Summary and Question Answer of NCERT Class 8 Science Chapter 6, Titled “Combustion and Flame"
Summary of Combustion
and Flame
In the chapter "Combustion and Flame" for Class 8
students, you learn about the process of combustion, which is the chemical
reaction between a substance and oxygen that gives off heat and light. Here is
a summary of the key points covered in the chapter:
Introduction to Combustion:
The chapter starts by defining combustion as a chemical
process in which a substance reacts rapidly with oxygen and releases energy in
the form of heat and light. The essential components for combustion are fuel,
oxygen, and heat.
Types of Combustion:
- Rapid
Combustion: This type of combustion is characterized by the evolution
of heat and light. Examples include burning of LPG, kerosene, and wood.
- Spontaneous
Combustion: Some substances catch fire spontaneously due to internal
heat generated by chemical reactions. For example, phosphorus can catch
fire spontaneously when exposed to air.
- Explosive
Combustion: Combustion accompanied by the evolution of a large amount
of heat and gas rapidly is termed explosive combustion. Examples include
the combustion in engines.
Ignition Temperature:
Every substance has a specific temperature at which it
starts to burn. This temperature is called the ignition temperature. Substances
with low ignition temperatures catch fire easily.
Combustion and Pollution:
Incomplete combustion leads to the formation of harmful
gases like carbon monoxide (CO) and unburnt carbon particles, which pollute the
environment. These pollutants can cause respiratory problems and contribute to
global warming.
Flames:
A flame is a region of a fire where combustion is taking
place. Flames have different parts, such as the outermost part called the outer
cone, the inner bluish part called the inner cone, and the
non-luminous zone called the dark zone.
Fire Extinguishers:
Different types of fire extinguishers are used to control
different types of fires. For example, water is effective for fires caused by
materials like wood and paper, but it is not suitable for oil and electrical
fires.
Safety Measures:
The chapter emphasizes the importance of safety measures to
prevent accidental fires. It also discusses the Stop, Drop, and Roll technique
to be used if someone's clothes catch fire.
Conclusion:
Understanding the process of combustion, types of flames,
and safety measures is crucial for students to be aware of fire hazards and to
promote fire safety in various situations.
This summary covers the fundamental concepts discussed in
the chapter "Combustion and Flame" for Class 8 students.
"Combustion and Flame" is an essential chapter in
science that deals with the process of combustion, which is the chemical
reaction between a substance and oxygen that results in the release of energy
in the form of heat and light. This process is vital for various activities in
our daily lives, from cooking food to powering vehicles and generating
electricity. Let me break down the key concepts of this chapter for you:
1. Combustion:
- Definition:
Combustion is a chemical reaction in which a substance combines rapidly
with oxygen and releases energy in the form of heat and light.
- Requirements
for Combustion: For combustion to occur, three things are necessary,
often referred to as the fire triangle:
- Fuel:
Any substance that undergoes combustion is called a fuel. Common fuels
include wood, coal, petrol, and natural gas.
- Oxygen:
Oxygen from the air is needed for combustion.
- Heat:
Initial heat is required to start the combustion process. Once started,
the process can sustain itself if there is enough heat, fuel, and oxygen.
2. Types of Combustion:
- Rapid
Combustion: It occurs with substances like petrol and diesel, leading
to a quick release of heat and light.
- Spontaneous
Combustion: Some substances can catch fire spontaneously due to
internal heat generated by chemical reactions. For example, oily rags can
spontaneously combust.
- Explosive
Combustion: This type of combustion occurs rapidly with the evolution
of a large amount of gas and heat. Explosions are examples of this type of
combustion.
3. Flame:
- Definition:
A flame is the visible, gaseous part of a fire. It consists of hot gases
and fine particles, emitting light and heat.
- Parts
of a Flame:
- Inner
Cone: The innermost part of the flame where complete combustion
occurs due to the availability of sufficient oxygen.
- Outer
Cone: Surrounding the inner cone, it is less hot and contains unburnt
carbon particles, making it appear yellow.
- Luminous
Zone: The outermost layer of the flame, which is least hot and
appears blue.
