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Revision Notes for Class 8 Science Chapter 6 Combustion and Flame
Class 8 Science students should refer to the following concepts and notes for Chapter 6 Combustion and Flame in Class 8. These exam notes for Class 8 Science will be very useful for upcoming class tests and examinations and help you to score good marks
Chapter 6 Combustion and Flame Notes Class 8 Science
COMBUSTION & FLAME
Disadvantages of incomplete combustion :
During incomplete combustion following problems may arise.
1 During incomplete combustion, a part of the unburnt carbon passes into the atmosphere in the form of soot. This not only wastes the fuel, but also pollutes the atmosphere.
2. During incomplete combustion, carbon monoxide is formed. This gas is highly poisonous in nature and can cause respiratory problems.
Flame :A region of burning gases is called flame. LPG is gas, the region in which it burns constitutes a flame. Wax is solid but it produce flame, when we light a candle with a match stick the heat of burning match stick not only melts wax but also changes it into wax vapour. The wax vapour mixes with air and then catches fire to produce a flame.
Structure of flame : The candle flame is divided into four different zones.
Blue zone : Near the bottom of the flame, there is a zone of blue colour. This is called blue zone. There fresh air rapidly mixes with wax vapour formed from the molten wax. A part of wax vapour almost completely burns and gives rise to blue flame.
Dark inner zone : This zone consists of unburnt wax vapour given off by the molten wax. It is coldest part of the flame. It is slightly dark because a part of wax vapour decompose on heating to form carbon particles.
Luminous zone : In this zone a partial combustion of wax vapour takes place with the liberation of lot of heat energy. The heat energy so liberated partly decomposes the wax vapour into carbon particles. The carbon particles in turn get very hot and emit yellowish light. So the colour of this zone is pale yellow. The unburnt carbon particles are present in this zone.
Non Luminous Zone :
It is a zone of complete combustion of wax vapour and carbon particles. The air form the sides of the flame mixes with unburnt wax vapour and carbon particles from the luminous zone and completely burns them to form carbon dioxide gas and water vapour. It is the hottest part of the flame. Which hardly visible.
DO YOU KNOW?
The Chemical History Of A Candle
The credit for inventing the first primitive candles should go to Egyptians; it was they who first started using papyrus reeds soaked in tallow (animal fat). These were often used to light their homes and cliff caves.
In 1826 Michael Faraday first made the world aware of the full details process by which a candle works. His brilliant series of research-based lectures on (and entitled). The Chemical History of a candle at the Royal institution were delivered as a part of Christmas lecture - a tradition that still continues. This lecture was later printed in book form in 1860. What faraday basically said was that when you burn a candle you see the formation of "A beautiful cup". This cup is formed as the air near the candles moves upward due to the force of the current produced by heat of the candle.
1. List conditions under which combustion can take place.
2. Explain how the use of CNG in automobiles has reduced pollution in our cities.
3. Compare LPG and wood as fuels.
4. 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.
5. Explain how CO2 is able to control fires.
6. It is difficult to burn a heap of green leaves but dry leaves catches fire easily. Explain.
Fuel : Any substance which is easily available and burn in air at moderate rate producing large amount of heat energy without leaving behind any undesirable residue is called a fuel.
Characteristics of a good fuel :
1. It should be cheap and readily available.
2. It should be easy to store
3. It should burn at a slow rate and its combustion should be controllable.
4. It should have low ignition temperature.
5. It should produce very small amount of residues such as ash.
6. It should have large calorific value.
7. It should not produce gases which pollute the air.
8. It should not produce any hazards during transportation.
Classifications of fuels :
On the basis of physical state fuels are classified into three parts.
1. Solid fuels : The fuels which occur in a solid state at room temperature are called solid fuels.
Example : Wood, agricultural residues, charcoal ,coal, coke etc.
2. Liquids fuels : The fuels which occur in a liquid state at room temperature are called liquid fuels.
Example : Liquefied hydrogen, petrol, oil, kerosene, diesel etc.
