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Combustion and Flame – CBSE NCERT Study Resources

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Combustion and Flame

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8th

8th - Science

Combustion and Flame

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Overview of the Chapter: Combustion and Flame

This chapter explores the process of combustion, the types of flames produced during combustion, and the importance of fire safety. Students will learn about the conditions necessary for combustion, different types of combustion, and the structure of a flame. The chapter also discusses fuels, their efficiency, and environmental impacts.

Combustion: A chemical process in which a substance reacts with oxygen to give off heat and light.

Conditions Necessary for Combustion

For combustion to occur, three essential conditions must be met:

  • Presence of a combustible substance (fuel)
  • Presence of a supporter of combustion (usually oxygen)
  • Heating the fuel to its ignition temperature

Types of Combustion

Combustion can be classified into different types based on the rate and nature of the process:

  • Rapid Combustion: Burns quickly with the release of heat and light (e.g., burning of LPG).
  • Spontaneous Combustion: Occurs without an external heat source (e.g., burning of white phosphorus).
  • Explosion: A sudden reaction with the release of heat, light, and sound (e.g., fireworks).

Flame and Its Structure

A flame is the visible, gaseous part of a fire. It has distinct zones:

  • Innermost Zone (Dark): Unburnt vapors of the fuel.
  • Middle Zone (Yellow): Partial combustion with luminous flame.
  • Outermost Zone (Blue): Complete combustion with the highest temperature.

Ignition Temperature: The lowest temperature at which a substance catches fire.

Fuels and Their Efficiency

Fuels are substances that produce heat energy on combustion. The efficiency of a fuel is measured by its calorific value (heat produced per unit mass). Fuels can be:

  • Solid Fuels: Wood, coal
  • Liquid Fuels: Petrol, diesel
  • Gaseous Fuels: CNG, LPG

Environmental Impact of Burning Fuels

Burning fuels releases harmful products like carbon dioxide (global warming), carbon monoxide (toxic), and sulfur dioxide (acid rain). Using cleaner fuels and improving combustion efficiency can reduce pollution.

Fire Safety and Control

Fire can be controlled by removing one or more conditions required for combustion. Methods include:

  • Using water or fire extinguishers
  • Cutting off the supply of air (oxygen)
  • Cooling the fuel below ignition temperature

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 combustion?
Answer:

A chemical process where a substance reacts with oxygen to produce heat and light.

Question 2:
Name the three essential requirements for combustion.
Answer:
  • Fuel
  • Oxygen
  • Ignition temperature
Question 3:
Which gas is necessary for combustion?
Answer:

Oxygen is necessary for combustion.

Question 4:
What is the ignition temperature of a substance?
Answer:

The minimum temperature at which a substance catches fire.

Question 5:
Give an example of a combustible substance.
Answer:

Wood is a combustible substance.

Question 6:
What is the main product of combustion of hydrocarbons?
Answer:

Carbon dioxide and water are the main products.

Question 7:
Which zone of a candle flame is the hottest?
Answer:

The non-luminous zone is the hottest.

Question 8:
What is a flame?
Answer:

A visible, gaseous part of fire where combustion occurs.

Question 9:
Name a fuel that burns without a flame.
Answer:

Charcoal burns without a flame.

Question 10:
What is incomplete combustion?
Answer:

Combustion with insufficient oxygen, producing soot and carbon monoxide.

Question 11:
Which substance is used in fire extinguishers to control fires?
Answer:

Carbon dioxide is used in fire extinguishers.

Question 12:
What is the color of the luminous zone of a candle flame?
Answer:

The luminous zone is yellow.

Question 13:
Why does a paper cup filled with water not burn?
Answer:

Water absorbs heat, keeping the cup below ignition temperature.

Question 14:
What is the purpose of the inflammable substance warning symbol?
Answer:

To indicate substances that catch fire easily.

Question 15:
Name the three essential requirements for combustion.
Answer:
  • Fuel (combustible substance)
  • Oxygen (from air)
  • Ignition temperature (minimum temperature to start burning)
Question 16:
What is the main product of combustion of hydrocarbons?
Answer:

The main products are carbon dioxide (CO2) and water (H2O), along with heat and light.

Question 17:
Define ignition temperature.
Answer:

Ignition temperature is the lowest temperature at which a substance catches fire and starts burning.

Question 18:
Why does a matchstick not catch fire on its own at room temperature?
Answer:

Because its ignition temperature is higher than room temperature. Friction against the matchbox provides the needed heat to reach ignition temperature.