4. Fire Extinguishing:
- Methods:
Fires can be extinguished by removing any one of the elements of the fire
triangle—fuel, oxygen, or heat. Common methods include using water
(removes heat), fire extinguishers (removes oxygen), or sand/blanket
(blocks oxygen supply).
5. Uses and Hazards:
- Uses:
Combustion is used for various purposes like cooking, heating, and
generating energy.
- Hazards:
Uncontrolled combustion can lead to fires, causing damage to property and
posing risks to life. Prevention and safety measures are crucial to avoid
accidents.
Understanding the concepts of combustion and flame is
essential not only for academic knowledge but also for personal safety and
environmental awareness. It's important to handle and use combustible materials
responsibly, considering the principles outlined in this chapter.
Question and Answer
List
conditions under which combustion can take place.
In the context of combustion, there are three essential
conditions that must be met for combustion to take place. These are often referred
to as the fire triangle:
- Fuel:
Combustion requires a substance that can burn. This substance is called
fuel. It can be in the form of solids (like wood or paper), liquids (such
as gasoline or oil), or gases (like natural gas).
- Oxygen:
Oxygen gas (O2) from the air is necessary for combustion to occur. Without
oxygen, the combustion process cannot take place. Sufficient oxygen must
be present in the surrounding environment for the fuel to burn.
- Heat:
Heat is the energy required to initiate the combustion process. It
provides the activation energy needed to start the chemical reaction
between the fuel and oxygen. Once the combustion process begins, it
releases more heat, which can sustain the reaction as long as there is a
continuous supply of fuel and oxygen.
These three conditions—presence of fuel, oxygen, and
sufficient heat—must all be met simultaneously for combustion to occur. If any
one of these conditions is removed or reduced significantly, the combustion
process will stop. Understanding and controlling these factors are crucial for
fire safety and the effective use of combustion in various applications.
Fill in the blanks.
(a) Burning of wood and coal causes _____ of air.
(b) A liquid fuel, used in homes is ______
(c) Fuel must be heated to its ______ before it starts
burning.
(d) Fire produced by oil cannot be controlled by
______"
Answer
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel used in homes is LPG (Liquefied
Petroleum Gas).
(c) Fuel must be heated to its ignition temperature
before it starts burning.
(d) Fire produced by oil cannot be controlled by water.
Question : Explain
how the use of CNG in automobiles has reduced pollution in our cities.
he adoption of Compressed Natural Gas (CNG) as a fuel for
automobiles has significantly contributed to reducing pollution in our cities.
This change has occurred due to several scientific reasons and environmental
benefits associated with CNG:
- Cleaner
Combustion:
- CNG
is composed mainly of methane (CH4). When compared to conventional fuels
like gasoline and diesel, methane contains fewer carbon atoms. During
combustion, it produces fewer carbon dioxide (CO2) emissions. Also, it
generates minimal carbon monoxide (CO) and almost no particulate matter
(PM), which are harmful pollutants.
- Lower
Emission of Harmful Gases:
- CNG
combustion produces significantly lower levels of nitrogen oxides (NOx)
compared to gasoline and diesel engines. NOx gases are major contributors
to air pollution and are associated with respiratory problems and smog
formation.
- Reduced
Greenhouse Gas Emissions:
- Methane,
the primary component of CNG, is a hydrocarbon. While it does produce CO2
when burned, the overall greenhouse gas emissions are lower compared to
other fossil fuels. Moreover, the carbon footprint of CNG is lower
because it has a simpler chemical structure and burns more efficiently.
- Abundance
and Domestic Production:
- Natural
gas, from which CNG is derived, is abundant and domestically available in
many countries. This reduces dependency on imported oil, leading to
energy security and economic stability.
- Promotion
of Public Transport:
- Many
urban areas have shifted their public transport systems, such as buses
and taxis, to CNG. This widespread adoption has a substantial impact on
overall emissions as public transport vehicles cover significant
distances daily.
- Incentives
and Regulations:
- Governments
in various countries have implemented policies to promote the use of
cleaner fuels. Subsidies, tax incentives, and stringent emission norms
have encouraged the adoption of CNG vehicles.
- Improved
Air Quality:
- By
reducing emissions of harmful pollutants, CNG-powered vehicles contribute
to improving the air quality in cities. This has direct health benefits
for the residents, especially for those with respiratory conditions.