3. Gaseous fuels : The fuels which occur in a gaseous state at room temperature are called gaseous fuels.
Example : Water gas, producer gas, coal gas, compressed natural gas (CNG) and gobar gas etc.
(i) Natural gas (CH4) : Natural gas is obtained from petroleum mills and mainly contains methane and small quantities of ethane, gobar gas plants contain mainly methane and is a kind of bio-gas produced from animal & plant waste degradation.
(ii) Producer gas (N2 + CO) : Producer gas is a mixture of nitrogen and carbon monoxide in the ratio of 2 : 1. It is prepared by passing air over red hot coke. It is used as a fuel in industries.
2C + O2+ 4H2 → 2CO + 4H2 (Producer gas)
(iii) Water gas (H2 + CO) : Water gas is mainly a mixture of hydrogen and carbon monoxide in the ratio of 1 : 1. It is prepared by passing steam over red hot coke at 1000-1400°C. Water gas is also known as blue gas because it burns with blue flame.
C + H2O(steam) → CO + H2(water gas)
water gas is a much better fuel than producer gas because both hydrogen and carbon monoxide of water gas are combustible.
(iv) Coal gas (CH4 + H2 + CO) : Coal gas is a mixture of methane, hydrogen and carbon monoxide. It is obtained by destructive distillation of coal.
(v) Liquefied petroleum gas (LPG) : LPG is mainly mixture of two hydrocarbon, butane and isobutane which are Liquefied by compressing. A strong smelling substance called ethyl mercaptan (C2H5SH) is added to LPG gas cylinders to help in the detection of gas leakage.
Advantages of LPG :
(i) LPG has high calorific value, hence it is a good fuel.
(ii) It burns with a smokeless flame, so it does not cause pollution.
(iii) It does not produce any poisonous gas on burning.
(iv) It is very easy to handle and convenient to store.
(v) It is a very neat and clean domestic fuel.
Calorific value of a fuel :
The amount of heat energy produced on completely burning one kilogram of fuel in pure oxygen is called calorific value of the fuel.
O R
The amount of heat energy produced on completely burning one gram of fuel in pure oxygen is called calorific value of the fuel.
The calorific value of the fuels is expressed in kilojoules per kilogram (kJ/kg) or (kJ/gm). This value of a fuel depends on its chemical composition and on the amount of water and ash it contains. For example peat has more water than coal, so it has a lower calorific value than coal.
Some common fuels and the ircalo rific values
Uses of fuels :
1. For cooking food in our homes in the form of a kerosene, coal, liquefied petroleum gas (LPG) etc.
2. For transporting men and materials by road, sea and air.
3. For heating boilers in the industries.
4. For generating electricity in thermal power plants.
DO YOU KNOW?
Burning Of Candle An Irreversible Phenomenon
When a candle is lit, the candle wax melts and burns at the same time. While the melted wax can be solidified, we cannot get back the gases evolved in the burning process. So burning of a candle is a irreversible process.
Flame At Zero Gravity
In zero gravity, such as the environment of outer space, the hot combustion products move away from the fuel source, resulting in a spherical flame front. The flame is also believed to be more bluish and efficient in such conditions.
Harmful effects of burning fuels :
The increasing fuel consumption has harmful effects on the environment. The main products formed during the fuel combustion which produce harmful effect are.
1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory disease, such as asthma.
2. Incomplete combustion forms carbon monoxide gas. It is very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that rooms.
3. Combustion of most fuels releases carbon dioxide in the environment. Increased percentage of carbon dioxide in the air cause global warning.
Global warming is the rise in temperature of the earth. This result in melting of polar glaciers. This leads to rise in sea level and floods in the sea coast.
4. Burning of coal and diesel release sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Sulphur dioxide and nitrogen oxide dissolve in rain water to form acid. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
• Make a labelled diagram of a candle flame.
• Name the unit in which the calorific value of fuel is expressed.
• Which zone of a flame does a goldsmith use of melting gold and silver and why ?