Question 19:
What is the difference between rapid combustion and spontaneous combustion?
Answer:
  • Rapid combustion: Burns quickly with flame (e.g., LPG).
  • Spontaneous combustion: Catches fire without external heat (e.g., phosphorus).
Question 20:
Why is water not used to control fires caused by electrical equipment?
Answer:

Water conducts electricity, which can cause electric shocks. Instead, CO2 extinguishers are used.

Question 21:
Why does a candle flame have different zones?
Answer:

Because different parts of the flame have varying temperatures and oxygen availability, leading to incomplete or complete combustion.

Question 22:
Name a fuel that burns without producing a flame.
Answer:

Charcoal burns without a flame as it does not vaporize during combustion.

Question 23:
What is the role of a fire extinguisher?
Answer:

A fire extinguisher cuts off the oxygen supply or cools the fuel below its ignition temperature to stop combustion.

Question 24:
Why is CNG considered a cleaner fuel than petrol?
Answer:

CNG produces less pollutants (like CO2 and soot) compared to petrol, making it environmentally friendly.

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 combustion.
Answer:

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light, usually in the form of a flame.

Question 2:
Name the three essential requirements for combustion to occur.
Answer:
  • Fuel (combustible substance)
  • Oxygen (from air)
  • Ignition temperature (minimum heat required to start combustion)
Question 3:
Why is water not used to extinguish fires caused by electrical equipment?
Answer:

Water is a conductor of electricity and may cause electric shocks. Instead, CO2 fire extinguishers are used as they cut off oxygen supply without conducting electricity.

Question 4:
What is the role of a flame in combustion?
Answer:

A flame is the visible, gaseous part of fire where combustion occurs, producing heat and light due to the burning of vapors.

Question 5:
How does increasing wind speed affect a fire?
Answer:

Wind supplies more oxygen, increasing the fire's intensity by aiding faster combustion.

Question 6:
What is global warming? How is it linked to combustion?
Answer:

Global warming is the rise in Earth's temperature due to excess greenhouse gases like CO2.
Combustion of fuels releases CO2, contributing to this effect.

Question 7:
Why is LPG a better fuel than wood?
Answer:

LPG has a higher calorific value, burns cleaner (less smoke), and is easier to store/transport compared to wood.

Question 8:
Name two pollutants released during incomplete combustion.
Answer:
  • Carbon monoxide (CO)
  • Soot (unburnt carbon particles)
Question 9:
What safety measures should be taken while handling fuels?
Answer:
  • Store fuels away from heat sources
  • Ensure proper ventilation
  • Use fire extinguishers nearby
  • Avoid leakage checks with flames

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 combustion and list the three essential conditions required for it to occur.
Answer:

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. The three essential conditions for combustion are:

1. Presence of a combustible substance (fuel) - A material that can burn.
2. Supply of oxygen (air) - Required to support the burning process.
3. Ignition temperature - The minimum temperature at which a substance catches fire.

Question 2:
Explain why water is not used to control fires involving electrical equipment or oils.
Answer:

Water is not used to control fires involving electrical equipment or oils because:

1. Electrical fires: Water conducts electricity, which can lead to electric shocks or short circuits.
2. Oil fires: Water is heavier than oil and sinks below it, causing the oil to splash and spread the fire further. Instead, fire extinguishers like CO2 or dry chemical extinguishers are used.

Question 3:
Differentiate between rapid combustion and spontaneous combustion with examples.
Answer:

Rapid combustion is when a substance burns quickly with the help of an external source, producing heat and light (e.g., burning of LPG in a stove).

Spontaneous combustion occurs when a substance catches fire on its own without any external heat source due to slow oxidation (e.g., burning of white phosphorus at room temperature).

Question 4:
Describe the structure of a flame and label its different zones.
Answer:

A flame consists of three zones:

1. Innermost zone (dark zone): Unburnt vapors of the fuel due to insufficient oxygen.
2. Middle zone (luminous zone): Partial combustion occurs, producing a yellow glow due to carbon particles.
3. Outermost zone (non-luminous zone): Complete combustion occurs, appearing blue due to high temperature and sufficient oxygen.

Question 5:
Why is the use of CNG as a fuel considered more environmentally friendly than petrol or diesel?
Answer:

CNG (Compressed Natural Gas) is more environmentally friendly than petrol or diesel because:

1. It produces fewer harmful gases like carbon monoxide and nitrogen oxides.
2. It burns cleaner, reducing air pollution.
3. It does not release unburnt carbon particles, which cause respiratory problems.

Question 6:
What is global warming? How does the burning of fossil fuels contribute to it?
Answer:

Global warming is the gradual increase in Earth's temperature due to the trapping of heat by greenhouse gases like carbon dioxide.