In conclusion, the use of CNG in automobiles has
substantially reduced pollution in our cities by promoting cleaner combustion,
lowering emissions of harmful gases, reducing greenhouse gas emissions,
utilizing domestic resources, and improving air quality. It stands as a prime
example of how scientific advancements in fuel technology can positively impact
our environment and public health.
Question : Compare
LPG and wood as fuels.
When comparing Liquefied Petroleum Gas (LPG) and wood as
fuels, several factors need to be considered. Here's a detailed comparison
based on various aspects:
1. Efficiency:
- LPG:
LPG is highly efficient as a fuel. It burns cleanly and releases a
significant amount of heat energy when combusted. It is easy to control
the flame, making it suitable for various applications such as cooking and
heating.
- Wood:
Wood is less efficient compared to LPG. It produces a substantial amount
of smoke and ash when burned. The heat produced may not be as consistent
or easy to control as LPG.
2. Environmental Impact:
- LPG:
LPG is a cleaner fuel compared to wood. It produces fewer pollutants and
greenhouse gases, making it more environmentally friendly. It helps in
reducing air pollution and deforestation.
- Wood:
Burning wood releases carbon dioxide, a greenhouse gas, into the
atmosphere. Deforestation, a result of wood usage, contributes to
environmental imbalances. Also, the smoke from burning wood can cause air
pollution and health problems.
3. Convenience and Availability:
- LPG:
LPG is convenient to use. It comes in pressurized cylinders, making it
easy to store and transport. It is readily available in urban and rural
areas, providing a consistent energy source.
- Wood:
Wood is widely available, especially in rural areas. However, it requires
proper storage, seasoning (drying), and handling. In urban areas, the
availability of quality wood might be limited.
4. Cost:
- LPG:
LPG is a commercial fuel and needs to be purchased. Its cost can vary
based on market conditions and government subsidies.
- Wood:
Wood is often cheaper or even free, especially in rural areas where it can
be sourced locally. However, the cost of equipment for burning wood, like
stoves or fireplaces, needs to be considered.
5. Safety:
- LPG:
LPG is safe to use when handled properly. However, it is flammable and
requires careful storage and usage to prevent accidents.
- Wood:
Wood fires can be hazardous if not managed correctly. There is a risk of
sparks, and improper ventilation can lead to indoor air pollution.
In summary, LPG is a more efficient, environmentally
friendly, and convenient fuel compared to wood. While wood is readily available
and low-cost, it poses environmental challenges and requires careful handling.
The choice between LPG and wood depends on factors such as availability,
budget, environmental concerns, and the availability of proper equipment and
infrastructure for usage.
Question : Give
reasons.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around
an aluminium pipe does not.
Answer
a) Water is not used to control fires involving
electrical equipment.
Reason: Water is not used to control fires involving
electrical equipment because water is a good conductor of electricity. When
water is sprayed on an electrical fire, it can conduct the electric current,
potentially leading to electric shock to the person attempting to extinguish
the fire. Moreover, water may not effectively extinguish electrical fires involving
live electrical equipment. Special fire extinguishers, known as Class C or
Class E fire extinguishers, are designed for electrical fires. These
extinguishers contain materials that do not conduct electricity, making them
safe and effective for putting out electrical fires.
(b) LPG is a better domestic fuel than wood.
Reasons:
- Efficiency:
LPG (liquefied petroleum gas) has a higher calorific value compared to
wood. This means that LPG produces more heat energy when burnt, making it
more efficient for cooking and heating purposes.
- Cleanliness:
LPG burns cleanly and produces minimal smoke and soot, leading to less
pollution and easier maintenance of household appliances.
- Control:
LPG stoves offer better control over the flame, allowing for precise
temperature regulation. This control is crucial for cooking various dishes
effectively.
- Convenience:
LPG is available in sealed cylinders, which are easy to transport and
store. Unlike wood, it doesn't require chopping, storing, or frequent
replenishment.
- Safety:
LPG cylinders are equipped with safety features, such as pressure
regulators and safety valves, making them safer to use in households
compared to open fires using wood.
(c) Paper by itself catches fire easily whereas a piece
of paper wrapped around an aluminium pipe does not.