• In experiment 4.5kg of fuel was completely burnt. The heat produced was measured to be 180000kJ. Calculate the calorific value of the fuel.
• Can the process of rusting be called combustion ? Discuss.
• 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.
QUICK REVISION
• Combustion is a chemical process in which a substance reacts with oxygen to give off heat and light.
• A fuel is a substance which gives heat on burning.
• Combustible substances are those which burn easily, e.g., paper, wood.
• Non-combustible substances are those which do not burn easily, e.g., iron, glass.
• Ignition temperature : The minimum temperature to which a substance is heated before it starts burning.
• Inflammable substances are those which have a very low ignition temperature and can easily catch fire with a flame.
• Fire extinguisher is based on the principle that either they cut off the supply of air to the combustible substance or it brings down the temperature of the combustible substance below its ignition temperature.
• Types of combustion :
(a) Rapid combustion : In this type of combustion, the substance burns rapidly till the whole substance burns completely.
(b) Spontaneous combustion : In such a combustion, a substance with low ignition temperature catches fire on its own.
(c) Explosion : A reaction in which substance burns with the evolution of heat, light and a large amount of gas is called explosion.
• Calorific value is the amount of heat energy produced on complete combustion of 1 kg of fuel. It is expressed as kJ/kg.
• Conditions essential for combustion :
(a) Presence of combustible material.
(b) Presence of supporter of combustion, i.e., oxygen.
(c) Attainment of ignition temperature.
• Characteristics of a good fuel :
(a) It should have high calorific value.
(b) It should not leave ash behind.
(c) It should be easy to store, transport and handle.
(d) It should have a convenient ignition temperature.
• Acid Rain : Oxides of sulphur and nitrogen dissolve in rain water to form acids. This is known as acid rain.
• Flame is a region where combustion of gases takes place.
• Global warming is the rise in temperature of the environment of the earth
• Fuels are of three types :
(a) Solid fuel : wood, coal
(b) Liquid fuel : Kerosene, petrol.
(c) Gaseous fuel : LPG, Coal gas, CNG.
• Zones of a flame :
(a) Non luminous zone or zone of complete combustion
(b) Luminous zone or zone of incomplete combustion
(c) Innermost zone or zone of no combustion.
EXERCISE-1 COMBUSTION & FLAME
Question. Which fuels are used for running automobiles ?
(A) Cowdung
(B) Petrol
(C) Diesel
(D) (B) and (C)
Answer : D
Question. Which of the following fuel is used in industry ?
(A) Coal
(B) CNG
(C) Petrol
(D) All of the above
Answer : A
Question. Which of the following fuel burns without a flame ?
(A) Candle
(B) Coal
(C) Kerosene oil
(D) None of these
Answer : B
Question. When magnesium burns with oxygen to form ?
(A) Mg(OH)2
(B) MgCO3
(C) MgO
(D) All of the above
Answer : C
Question. Which of the following are produced by nuclear reactions ?
(A) Sun
(B) Heat
(C) Light
(D) All of the above
Answer : B
Question. Which is necessary for combustion ?
(A) Water
(B) Air
(C) Both
(D) None of these
Answer : B
Question. Which gas is the best extinguisher ?
(A) CO2
(B) CO
(C) Water vapour
(D) None of these
Answer : A
Question. Which fuel is an extremely good domestics fuel ?
(A) Charcoal
(B) LPG
(C) Gobar gas
(D) All of the above
Answer : B
Question. Incomplete combustion of fuels gives -
(A) CO2gas
(B) CO gas
(C) CO + H2
(D) None of these
Answer : B
Question. Which fuel have more calorific value ?
(A) Wood
(B) Coal
(C) Kerosene
(D) LPG
Answer : D
Question. The unburnt Carbon particles causing respiratory diseases such as -
(A) TB
(B) Asthma
(C) Both
(D) None of these
Answer : C
Question. The substances which have very low ignition temperature and can easily catch fire with a flame are called :
(A) Luminous substances
(B) Combustible substances
(C) Inflammable substances
(D) All of these
Answer : C
Question. The heat and light that come from burning materials is known as :
(A) Fire
(B) Fuel
(C) Flame
(D) All of these
Answer : C
Question. The conditions which are required for combustion to take place are:
(A) Fuel
(B) Ignition temperature
(C) Oxygen
(D) All of these
Answer : D
Question. How fire can be put off?