Burning fossil fuels releases large amounts of CO2, which enhances the greenhouse effect, leading to:

1. Rising temperatures.
2. Melting of polar ice caps.
3. Climate change and extreme weather conditions.

Question 7:
Explain the conditions necessary for combustion to occur.
Answer:

For combustion to occur, three essential conditions must be met:

  • Presence of a combustible substance (fuel): A material that can burn, such as wood, paper, or LPG.
  • Supply of oxygen: Combustion is an oxidation reaction, so oxygen from the air is required.
  • Attainment of ignition temperature: The fuel must be heated to its minimum temperature at which it catches fire.

Without any of these, combustion cannot take place. For example, a candle goes out when covered because oxygen supply is cut off.

Question 8:
Why is water not used to control fires involving electrical equipment or oils?
Answer:

Water should not be used in electrical or oil fires because:

1. Electrical fires: Water conducts electricity, risking electric shocks or spreading the fire.
2. Oil fires: Oil floats on water, causing flames to spread instead of extinguishing.

Instead, CO2 fire extinguishers or sand are used, as they cut off oxygen supply without conducting electricity.

Question 9:
Describe how the global warming is linked to the combustion of fuels.
Answer:

Combustion of fuels releases carbon dioxide (CO2) and other greenhouse gases.

These gases trap the sun's heat in the atmosphere, increasing Earth's temperature—a phenomenon called global warming.

Excessive burning of fossil fuels (coal, petrol) intensifies this effect, leading to climate change, melting glaciers, and extreme weather.

Question 10:
What is a flame? Why does a goldsmith use a luminous flame?
Answer:

A flame is the visible, gaseous part of a fire where combustion occurs.

Goldsmiths use a luminous flame (e.g., from a kerosene lamp) because:

1. It produces more light due to incomplete combustion, helping them see intricate designs clearly.
2. The lower temperature prevents excessive melting of delicate gold ornaments.

Question 11:
List three ways to reduce harmful effects of burning fuels.
Answer:

To minimize the harmful effects of burning fuels:

  • Use cleaner fuels: Switch to CNG or LPG instead of coal/wood.
  • Maintain vehicles: Regular servicing reduces smoke emissions.
  • Adopt renewable energy: Solar/wind energy produce no pollution.

These steps reduce air pollution and health risks like asthma.

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 the conditions required for combustion with examples from daily life and NCERT.
Answer:
Concept Overview

Combustion requires three conditions: fuel, oxygen, and ignition temperature. Our textbook shows how a matchstick burns when rubbed due to heat and oxygen.

Process Explanation
  • Fuel: Like wood or LPG.
  • Oxygen: From air.
  • Ignition temperature: Heat from a match.
Real-world Application

In gas stoves, LPG burns with a blue flame when all conditions are met. Without oxygen, like under a glass, flames extinguish.

Question 2:
Describe the types of combustion with one NCERT and one real-life example for each.
Answer:
Concept Overview

Combustion can be rapid, spontaneous, or explosive. Our textbook explains burning paper as rapid combustion.

Process Explanation
  • Rapid: Burning matchstick (NCERT).
  • Spontaneous: Coal dust in mines.
  • Explosive: Firecrackers (real-life).
Real-world Application

Spontaneous combustion occurs in haystacks due to heat buildup, while explosions happen in fuel tanks if ignited.

Question 3:
How does a flame form? Compare luminous and non-luminous flames using NCERT examples.
Answer:
Concept Overview

A flame forms when vapors of a flammable substance burn. Our textbook shows candle wax vaporizing to form a luminous flame.

Process Explanation
  • Luminous: Candle (yellow, sooty).
  • Non-luminous: Bunsen burner (blue, hotter).
Real-world Application

Kerosene lamps give luminous flames, while LPG stoves produce non-luminous flames for efficient cooking.

Question 4:
What is global warming? Explain its link to combustion using NCERT and a real-world scenario.
Answer:
Concept Overview

Global warming is Earth's temperature rise due to gases like CO2. Our textbook links it to burning fossil fuels.

Process Explanation
  • Combustion releases CO2.
  • Example: Burning coal in factories (NCERT).
Real-world Application

Car engines burn petrol, increasing CO2 levels, causing ice caps to melt.

Question 5:
List the zones of a candle flame and their characteristics. Include a diagram description.
Answer:
Concept Overview

A candle flame has three zones: innermost, middle, and outer. Our textbook describes their colors and temperatures.