Reason: When a piece of paper is wrapped around an
aluminum pipe, it does not catch fire easily due to the following reasons:
- Heat
Dissipation: Aluminum is an excellent conductor of heat. When the
paper is wrapped around the aluminum pipe, the heat generated during
combustion is quickly conducted away from the paper. This rapid
dissipation of heat prevents the paper from reaching its ignition
temperature, which is necessary for it to catch fire.
- Isolation
from Oxygen: Wrapping the paper around the aluminum pipe may restrict
the oxygen supply to the paper. Combustion requires oxygen, and if the
paper is not in direct contact with the surrounding air, it may not receive
enough oxygen to sustain the combustion process. Lack of oxygen prevents
the paper from catching fire or slows down the process significantly.
These reasons demonstrate how the combination of heat
dissipation and limited oxygen supply prevents the paper from catching fire
easily when wrapped around an aluminum pipe.
Question : Make a
labelled diagram of a candle flame.
- Wick:
The wick is the central part of the candle that absorbs and draws up the
liquid wax. It is the primary source of fuel for the candle flame.
- Blue
Zone (Inner Cone): This is the hottest part of the flame where wax
vaporizes and mixes with oxygen to undergo complete combustion. In this
zone, all the wax is burnt completely, and no soot is produced.
- Yellow
Zone (Outer Cone): Surrounding the inner cone, the outer cone is less
hot than the inner cone. In this zone, the wax vapors do not burn
completely, leading to the formation of tiny soot particles, which are
responsible for the yellow color.
- Luminous
Zone: The luminous zone is the outermost layer of the flame, visible
as a faint blue outer envelope. This is the coolest part of the flame,
where combustion is incomplete and unburnt carbon particles are released,
causing the yellow color.
- Heat
Flow: Heat is transferred upward and outward from the flame. The
highest temperature is at the tip of the inner cone.
Question : Name
the unit in which the calorific value of a fuel is expressed.
The unit in which the calorific value of a fuel is expressed
is "kilojoules per kilogram" (kJ/kg) or sometimes in "joules
per gram" (J/g). Calorific value represents the amount of heat energy
released when a specific quantity of a substance is burned. It's a crucial
measure in understanding the efficiency and potential uses of different fuels.
Question : Explain how CO2 is able to control fires.
Answer: Carbon dioxide (CO2) is able to control fires
because it works by smothering the fire. Fire requires oxygen to sustain
itself, and CO2 is a gas that is denser than air. When CO2 is released onto a
fire, it forms a layer above the fire, preventing oxygen from reaching the
fuel. Without oxygen, the fire cannot continue to burn, and it is extinguished.
Question : It is difficult to burn a heap of green leaves
but dry leaves catch fire easily. Explain.
Answer: Green leaves have a high water content, which
makes them difficult to burn. Water is an excellent heat absorber, and the
moisture in green leaves absorbs the heat from the fire, making it challenging
for the leaves to reach the temperature required for ignition. Dry leaves, on
the other hand, have very little moisture content. Therefore, they can reach
the ignition temperature more quickly, allowing them to catch fire easily.
Question : Which zone of a flame does a goldsmith use for
melting gold and silver and why?
Answer: A goldsmith uses the outermost zone of a
flame for melting gold and silver. The outermost zone of a flame is less hot
compared to the inner blue zone but is more steady and directed. This makes it
ideal for delicate operations like melting gold and silver, as it provides a
consistent and controlled heat without the risk of overheating or burning the
precious metals.
Question : In an experiment, 4.5 kg of a fuel was
completely burnt. The heat produced was measured to be 180,000 kJ. Calculate
the calorific value of the fuel.
Answer: Calorific value (CV) is the amount of heat
energy produced when a specific amount of a substance is burned completely. It
is calculated using the formula:
Calorific Value=Heat Produced (in kJ)Mass of Fuel Burnt (in kg)Calorific Value=Mass of Fuel Burnt (in kg)Heat Produced (in kJ)
Using the given values: Calorific Value=180,000 kJ4.5 kg=40,000 kJ/kgCalorific Value=4.5kg180,000kJ=40,000kJ/kg
Question : Can the process of rusting be called
combustion? Discuss.
Answer: Rusting is a chemical process in which iron
reacts with oxygen and water to form iron oxide (rust). While both rusting and
combustion involve reactions with oxygen, they are different processes.