(A) By throwing water on it
(B) By using fire extinguishers
(C) By throwing sand or soil over it
(D) Any of these methods.
Answer : D
FILL IN THE BLANKS :
Question. On burning, solid fuels leave_____and______.
Answer : Unburnt carbon, particles smoke
Question. Fuels are compounds of____and_____
Answer : Carbon, hydrogen
Question. 4P + 5O2 → ____
Answer : 2P2O5
Question. _____and_____are non-combustible substances.
Answer : Glass, Iron
Question. Substances that burn easily are called______substances.
Answer : Combustible
Question. A candle flame consists of______important zones.
Answer : Four
Question. A liquid fuel used in home is______
Answer : LPG
Question. Combustion is of____types.
Answer : Three
Question. The innermost zone of flame is known as_______
Answer : Dark
Question. Burning of wood and coal causes ...............of air.
Answer : Pollution
Question. Fuel must be heated to its .....................before its starts burning.
Answer : Kindling temperature
Question. Fire produced by oil cannot be controlled by .....................
Answer : Water
TRUE AND FALSE :
Question. Substances burn in air and form carbon dioxide.
Answer : True
Question. Burnt fuel should always leave some residue.
Answer : False
Question. Water gas is a mixture of carbon monoxide and nitrogen.
Answer : False
Question. Calorific value of coke is more than that of coal.
Answer : True
Question. Natural gas mainly contains methane and traces of ethane and propane.
Answer : True
Question. Wood burns with a flame.
Answer : True
Question. There is a blue colour zone at the base of the wick.
Answer : True
Question. Presence of a combustible substance is not essential for a substances to burn.
Answer : False
Question. Red phosphorus can be stored in a bottle.
Answer : True
Question. Coal gas is obtained from destructive distillation of coal.
Answer : False
Question. Only gases or vapours burn with a flame.
Answer : True
Question. Burning of LPG is slow combustion.
Answer : False
Question. Coal is an inflammable fuel.
Answer : False
Question. Blue zone of a candle flame is the coolest.
Answer : False
Question. CO in air causes acid rain.
Answer : False
MATCH THE FOLLOWING :
Question. Match the substances given in column A with their uses given in column B.
Column–A Column–B
(1) Candle (i) Kerosene
(2) Sodium (ii) Destructive distillation
(3) Blown out candle (iii) Fractional distillation of petroleum
(4) Coal (iv) Methane
(5) Diesel (v) Rusting
(6) Paper (vi) Coal
(7) Natural gas (vii) Fowl smell
(8) Slow combustion (viii) Smelly stream of gas
(9) Thermal power (ix) Combustible
(10) Ethyl mercaptan (x) Flame
Answer : (1) → (x), (2) → (i), (3) → (viii), (4) → (ii), (5) → (iii), (6) → (ix), (7) → (iv), (8) → (v), (9) → (vi), (10) → (vii).
Question. Match the substances given in column A with their uses given in column B.
Column–A Column–B
(1) Combustible (i) Kindling temperature
(2) Non-combustible (ii) Zone of combustion
(3) Ignition temperature (iii) Fire extinguisher
(4) Flame (iv) Petrol
(5) Water (v) Diamond
(vi) Sand
Answer : (1) → (iv), (2) → (v), (3) → (i), (4) → (ii), (5) → (iii)
EXERCISE-2 FOR SUMMATIVE ASSESSMENT
Question. Which of the following is not the attribute of a good fuel?