Process Explanation
  • Innermost: Dark, unburnt wax vapors.
  • Middle: Yellow, luminous.
  • Outer: Blue, hottest.
Diagram
[Diagram: Labeled candle flame with three zones]
Real-world Application

This helps in understanding efficient burning, like in gas stoves where the blue zone is used for heating.

Question 6:
Explain the combustion process with an example from NCERT and a real-world application.
Answer:
Concept Overview

Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. Our textbook shows burning wood as an example.

Process Explanation
  • Fuel (wood) heats up.
  • It reacts with oxygen to form CO2 and water.
  • Heat and light are released.
Real-world Application

In LPG stoves, combustion of gas provides flame for cooking. [Diagram: Combustion of LPG in a stove]

Question 7:
Describe the types of combustion with one NCERT example and a daily-life instance.
Answer:
Concept Overview

Combustion can be rapid, spontaneous, or explosive. NCERT mentions burning matchstick as rapid combustion.

Process Explanation
  • Rapid: Quick burning (matchstick).
  • Spontaneous: No external heat (coal dust).
  • Explosive: Sudden release of gas (firecrackers).
Real-world Application

Forest fires are spontaneous combustion due to heat accumulation. [Diagram: Forest fire ignition]

Question 8:
How does a flame form? Use the NCERT candle example and a household application.
Answer:
Concept Overview

A flame is the visible part of combustion. Our textbook explains candle flame zones.

Process Explanation
  • Wax vaporizes and reacts with oxygen.
  • Forms inner (dark), middle (luminous), and outer (hottest) zones.
Real-world Application

Gas stoves also have flame zones, with blue being the hottest. [Diagram: Candle vs. gas stove flame]

Question 9:
What are fuel efficiency and calorific value? Compare NCERT’s diesel example with a modern fuel.
Answer:
Concept Overview

Calorific value is energy per kg of fuel. NCERT states diesel has 45,000 kJ/kg.

Process Explanation
  • Higher calorific value = more efficient.
  • Diesel > Petrol in efficiency.
Real-world Application

CNG (50,000 kJ/kg) is now preferred for vehicles due to higher efficiency. [Diagram: Fuel efficiency comparison table]

Question 10:
Explain fire control methods using the NCERT ‘fire triangle’ and a real-life safety measure.
Answer:
Concept Overview

Fire needs fuel, heat, and oxygen (fire triangle). NCERT shows removing one stops fire.

Process Explanation
  • Water cools (removes heat).
  • Sand cuts oxygen.
Real-world Application

Fire extinguishers use CO2 to smother flames in kitchens. [Diagram: Fire extinguisher in action]

Question 11:
Explain the process of combustion with a suitable example. Discuss the conditions necessary for combustion to occur and how they influence the process.
Answer:

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. For example, when a candle burns, the wax (hydrocarbon) reacts with oxygen in the air to form carbon dioxide, water vapor, and releases energy in the form of heat and light.

The three essential conditions for combustion are:

  • Presence of a combustible substance (fuel): A material that can burn, like wood, paper, or LPG.
  • Supply of oxygen (air): Combustion cannot occur without oxygen, as it supports the chemical reaction.
  • Ignition temperature: The minimum temperature at which a substance catches fire. For example, a matchstick ignites when rubbed against a rough surface, reaching its ignition temperature.

If any of these conditions are removed, combustion stops. For instance, covering a burning candle with a glass cuts off oxygen, extinguishing the flame. Understanding these conditions helps in fire safety and controlling fires.

Question 12:
Describe the structure of a flame with a labeled diagram. Explain the different zones of a flame and their characteristics.
Answer:

A flame is the visible, gaseous part of a fire. It consists of several zones, each with distinct characteristics. Below is a description of the zones in a candle flame:

  • Innermost Zone (Dark Zone): This is the coldest part of the flame, containing unburnt wax vapors due to insufficient oxygen.
  • Middle Zone (Luminous Zone): Here, partial combustion occurs, producing a yellow glow due to the presence of glowing carbon particles.
  • Outermost Zone (Non-Luminous Zone): This is the hottest zone where complete combustion takes place, producing a blue color due to the presence of sufficient oxygen.

Characteristics of flame zones:

  • The innermost zone is the least hot and appears dark.
  • The middle zone is moderately hot and emits light, making it luminous.
  • The outermost zone is the hottest and is almost invisible in daylight.

Here is a simple labeled diagram of a candle flame:

[Diagram: A vertical candle flame divided into three concentric zones labeled as Innermost (Dark), Middle (Luminous), and Outermost (Non-Luminous).]

Understanding the structure of a flame helps in studying efficient burning and fire safety measures.