Combustion is a rapid chemical reaction that produces heat and light, while
rusting is a slow oxidation process that occurs over time. Therefore, rusting
cannot be called combustion because it doesn't involve a rapid release of
energy in the form of heat and light.
Question : Abida and Ramesh were doing an experiment in
which water was to be heated in a beaker. Abida kept the beaker near the wick
in the yellow part of the candle flame. Ramesh kept the beaker in the outermost
part of the flame. Whose water will get heated in a shorter time?
Answer: Ramesh's water will get heated in a shorter
time. The outermost part of the flame is the hottest part. Placing the beaker
in this zone ensures that it receives the maximum heat energy, leading to
faster heating compared to the yellow part of the flame, where the temperature
is lower.
1 Mark Question
Question 1: An important liquid fuel, used in homes is ………..
Answer: An important liquid fuel used in homes is kerosene.
Question 2: What are the main constituents of biogas and
kitchen gas (L.P.G.) ?
Answer: The main constituents of biogas are methane and
carbon dioxide. Kitchen gas (L.P.G.) primarily consists of propane and butane.
Question 3: What is biogas ?
Answer: Biogas is a renewable energy source produced by the
breakdown of organic matter in the absence of oxygen. It mainly contains
methane gas and is used for cooking and lighting.
Question 4: Fuel must be heated to its ……….. temperature
before it starts burning.
Answer: Fuel must be heated to its ignition temperature
before it starts burning.
Question 5: ……….. is a liquid fuel.
Answer: Petrol is a liquid fuel.
Question 6: When fuels burn, what do they produce?
Answer: When fuels burn, they produce heat, light, and
various gases.
Question 7: List conditions under which combustion can take
place.
Answer: Combustion can take place under the following
conditions:
Presence of fuel
Presence of oxygen
Ignition temperature of the fuel must be reached.
Question 8: Define combustion.
Answer: Combustion is a chemical process in which a
substance reacts rapidly with oxygen and releases energy in the form of heat
and light.
Question 9: What name is given to substances which can burn
easily?
Answer: Substances which can burn easily are called
combustible substances.
Question 10: Give two examples of combustible substances.
Answer: Two examples of combustible substances are wood and
paper.
Question 11: What name is given to substances which do not
burn?
Answer: Substances which do not burn are called
non-combustible substances.
Question 12: Classify the following as combustible and
non-combustible substances- Paper, iron nails, cloth, glass.
Answer:
Combustible: Paper, cloth
Non-combustible: Iron nails, glass
Question 13: How is heat and light produced in the sun?
Answer: Heat and light in the sun are produced due to
nuclear fusion, where hydrogen atoms combine to form helium, releasing a
tremendous amount of energy.
Question 14: When does a substance start burning?
Answer: A substance starts burning when it reaches its
ignition temperature and is exposed to oxygen.
Question 15: Give two examples of inflammable substances.
Answer: Two examples of inflammable substances are gasoline
and natural gas.
Question 16: When does a fire brigade arrive?
Answer: The fire brigade arrives when there is an emergency
call reporting a fire incident.
Question 17: How does the fire brigade put off fire?
Answer: The fire brigade puts off fire by using water, foam,
or fire extinguishers to remove one of the elements necessary for combustion,
typically the heat.
Question 18: When water is poured on a fire, which condition
of combustion is not fulfilled?
Answer: When water is poured on a fire, the condition of
heat is not fulfilled, as water cools down the temperature and prevents the
fire from spreading.
Question 19: What is meant by rapid combustion?
Answer: Rapid combustion is a type of combustion that occurs
very quickly and is often accompanied by the production of heat and light.
Explosions are examples of rapid combustion.
Question 20: What name is given to combustion which takes
place on its own?
Answer: Combustion that takes place on its own is called
spontaneous combustion.
Question 21: When crackers are ignited, what type of
combustion takes place?
Answer: When crackers are ignited, explosive combustion
takes place due to rapid and intense reactions producing heat, light, and
sound.
Question 22: What is a flame?
Answer: A flame is the visible, gaseous part of a fire. It
consists of hot gases and fine particles emitting light and heat during
combustion.