(A) Low calorific value
(B) Moderate rate of combustion
(C) Fairly cheap and easily available
(D) Safe to handle, store, and transport
Answer : A
Question. Calorific value of a fuel may be defined as :
(A) The amount of heat produced when 1 kg of a fuel is completely burnt
(B) The amount of heat produced when 1g of a fuel is completely burnt
(C) The amount of heat produced when 10 g of a fuel is completely burnt
(D) The amount of heat produced in kilojoules when 1 g of a fuel is completely burnt
Answer : A
Question. Which of the following fuels has the highest calorific value?
(A) Petrol
(B) Hydrogen
(C) LPG
(D) Natural gas
Answer : B
Question. In which zone of a candle flame does complete combustion take place?
(A) Inner
(B) Outer
(C) Middle
(D) All three zones
Answer : B
Question. Which of the following is not a necessary condition of combustion?
(A) Presence of combustible substance
(B) Presence of a supporter of combustion
(C) Attainment of ignition temperature of the fuel
(D) Presence of carbon dioxide.
Answer : D
Question. A luminous flame appears :
(A) Red
(B) Green
(C) Yellow
(D) Blue
Answer : C
Question. Which one of the following is a non-combustible substance?
(A) Coal
(B) Iron
(C) Straw
(D) Wood
Answer : B
Question. Which zone is hotter than luminous zone?
(A) Non-luminous zone
(B) Dark zone
(C) Blue zone
(D) None of these
Answer : C
Question. Water cannot be used in :
(A) Oil fire
(B) Forest fire
(C) Building fire
(D) Electrical fire
Answer : D
Question. Which is a zone of no combustion?
(A) Blue zone
(B) Dark zone
(C) Luminous zone
(D) All of the above
Answer : B
Question. Which is not used as a fire extinguisher?
(A) Water
(B) Carbon dioxide
(C) Oxygen
(D) All of the above
Answer : C
Question. Glass is :
(A) Non-combustible
(B) Combustible
(C) None of these
(D) All of the above
Answer : A
Question. Camphor
(A) Vaporizes in air
(B) Condenses in air
(C) Both (A) & (B)
(D) None of these
Answer : A
Question. Carbon monoxide burns with :
(A) No flame
(B) Yellow flame
(C) Blue flame
(D) All of the above
Answer : B
Question. For combustion reactions :
(A) Air is essential
(B) Combustible substance is essential
(C) Both (A) & (B)
(D) None of these
Answer : C
Question. Dark zone is :
(A) Visible
(B) Invisible
(C) Green in colour
(D) None of these
Answer : B
FILL IN THE BLANKS :
Question. Burning of wood and coal causes_______of air.
Answer : Pollution
Question. A liquid fuel used in homes is______
Answer : LPG
Question. Fuel must be heated to its______before it starts burning.
Answer : Kindling temperature
Question. Fire produced by oil cannot be controlled by_______
Answer : Water
Question. The combustion caused by gas is known as______combustion.
Answer : Spontaneous
Question. _____substances have a low ignition temperature.
Answer : Inflammable
Question. _____wax burns in a candle flame.
Answer : Vapour of
Question. Combustion does not take place in the _____zone of the candle flame.
Answer : Dark
Question. Burning of white phosphorus is an example of_______combustion.
Answer : Spontaneous
Question. Respiration is a type of _____combustion.
Answer : Slow
TRUE AND FALSE :
Question. Incomplete combustion of wax vapours take place in the outer zone of the candle flame.
Answer : False
Question. The outer zone of the candle flame is also known as luminous zone.
Answer : False
Question. Carbon soot and carbon monoxide are produced during incomplete combustion of hydrocarbon fuels.
Answer : True
Question. Water is a combustible substance.
Answer : False
Question. Wood is a combustible substance.
Answer : True
Question. Oxygen is a supporter of combustion.
Answer : True
Question. Combustion is a chemical process in which a substance reacts to give off heat.
Answer : True
Question. Combustion cannot take place in the sun since there is no air in the sun.
Answer : True
Question. Usually substances catch fire at one fixed temperature.
Answer : True
Question. The substances which have very high ignition temperature are called inflammable substances.
