda.
Question 13:

During a science experiment, Riya observed that when a piece of zinc is added to dilute hydrochloric acid, bubbles of a gas are formed. The gas burns with a pop sound when a burning matchstick is brought near it.

(a) Identify the gas evolved during the reaction.

(b) Write the balanced chemical equation for the reaction.

(c) Why does the gas burn with a pop sound?

(d) What type of reaction is this?

Answer:

(a) The gas evolved is hydrogen (H2).

(b) The balanced chemical equation is:
Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g)

(c) The gas burns with a pop sound because hydrogen is highly flammable and reacts explosively with oxygen in the air, producing a small explosion.

(d) This is a displacement reaction (single displacement) as zinc displaces hydrogen from hydrochloric acid.

Question 14:

A student mixed a solution of silver nitrate (AgNO3) with a solution of sodium chloride (NaCl) and observed the formation of a white precipitate.

(a) Identify the white precipitate formed.

(b) Write the balanced chemical equation for the reaction.

(c) What type of reaction is this? Give one more example of this type of reaction.

(d) Why is this reaction important in daily life?

Answer:

(a) The white precipitate is silver chloride (AgCl).

(b) The balanced chemical equation is:
AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

(c) This is a double displacement reaction (also called precipitation reaction).
Another example: BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)

(d) This reaction is important in daily life for water purification and photography, as silver chloride is used in black-and-white photographic films.

Question 15:
A student observed that when a piece of zinc is added to a solution of copper sulfate, the blue color of the solution fades, and a reddish-brown deposit forms on the zinc.

Explain the type of reaction occurring here and write the balanced chemical equation for the same. Also, identify the oxidizing agent and reducing agent in this reaction.

Answer:

This is an example of a displacement reaction, where a more reactive metal (zinc) displaces a less reactive metal (copper) from its compound (copper sulfate).


The balanced chemical equation is:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

In this reaction:

  • Oxidizing agent: Copper sulfate (CuSO4), as it gains electrons (reduced) to form copper.
  • Reducing agent: Zinc (Zn), as it loses electrons (oxidized) to form zinc sulfate.

The reddish-brown deposit is copper, which confirms the displacement of copper by zinc.

Question 16:
A teacher demonstrated the reaction between sodium hydroxide and hydrochloric acid in the lab. The students observed that the temperature of the mixture increased, and a neutral solution was formed.

Identify the type of reaction, write its balanced chemical equation, and explain why this reaction is considered exothermic. Also, suggest a practical application of this reaction.

Answer:

This is an example of a neutralization reaction, where an acid (hydrochloric acid) reacts with a base (sodium hydroxide) to form salt and water.


The balanced chemical equation is:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)

This reaction is exothermic because heat is released during the process, causing the temperature of the mixture to rise.

A practical application of this reaction is in antacid tablets, where bases like magnesium hydroxide neutralize excess stomach acid (hydrochloric acid) to relieve acidity.

Question 17:
A student observed that when a piece of zinc is added to a solution of copper sulphate, the blue color of the solution fades and a reddish-brown deposit forms on the zinc.

Based on this observation, answer the following:

  • Identify the type of reaction.
  • Write the balanced chemical equation for the reaction.
  • Explain why the blue color fades.
  • What is the reddish-brown deposit?
Answer:

The reaction observed is a displacement reaction, where a more reactive metal (zinc) displaces a less reactive metal (copper) from its compound.


The balanced chemical equation is:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

The blue color fades because copper sulphate (blue) is being converted to zinc sulphate (colorless) as the reaction progresses.


The reddish-brown deposit is copper metal, which forms as a solid precipitate during the reaction.
Question 18:
During a laboratory experiment, a student heated ferrous sulphate crystals in a test tube. She noticed a color change from green to brown and a pungent smell.

Based on this observation, answer the following:

  • What type of reaction is this?
  • Write the balanced chemical equation for the reaction.
  • Why does the color change occur?
  • Identify the gas responsible for the pungent smell.
Answer:

This is a decomposition reaction, where ferrous sulphate breaks down into simpler substances on heating.


The balanced chemical equation is:
2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)

The color changes from green to brown because ferrous sulphate (green) decomposes to form ferric oxide (brown).


The pungent smell is due to the release of sulphur dioxide (SO2) gas, which is a byproduct of the decomposition.
Question 19:
A student observed that when a piece of zinc is added to a solution of copper sulphate, the blue color of the solution fades gradually, and a reddish-brown deposit forms at the bottom of the beaker.

Based on this observation, answer the following:

  • Identify the type of reaction.
  • Write the balanced chemical equation for the reaction.
  • Explain why the blue color fades.
Answer:

The reaction observed is a displacement reaction, where more reactive zinc displaces less reactive copper from its compound.


The balanced chemical equation is:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)


The blue color fades because copper sulphate solution (blue) is converted to zinc sulphate solution (colorless) as the reaction progresses. The reddish-brown deposit is copper metal formed due to the displacement.

Question 20:
During a laboratory experiment, a student heated lead nitrate in a test tube and observed the formation of a yellow residue and a gas that turned moist litmus paper red.

Based on this, answer:

  • Name the type of reaction.
  • Write the balanced chemical equation.
  • Why does the gas turn litmus paper red?
Answer:

The reaction is a decomposition reaction, where lead nitrate breaks down into simpler substances on heating.


The balanced chemical equation is:
2Pb(NO3)2 (s) → 2PbO (s) + 4NO2 (g) + O2 (g)


The yellow residue is lead oxide (PbO), and the gas evolved is nitrogen dioxide (NO2), which is acidic. It turns moist litmus paper red because NO2 dissolves in water to form nitric acid, which is acidic in nature.

Question 21:
A student observed that when a piece of zinc is added to a solution of copper sulphate, the blue color of the solution fades and a reddish-brown deposit forms. Identify the type of reaction and write the balanced chemical equation. Explain why the color change occurs.
Answer:

The reaction observed is a displacement reaction, where more reactive zinc displaces less reactive copper from its compound.


Balanced Chemical Equation:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)


Explanation of Color Change:
The blue color of copper sulphate solution is due to the presence of Cu2+ ions. When zinc is added, it displaces copper ions, forming colorless zinc sulphate and reddish-brown copper metal. Hence, the blue color fades.

Question 22:
A farmer noticed that his iron tools developed a reddish-brown coating when left exposed to moisture. Identify the chemical process and write the reaction involved. Suggest two methods to prevent this process.
Answer:

The reddish-brown coating is due to rusting, a chemical process where iron reacts with oxygen and water to form hydrated iron(III) oxide.


Chemical Reaction:
4Fe (s) + 3O2 (g) + xH2O (l) → 2Fe2O3.xH2O (s)


Prevention Methods:

  • Galvanization: Coating iron with a layer of zinc to prevent contact with moisture and oxygen.
  • Painting/Oiling: Applying a protective layer to block exposure to air and water.

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