Question 13:
Explain the process of combustion with the help of an example. Discuss the conditions necessary for combustion to occur and how they influence the process.
Answer:

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. For example, when wood burns, it reacts with oxygen in the air to produce carbon dioxide, water vapor, heat, and light.

The conditions necessary for combustion are:

  • Presence of a combustible substance (fuel): A material that can burn, like wood, paper, or LPG.
  • Presence of oxygen (supporter of combustion): Combustion cannot occur without oxygen, as it is essential for the chemical reaction.
  • Ignition temperature: The fuel must reach its minimum temperature at which it catches fire.

These conditions influence combustion because:

  • If the fuel is insufficient, combustion stops once it is exhausted.
  • Without oxygen, the fire will extinguish.
  • If the temperature drops below the ignition point, combustion ceases.

Understanding these conditions helps in fire safety and controlling fires effectively.

Question 14:
Explain the process of combustion with the help of an example. Also, discuss the conditions necessary for combustion to occur.
Answer:

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. For example, when a candle burns, the wax (hydrocarbon) reacts with oxygen in the air to form carbon dioxide, water vapor, and releases energy in the form of heat and light.

The three essential conditions for combustion are:

  • Presence of a combustible substance (fuel): A material that can burn, such as wood, paper, or LPG.
  • Presence of a supporter of combustion (oxygen): Oxygen is required for burning, as it helps in the oxidation process.
  • Attainment of ignition temperature: The substance must be heated to its minimum temperature at which it catches fire.

Without any of these conditions, combustion cannot occur. For instance, a fire extinguisher works by removing one or more of these conditions.

Question 15:
Differentiate between rapid combustion and spontaneous combustion with suitable examples. How can spontaneous combustion be prevented?
Answer:

Rapid combustion is a type of combustion where a substance burns quickly with the help of an external source, producing heat and light. For example, burning of LPG in a stove or a matchstick catching fire.

Spontaneous combustion, on the other hand, occurs when a material suddenly catches fire without any external heat source due to internal heat buildup. For example, a heap of wet hay or coal dust can self-ignite due to slow oxidation.

To prevent spontaneous combustion:

  • Store flammable materials like hay, coal, or oily rags in well-ventilated areas to avoid heat accumulation.
  • Keep such materials away from direct sunlight or heat sources.
  • Regularly check storage areas for temperature rise.

Understanding these types of combustion helps in fire safety and proper handling of combustible materials.

Question 16:
Differentiate between rapid combustion and spontaneous combustion with examples. Explain why spontaneous combustion can be dangerous and how it can be prevented.
Answer:

Rapid combustion is when a substance burns quickly with the help of an external source, producing heat and light. For example, burning LPG in a stove or lighting a matchstick. On the other hand, spontaneous combustion occurs without any external ignition source when a material self-heats due to slow oxidation, reaching its ignition temperature. An example is the sudden burning of a heap of wet hay or coal dust in mines.

Spontaneous combustion is dangerous because:

  • It happens unexpectedly, leading to fires in storage areas.
  • Materials like oily rags or agricultural waste can ignite without warning, causing damage.

Prevention methods include:

  • Storing flammable materials in cool, dry, and well-ventilated spaces.
  • Avoiding piling up waste that can undergo slow oxidation.
  • Regularly checking storage areas for heat buildup.

Understanding these types of combustion helps in handling flammable materials safely.

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:
During a campfire, Riya observed that wood burns with a yellow flame while a candle burns with a blue flame. Explain why different fuels produce flames of different colors.
Answer:
Case Summary

Riya noticed wood and candle flames differ in color due to combustion properties.

Scientific Principle
  • Incomplete combustion (wood) produces a yellow flame due to unburnt carbon particles.
  • Complete combustion (candle) forms a blue flame as hydrocarbons burn efficiently.
Solution Approach

Our textbook shows combustion efficiency affects flame color. Real-world example: LPG burns blue, indicating clean fuel.

Question 2:
A teacher demonstrated that covering a burning candle with a glass tumbler extinguishes it. Identify the condition necessary for combustion and relate it to the experiment.
Answer:
Case Summary

The candle flame went out due to lack of a combustion condition.

Scientific Principle
  • Combustion requires oxygen (NCERT example: Fire triangle).
  • Glass tumbler cuts off oxygen supply.
Solution Approach

We studied that fuels need oxygen to burn. Real-world application: Fire blankets smother fires by blocking oxygen.

Question 3:
In an experiment, three fuels (A, B, C) were tested for calorific value. Results:
FuelCalorific Value (kJ/g)
A30
B45
C25
Which fuel is most efficient? Justify with a real-world example.
Answer:
Case Summary

Fuel B has the highest calorific value among tested fuels.