Answer : False
Question. The combustion in which a substance burns rapidly and produces heat and light is called rapid combustion.
Answer : True
MATCH THE FOLLOWING :
Question.
Column–A Column–B
(1) Acid Rain (i) CO2
(2) Supporter of Combustion (ii) Global warming
(3) CO2 (iii) Iron nails
(4) Inflammable substance (iv) Matchsticks
(5) Combustible substance (v) Petrol
(6) Non-combustible substance (vi) O2
Answer : (1) → (i), (2) → (vi), (3) → (ii), (4) → (v), (5) → (iv), (6) → (iii)
EXERCISE-3 SOLVED EXERCISE OF NCERT
Question. List conditions under which combustion can take place.
Answer : There are three essential conditions of combustion:
(a) Presence of a combustible substance.
(b) Presence of oxygen, i.e., supporter of combustion.
(c) Attainment of ignition temperature.
Question. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer : CNG is cheap, readily available and highly combustible. It has high calorific value.
It does not produce gases or residues when used in automobiles, so it reduced the pollution in our cities.
The use of CNG in automobiles has reduced pollution in our cities because CNG produces the harmful product in very small amount and it is a cleaner fuel.
Question. Compare LPG and wood as fuels.
Answer : 
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 the fire involving electric equipment because it is a good conductor of electricity and harm those trying to douse the fire.
(b) LPG is better domestic fuel than wood because it neither produce gases nor residues that pollute the environment.
(c) The paper by itself catches fire easily because its ignition temperature is low, while a piece of paper wrapped around an aluminium pipe does not catches fire, because its ignition temperature rises.
Question. Make a labelled diagram of a candle flame.
Answer : 
Question. Name the unit in which the calorific value of a fuel is expressed.
Answer : The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Question. Explain how CO2 is able to control fires.
Answer : CO2, being heavier than oxygen, covers the fire like blanket and also brings down the temperature of fuel.
Since the contact between the fuel and oxygen is cut off, the fire is controlled.
Question. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer : To burn a heap of green leaves is difficult, because its ignition temperature is high, but dry leaves catch fire easily as its ignition temperature switches to low.
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 because it is the hottest zone of the flame (temperature"" 800°C) and is non-luminous in nature.
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 : The calorific value of the fuel:
Calorific value = kJ/kg = 180,000/4.5 kJ/kg = 40,000 kJ/kg.
Question. Can the process of rusting be called combustion? Discuss.
Answer : The process of rusting cannot be called as combustion because neither release of energy nor heat and light are produced during it, while in combustion- release of energy takes place with 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.
IMPORTANT QUESTIONS :
Question. Why water is not suitable as fire extinguisher for fires involving oil and petrol?
Answer : As we know, water is heavier than oil so it sinks below the oil and oil keep burning on the top.
Question. Define combustible substances.
Answer :. The substance that can reacts with oxygen and gives off heat and light i.e., undergo combustion, is called combustible substances.
Question. With the help of given table, mark the combustible and non-combustible substances of your daily life.
Answer :
Question. Why we wrap a blanket around a person who caught fire?
Answer : We wrap a blanket around a person who caught fire, because the fire stops getting the oxygen in required amount for combustion and it extinguishes.
Question. In what form (solid, liquid, gas) carbon dioxide be stored in cylinders?
Answer : Carbon dioxide can be stored as a liquid in cylinders.
Question. With the help of an experiment, show that oxygen (air) is necessary for the burning of a candle.
Answer : To show the experiment we have to fix three lighted candle on a table, and put a glass chimney over it.
The chimney (Fig. a) rested on wooden blocks in such a way that air can enter the chimney-the candle will continue burning, because air can enter the chimney from below.
The chimney (Fig. b) rested on the table with no way for air-the flame flickers and produces smoke.
The chimney (Fig. c) rested on table and covered from top with glass plate-the flame goes off because the air is not available.
Question. What do you mean by inflammable substances?
Answer : The substance which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
Question. What is rapid combustion?
Answer : A combustion that takes place at a high speed is known as rapid combustion.