Question 23: Which substances burn with a flame?
Answer: Substances that produce flammable gases during
combustion burn with a flame. For example, wood, paper, and candles.
Question 24: What is meant by global warming?
Answer: Global warming refers to the long-term increase in
Earth's average surface temperature due to human activities, primarily the
emission of greenhouse gases, leading to climate change.
Question 25: What is acid rain?
Answer: Acid rain is rainwater with a pH level below 5.6,
caused by the emission of sulfur dioxide and nitrogen oxides into the
atmosphere, leading to environmental damage.
Question 26: How is acid rain harmful?
Answer: Acid rain is harmful as it damages vegetation,
aquatic life, buildings, and soil, leading to deforestation, water pollution,
and corrosion of structures.
Question 27: What is the full form of LPG?
Answer: The full form of LPG is Liquefied Petroleum Gas.
Question 28: Why is a smelling agent added to LPG?
Answer: A smelling agent is added to LPG to detect leaks
easily. LPG is colorless and odorless, so the smell is added for safety
purposes to alert people in case of gas leaks.
Question 29: Why is respiration referred to as slow
combustion?
Answer: Respiration is referred to as slow combustion
because it is a gradual process where food molecules react with oxygen in
cells, releasing energy slowly over time.
Question 30: Give one reason why LPG is a better fuel than
kerosene?
Answer: LPG is a better fuel than kerosene because it
produces more heat energy per unit and is cleaner as it produces fewer
pollutants when burned.
Question 31: Why does yellow phosphorus catch fire on its
own?
Answer: Yellow phosphorus catches fire on its own in the
presence of air (oxygen) due to its high reactivity, leading to spontaneous
combustion.
Question 32: A person sleeping in a closed room with burning
coal feels suffocated after some time. Why?
Answer: A person feels suffocated in a closed room with
burning coal because burning coal consumes oxygen and produces harmful gases
like carbon dioxide, which depletes the oxygen level in the closed space,
leading to suffocation.
2 Marks Question
Question 1: Abida and Ramesh were doing an experiment
in which water was to be heated in a beaker. Abida kept the beaker near the
wick in the yellow part of the candle flame. Ramesh kept the beaker in the
outermost part of the flame. Whose water will get heated in a shorter time?
Why?
Answer: Abida's water will get heated in a shorter
time. The yellow part of the candle flame is the hottest zone where complete
combustion occurs and produces more heat. Placing the beaker in this part of
the flame transfers more heat energy to the water, heating it faster than if it
were placed in the outermost part of the flame where the temperature is lower.
Question 2: Explain how CO2 is able to control fires.
Answer: CO2 (carbon dioxide) controls fires by
displacing oxygen. Fire requires oxygen to sustain itself. When CO2 is released
onto a fire, it forms a layer above the fire, cutting off the supply of oxygen.
Without oxygen, the fire cannot continue burning, and it is extinguished.
Question 3: What is fuel? Name two liquid fuels.
Answer: Fuel is any substance that undergoes
combustion and releases energy. Two examples of liquid fuels are petrol and
diesel.
Question 4: Is food a fuel for our body?
Answer: Yes, food is a fuel for our body. It provides
the energy necessary for various bodily functions through a process similar to
combustion, known as cellular respiration.
Question 5: Distinguish between combustible and
non-combustible substances.
Answer: Combustible substances are those that can
catch fire and burn, while non-combustible substances are those that do not
burn or catch fire under normal conditions.
Question 6: Can you burn paper when a burning
matchstick is brought near it? Why?
Answer: Yes, paper can burn when a burning matchstick
is brought near it because paper is a combustible substance. When exposed to
heat, paper undergoes combustion and catches fire.
Question 7: What is a forest fire?
Answer: A forest fire is an uncontrolled fire that
occurs in a forest area, often causing significant damage to wildlife,
vegetation, and sometimes human settlements.
Question 8: Why do different substances catch fire at
different temperatures?
Answer: Different substances have different ignition
temperatures, which is the minimum temperature at which a substance catches
fire. This variation is due to differences in the chemical composition and structure
of the substances.
Question 9: Kerosene oil catches fire faster than
wood. Why?