Scientific Principle
  • Higher calorific value = more energy per gram (NCERT: Chap 6).
  • Efficiency depends on energy output.
Solution Approach

Our textbook shows CNG (50 kJ/g) is preferred over coal (25-30 kJ/g) for higher efficiency, similar to Fuel B.

Question 4:
Villagers use cow dung cakes as fuel despite their low calorific value. Analyze two reasons for this practice and suggest a cleaner alternative.
Answer:
Case Summary

Cow dung cakes are used despite inefficiency due to accessibility.

Scientific Principle
  • Readily available (NCERT: Traditional fuels).
  • Low cost but causes air pollution.
Solution Approach

We studied biogas as a cleaner alternative. Real-world example: Gobar Gas plants convert dung into methane efficiently.

Question 5:
During a campfire, Rohan noticed that dry wood burns easily while wet wood does not. Explain why and list two conditions necessary for combustion.
Answer:
Case Summary
Dry wood burns easily, but wet wood resists burning.
Scientific Principle
Combustion requires fuel, heat, and oxygen. Wet wood contains water, which absorbs heat, lowering temperature below ignition point.
Solution Approach
  • Dry wood meets combustion conditions: fuel (wood), heat (fire), oxygen (air).
  • Two conditions: presence of combustible material and ignition temperature.

Our textbook shows similar examples like burning paper vs wet paper.

Question 6:
A candle flame has three zones. Identify them and explain why the middle zone is the hottest.
Answer:
Case Summary
Candle flame has three distinct zones.
Scientific Principle
Zones are: outer (non-luminous), middle (luminous), inner (dark). Middle zone has partial combustion, releasing more heat.
Solution Approach
  • Outer zone: complete combustion (hottest).
  • Middle zone: partial combustion (yellow, bright).

We studied in NCERT that unburnt carbon particles glow here, making it luminous.

Question 7:
Priya observed that a glass jar placed over a burning candle extinguishes it. Explain the reason and name the type of combustion in this case.
Answer:
Case Summary
Candle extinguishes when covered with a jar.
Scientific Principle
Combustion stops due to lack of oxygen. This is rapid combustion (fast burning with flame).
Solution Approach
  • Oxygen depletes inside jar, stopping reaction.
  • Example: Similar to blowing out a matchstick.

Our textbook shows how fire extinguishers work on this principle.

Question 8:
In an experiment, three fuels (A, B, C) were tested for calorific value. Interpret the data and suggest the best fuel for domestic use.
[Table: Fuel A: 30 kJ/g | Fuel B: 45 kJ/g | Fuel C: 25 kJ/g]
Answer:
Case Summary
Comparing calorific values of three fuels.
Scientific Principle
Higher calorific value means more energy per gram. Ideal fuel should be efficient and safe.
Solution Approach
FuelCalorific Value
A30 kJ/g
B45 kJ/g
C25 kJ/g

Fuel B is best (highest value). Like LPG in NCERT examples.

Question 9:
During a campfire, Rohan noticed that wood burns with a yellow flame, while a candle burns with a blue flame. Explain why different fuels produce flames of different colors.
Answer:
Case Summary

Wood and candle wax burn differently due to their composition.


Scientific Principle
  • Yellow flame indicates incomplete combustion (soot particles glow).
  • Blue flame shows complete combustion (cleaner fuel like LPG).

Solution Approach

Our textbook shows that impurities in wood cause yellow flame, while wax burns cleaner. Real-world example: Bunsen burner adjusts air for blue flame.

Question 10:
Priya observed that a paper cup filled with water does not burn when held over a candle flame. Identify the scientific reason behind this observation.
Answer:
Case Summary

Water prevents paper cup from burning.


Scientific Principle
  • Combustion requires ignition temperature.
  • Water absorbs heat, keeping paper below this temperature.

Solution Approach

We studied that water acts as a coolant. NCERT example: Firefighters use water to extinguish fires by cooling fuels.

Question 11:
A gas stove produces heat instantly, while coal takes time to start burning. Compare the ignition temperatures of these fuels and their practical implications.
Answer:
Case Summary

Gases ignite faster than solid fuels like coal.


Scientific Principle
  • Gases have lower ignition temperature than solids.
  • Surface area affects combustion rate.

Solution Approach

Our textbook shows LPG (gas) ignites at ~600°C, while coal needs ~700°C. Real-world application: Gas stoves are quicker for cooking than wood fires.

Question 12:
In an experiment, three candles were covered with jars of different sizes. Analyze how the size of the jar affects the burning time of the candle.
Answer:
Case Summary

Larger jars allow longer burning.