Question. What is spontaneous burning?
Answer : In a heap of several substances, when they catches fire, they undergo slow oxidation with evolution of heat. At certain stage when the temperature becomes more than the ignition temperature of anyone of the constituentall of a sudden, it starts burning. This is called spontaneous burning.
Question. Give the characteristics of Ideal fuel?
Answer : 1. It is readily available.
2. It is cheap.
3. It burns easily at moderate rate.
4. It produces a large amount of heat.
5. It does not leave behind any undesirable substances.
Question. Charcoal does not burn with flame but glows only.
Answer : Charcoal does not burn with flame because it is not volatile in nature or does not vapourise on burning, so it glows only.
Question. What is meant by the term 'fuel'?
Answer : A fuel is a substance, which may be burnt to produce considerable heat without the formation of undesirable products.
Question. Why we are advised never to sleep in a room with burning coal fire in it?
Answer : The carbon monoxide gas produced by incomplete combustion of coal can kill persons sleeping in that room because it is a poisonous gas.
Question. Why water is not used to control the fire by electric short circuit?
Answer : Water is not used at all to control the fire by electric short circuit because it is a good conductor of electricity and may result in electrocution or electric shock.
Question. What are the constituents of a foam type fire extinguisher?
Answer : The foam type fire extinguisher is also called as 'Soda acid' fire extinguisher. The constituents are:
(a) Dilute sulphuric acid (b) Sodium bicarbonate (soda).
When it is operated, it comes out under pressure in the form of a foam, which covers the surface of fire and controls it.
Question. Describe the various zones of a flame.
Answer : A flame has four zones in it:
(a) The outermost thin transparent faint bluish nonluminous region of complete combustion. It is the hottest zone of the flame.
(b) The middle bright luminous zone of partial combustion. It is the moderately hot zone.
(c) The innermost, coldest dark zone, consists of hot vapour and called as zone of no combustion.
(d) Blue zone near the bottom of the flame is due to burning of CO. CO is formed due to incomplete combustion.
Question. What are the different harmful products formed by the burning of a fuel?
Answer : The harmful product formed by the burning of a fuel:
1. Carbon fuels release unburnt carbon particles. These fine particles are dangerous pollutants causes respiratory problems.
2. Incomplete combustion of carbon fuels gives carbon mono oxide a poisonous gas that can even kill a person sleeping in a closed room.
Question. Explain the term "global warming".
Answer : The combustion of fuels release carbon dioxide in the environment. When the percentage of carbon dioxide increases in the air and makes the earth surface hot-it is believed to cause global warming.
Question. What is acid rain?
Answer :The gases like nitrogen oxide, sulphur dioxide etc. present in the atmosphere as pollutants when these gases dissolve in rain water and form acids and reaches to the earth as rain called as acid rain.
Question. Why CNG is preferable for vehicles in respect to petrol and diesel?
Answer : CNG (compressed natural gas) is preferable for vehicles in respect to petrol and diesel, because:
(a) It leaves least residues.
(b) It is cheaper.
(c) It is eco-friendly.
Question. Why any of the fuel is not considered as an ideal fuel?
Answer : The fuel is not considered as an ideal fuel because all the fuels are differ in their cost and efficiency, i.e., its calorific value.
Question. "Food is a fuel for our body". Explain how?
Answer : In our body, food is broken down by reaction with oxygen and heat is produced that is why food is a type of fuel for our body.
Question. What chemicals are used in the preparation of matchsticks?
Answer : The head of safety match contains only antimony trisulphide and potassium chlorate.
Question. Which have higher ignition temperature - kerosene oil or wood.
Answer : The kerosene oil has lower ignition temperature than wood.
Question. What are the three essential requirements for producing fire?
Answer : The three essential requirements are
(i) Fuel
(ii) Air (to supply oxygen)
(iii) Heat (to raise the temperature of the fuel beyond the ignition temperature).
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CBSE Class 8 Science Chapter 6 Combustion and Flame Notes
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