Answer: Kerosene oil catches fire faster than wood
because it has a lower ignition temperature. Ignition temperature is the
minimum temperature required for a substance to catch fire. Kerosene oil has a
lower ignition temperature compared to wood, so it ignites more quickly when
exposed to a flame.
Question 10: Why is water poured to extinguish
burning wood?
Answer: When water is poured on burning wood, the
temperature of wood falls below its ignition temperature. This prevents fire
from spreading. Water also cuts off the oxygen supply to the wood, further
aiding in extinguishing the fire.
Question 11: Can you put off a fire due to electric
short circuit by using water? Why?
Answer: We cannot put off a fire due to electric
short circuit by using water because water is a conductor of electricity. Using
water can lead to electric shock and may worsen the situation. Fire due to
electrical short circuit should be extinguished using a fire extinguisher
designed for electrical fires.
Question 12: Supposing the kerosene oil catches fire.
How will you put it off?
Answer: A fire due to kerosene oil can be put off by
smothering it with a fire blanket, sand, or soil. These materials cut off the
oxygen supply to the fire, extinguishing it.
Question 13: How is fire extinguished at airports and
petrol pumps?
Answer: Fire at airports and petrol pumps is usually
extinguished using specialized fire extinguishing agents such as foam or dry
chemical powder. These agents are effective in controlling fires involving
flammable liquids like aviation fuel or petrol.
Question 14: What is meant by spontaneous combustion?
Give one example of a substance that shows spontaneous combustion.
Answer: Spontaneous combustion is a phenomenon where
a substance self-ignites without any external heat source. One example is oily
rags, which, when piled together, can undergo spontaneous combustion due to the
heat generated by oxidation processes in the presence of oil.
Question 15: Which substances burn with a flame?
Answer: Substances like wood, paper, petrol, and
candles burn with a flame when exposed to a fire source.
Question 16: Does magnesium burn with a flame? Why?
Answer: Yes, magnesium burns with a brilliant white
flame because it has a very high ignition temperature. When ignited, magnesium
reacts vigorously with oxygen in the air, producing intense heat and light.
Question 17: Camphor burns with a flame, but charcoal
glows. Why?
Answer: Camphor burns with a flame because it undergoes
rapid combustion, producing heat and light. Charcoal, on the other hand, glows
because it undergoes slow combustion (oxidation) at a lower temperature,
emitting a faint red glow without producing an open flame.
Question 18: Hydrogen has the highest calorific
value, but it is not used as a fuel. Why?
Answer: Hydrogen, despite having a high calorific
value, is not commonly used as a fuel because it is highly flammable and
requires special storage and handling precautions. It also tends to leak
easily, making it challenging to store and transport safely.
Question 19: Why cannot petrol be used as a fuel in
stoves at homes?
Answer: Petrol cannot be used as a fuel in stoves at
homes because it has a low ignition temperature and a high volatility, making
it highly flammable. Using petrol in household stoves can lead to dangerous
accidents and fires due to its rapid ignition and explosive nature.
Question 20: Amit and Sunil were doing an experiment
in which water was to be heated in a beaker. Amit kept the beaker near the wick
in the yellow part of the candle flame. Sunil kept the beaker in the outermost
part of the flame. Whose water will get heated in a shorter time? Why?
Answer: Amit's water will get heated in a shorter
time. The yellow part of the candle flame is the hottest zone where complete
combustion occurs and produces more heat. Placing the beaker in this part of
the flame transfers more heat energy to the water, heating it faster than if it
were placed in the outermost part of the flame where the temperature is lower.
MCQ Answer
Question 1: The gas required for combustion is (a)
oxygen
Question 2: Burning of hydrogen is an example of (c)
explosion
Question 3: The gas produced in the working of
soda-acid type fire extinguisher is (a) carbon dioxide
Question 4: The fuel used in the human body to
produce energy, is (b) food
Question 5: Burning of LPG at home is an example of (b)
rapid combustion
Question 6: Which one of the following will show
spontaneous combustion? (a) Sodium
Question 7: Which of the following can be used to
extinguish fire at the petrol pump? (b) Carbon dioxide
Question 8: While shaping gold into ornaments, which
part of the flame directly used by goldsmith? (a) Non-luminous
Question 9: Which fuel is the ideal fuel to be used
at home? (a) LPG