Scientific Principle
  • Combustion requires oxygen.
  • Limited oxygen extinguishes flame.

Solution Approach

We studied that NCERT Activity 6.1 shows this. Larger jars have more oxygen supply. Real-world example: Small spaces increase fire hazards due to oxygen depletion.

Question 13:

Rahul conducted an experiment to study the conditions necessary for combustion. He took three candles and placed them under different conditions:

  • Candle A: Covered with a glass jar
  • Candle B: Placed in open air
  • Candle C: Surrounded by water

After lighting all three candles, he observed their burning times. Based on this, answer:

(a) Which candle will burn the longest and why?
(b) Explain the role of oxygen in combustion with reference to the experiment.

Answer:

(a) Candle B (placed in open air) will burn the longest because it has a continuous supply of oxygen, which is essential for combustion. The other candles either have limited oxygen (Candle A) or no oxygen supply (Candle C).

(b) Oxygen is a critical supporter of combustion. In the experiment:
Candle A stops burning when the oxygen inside the jar is exhausted.
Candle C cannot burn as water blocks oxygen supply.
This proves that combustion requires a steady supply of oxygen to sustain the reaction. Without oxygen, the combustion process stops.

Question 14:

Priya noticed that when her mother lit a kerosene lamp, it produced a yellow flame, while the LPG stove at home produced a blue flame. Answer the following:

(a) Why does the kerosene lamp produce a yellow flame?
(b) Compare the efficiency of combustion in both fuels based on flame color.

Answer:

(a) The kerosene lamp produces a yellow flame due to incomplete combustion. The unburnt carbon particles in the flame get heated and emit a yellow glow, indicating inefficient burning.

(b) The LPG stove produces a blue flame, which indicates complete combustion. This means LPG burns more efficiently than kerosene because:
1. Blue flame signifies higher temperature and cleaner burning.
2. No soot or residue is left behind.
3. More energy is released per unit of fuel compared to kerosene's yellow flame.

Question 15:

Rahul conducted an experiment to study the conditions necessary for combustion. He took three candles of the same size and placed them under different conditions:

  • Candle A: Covered with a glass jar
  • Candle B: Placed in open air
  • Candle C: Covered with a glass jar but with a small hole at the top

After lighting all three candles, he observed their burning times. Based on this case, answer:

Q1. Which candle will burn the longest and why? Also, explain the role of oxygen in combustion.

Answer:

Candle B will burn the longest because it is placed in open air, allowing continuous supply of oxygen, which is essential for combustion.

Explanation:

  • Candle A stops burning quickly due to lack of oxygen as the glass jar traps the carbon dioxide produced, suffocating the flame.
  • Candle C burns longer than A but shorter than B because the small hole allows limited oxygen supply.

Role of Oxygen:

Oxygen is a supporting agent for combustion. It reacts with the fuel (candle wax) to produce heat, light, carbon dioxide, and water vapor. Without oxygen, combustion cannot occur.

Question 16:

Priya noticed that her cooking gas stove produces a blue flame while a candle produces a yellow flame. She wondered why there is a difference. Based on this case, answer:

Q2. Explain the reason behind the difference in flame color. Also, mention which flame is more efficient for cooking and why.

Answer:

Difference in Flame Color:

  • The blue flame in the gas stove indicates complete combustion due to sufficient oxygen supply, resulting in higher temperature and cleaner burning.
  • The yellow flame in the candle indicates incomplete combustion due to limited oxygen, leading to the formation of soot particles that glow yellow.

Efficiency for Cooking:

The blue flame is more efficient because:

  • It produces more heat due to complete combustion.
  • It does not leave soot or residue, making it cleaner for cooking.
Question 17:

Rahul conducted an experiment to study the conditions necessary for combustion. He took three candles and placed them under different conditions:

  • Candle A: Covered with a glass jar
  • Candle B: Placed in open air
  • Candle C: Kept in a jar filled with carbon dioxide

After lighting all three candles, he observed their burning time. Based on this, answer:

(a) Which candle stopped burning first? Give a reason.

(b) Why did Candle B burn the longest?

Answer:

(a) Candle C stopped burning first because combustion requires oxygen, and the jar was filled with carbon dioxide, which does not support burning.
(b) Candle B burned the longest as it was placed in open air, providing a continuous supply of oxygen, which is essential for sustaining combustion. The absence of any restricting cover allowed the flame to access oxygen freely.

Additional Insight: This experiment demonstrates the fire triangle concept, where combustion requires fuel, heat, and oxygen. Removing any one of these stops the process.

Question 18:

Priya noticed that her mother uses a blue flame stove for cooking instead of a traditional yellow flame stove. She wondered about the difference. Help her by answering:

(a) Why does a blue flame indicate complete combustion?

(b) What are the advantages of using a blue flame stove over a yellow flame one?

Answer:

(a) A blue flame indicates complete combustion because the fuel burns completely in the presence of sufficient oxygen, producing a high-temperature flame with minimal residue. The blue color is due to the excitation of carbon monoxide and other gases at high temperatures.
(b) Advantages of a blue flame stove include:

  • Higher efficiency: More heat is produced per unit of fuel.
  • Cleaner burning: Less soot or smoke is released, reducing indoor pollution.
  • Precise control: The flame is steady and adjustable, ideal for cooking.

Additional Insight: Incomplete combustion in a yellow flame stove releases unburnt carbon particles, which can cause respiratory issues and waste fuel.

Question 19:
Rahul conducted an experiment to study the conditions necessary for combustion. He took three candles of the same size and placed them under different conditions:
  • Candle A: Covered with a glass jar
  • Candle B: Placed in open air
  • Candle C: Covered with a glass jar but with a small hole at the top
After lighting all three candles, Rahul observed their burning times.

Based on this experiment, answer the following:

  • Which candle burned the longest and why?
  • Explain the role of oxygen in combustion with reference to the experiment.
Answer:

Candle B burned the longest because it was placed in open air, allowing a continuous supply of oxygen, which is essential for combustion.

Candle A stopped burning quickly as the glass jar cut off the oxygen supply, while Candle C had limited oxygen due to the small hole, causing it to burn for a shorter duration than Candle B but longer than Candle A.

The experiment demonstrates that oxygen is a critical supporter of combustion. Without sufficient oxygen, the flame extinguishes. This is why fire safety measures often focus on cutting off oxygen supply (e.g., using sand or fire blankets).

Question 20:
Priya noticed that her mother uses a blue flame for cooking instead of a yellow flame on their gas stove. She wondered why the blue flame is preferred.

Answer the following based on this observation:

  • What does the color of a flame indicate about its efficiency?
  • Why is a blue flame considered better for cooking than a yellow flame? Explain with respect to complete combustion.
Answer:

The color of a flame indicates its temperature and combustion efficiency. A blue flame signifies complete combustion, meaning the fuel is burning efficiently with sufficient oxygen, producing maximum heat and minimal soot.

A yellow flame, on the other hand, indicates incomplete combustion due to insufficient oxygen, leading to lower heat output and the release of unburnt carbon particles (soot).

For cooking, a blue flame is preferred because:

  • It provides higher heat, reducing cooking time.
  • It avoids soot deposition on utensils, keeping them cleaner.
  • It is more fuel-efficient, saving energy.
This is why gas stoves are adjusted to ensure a blue flame for optimal performance.

Question 21:
Rahul conducted an experiment to compare the burning of a candle in open air and under a glass jar. He observed that the candle flame extinguished quickly under the jar.

Explain the scientific reason behind this observation and discuss the role of combustion in this process.

Answer:

The candle flame extinguished under the glass jar because combustion requires oxygen to sustain the burning process. When the jar was placed over the candle, the available oxygen inside was gradually used up, forming carbon dioxide and water vapor.

Since the jar restricted the supply of fresh air, the oxygen level dropped, and the flame could no longer sustain itself. This demonstrates the essential condition for combustion: a continuous supply of oxygen (or a supporter of combustion).

Additionally, the experiment highlights how combustion stops in the absence of oxygen, which is a key concept in fire safety and controlled burning applications.

Question 22:
Priya noticed that a kerosene lamp produces a yellow flame, while a Bunsen burner produces a blue flame.

Analyze the differences between these flames in terms of combustion efficiency, soot production, and temperature. What does this indicate about the nature of their fuels?

Answer:

The yellow flame in the kerosene lamp indicates incomplete combustion, which occurs due to insufficient oxygen supply. This results in:

1. Lower temperature (as some energy remains trapped in unburnt carbon particles).
2. Soot production (visible as black smoke due to unburnt carbon).
3. Less efficient combustion (wastage of fuel).

In contrast, the blue flame in the Bunsen burner signifies complete combustion because of adequate oxygen supply, leading to:

1. Higher temperature (more energy released).
2. Minimal soot (cleaner burn as carbon fully reacts with oxygen).
3. Higher combustion efficiency (optimal fuel usage).

This difference shows that kerosene burns less efficiently in open lamps, while gaseous fuels (like in Bunsen burners) allow better air mixing for complete combustion.

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