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Page No 87:

Question 1:

Good fuels have
(a) low ignition temperature and high calorific value.
(b) low ignition temperature and low calorific value.
(c) high ignition temperature and high calorific value.
(d) high ignition temperature and low calorific value.

Answer:

(a) low ignition temperature and high calorific value.
Good fuels should have low ignition temperature (the lowest temperature at which a substance catches fire) and high calorific value (produce a large quantity of heat).

Page No 87:

Question 2:

Calorific value of a fuel is the heat energy produced when
(a) any amount of the fuel is completely burnt.
(b) one kilogram of the fuel is completely burnt.
(c) one milligram of the fuel is completely burnt.
(d) hundred grams of the fuel are completely burnt.

Answer:

(b) one kilogram of the fuel is completely burnt.

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).

Page No 87:

Question 3:

Which poisonous gas is formed as a result of incomplete combustion?
(a) coal gas
(b) carbon monoxide
(c) carbon dioxide
(d) nitrogen oxide

Answer:

(b) carbon monoxide

Incomplete combustion of fuels produces carbon monoxide gas. It is dangerous to burn coal in a closed room because it produces carbon monoxide gas, which can kill persons sleeping in that room.

Page No 87:

Question 4:

Acid rain can be caused by the burning of
(a) petrol.
(b) CNG.
(c) diesel.
(d) coal.

Answer:

(d) coal

Sulphur is found as a trace element in coal. When coal is burnt, sulphur combines with oxygen to form sulphur dioxide (SO2). SO2 does not react with most chemicals found in the atmosphere and can travel long distances. Further, it reacts with water vapors in the atmosphere and causes acid rains.

Page No 87:

Question 5:

To extinguish a flame, which of these methods can be used?
(a) reduce temperature to below ignition temperature
(b) remove combustible substance
(c) cut off air supply
(d) any one of these

Answer:

(d) any one of these

Job of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel, or both. In some cases, a flame can also be extinguished by just removing the combustible substance.

Page No 87:

Question 6:

The hottest zone of a candle flame is
(a) non-luminous zone.
(b) luminous zone.
(c) dark zone.
(d) blue zone at the base of the wick.

Answer:

(a) non-luminous zone.

Non-luminous zone is a zone of complete combustion. It is the outermost and the hottest zone of the flame with plenty of oxygen available from the surrounding atmospheric air.

Page No 87:

Question 7:

Which of these is a solid pollutant?
(a) SPM
(b) carbon monoxide
(c) nitrogen oxides
(d) sulphur dioxide

Answer:

(a) SPM

Suspended Particulate Matter (SPM) consists of mist, dust, fumes and smoke. Common chemicals that constitute SPM are nickel, arsenic and lead. When these particles are inhaled, they tend to accumulate in the lungs and cause respiratory problems and damage the lungs.

Page No 87:

Question 8:

Which of these fuels has the highest calorific value?
(a) CNG
(b) coal
(c) hydrogen
(d) petrol

Answer:

(c) hydrogen

The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).
The calorific values of coal, CNG, hydrogen and petrol are as follows:
Coal: 25,000-33,000 kJ/kg
CNG: 50,000 kJ/kg,
Hydrogen: 150,000 kJ/kg
Petrol: 45,000 kJ/kg

Page No 87:

Question 1:

What is the process of burning of substances with the evolution of heat and light called?

Answer:

The process of burning of substances with the evolution of heat and light is called combustion.

Page No 87:

Question 2:

Most fuels are made up of __________ and __________

Answer:

Most fuels are made up of carbon and hydrogen.



Page No 88:

Question 3:

Which gas is produced when a fuel burns in
(a) sufficient supply of air
(b) insufficient supply of air.

Answer:

 (a) Carbon dioxide gas is formed, when a fuel burns in sufficient supply of air.
 (b) Carbon monoxide gas is formed, when a fuel burns in insufficient supply of air.

Page No 88:

Question 4:

All combustible substances must be heated to the same temperature to make them burn. True of false?

Answer:

False.
Different combustible substances catch fire at different temperatures.
A combustible substance cannot catch fire or burn if its temperature is lower than its ignition temperature.

Page No 88:

Question 5:

Any substance that can burn is an inflammable substance. True of false?

Answer:

False.
Substances that have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

Page No 88:

Question 6:

Spontaneous combustion starts at room Temperature. True of false?

Answer:

True.
The combustion in which a material suddenly bursts into flames without the application of any apparent cause is called spontaneous combustion.
Example: Phosphorous burns in air at room temperature.

Page No 88:

Question 7:

During an explosion, a large amount of __________ is given out.

Answer:

During an explosion, a large amount of gas is given out.

Page No 88:

Question 8:

Water is useful in putting out all fires. True or false?

Answer:

False.
Water cannot be used if electrical equipment is on fire because it can conduct electricity and cause harm.
It is also not suitable to use water to extinguish fires involving inflammable materials like oil and petrol because water is heavier than oil and sinks below it. On the other hand, oil continues to burn on the top.

Page No 88:

Question 9:

In a soda-acid type fire extinguisher, __________ reacts with sulphuric acid to give out carbon dioxide.

Answer:

In a soda-acid type fire extinguisher, sodium hydrogen carbonate (NaHCO3) reacts with sulphuric acid to give out carbon dioxide.

Page No 88:

Question 10:

The __________ zone in a candle flame is the zone of no combustion.

Answer:

The central zone in a candle flame is the zone of no combustion.

Page No 88:

Question 11:

The amount of heat liberated when one kilogram of a fuel is completely burnt in sufficient supply of oxygen is called __________ __________ of the fuel.

Answer:

The amount of heat liberated when one kilogram of a fuel is completely burnt in sufficient supply of oxygen is called calorific value of the fuel.

Page No 88:

Question 12:

Name one unit in which the heat liberated by a fuel is measured.

Answer:

The heat liberated by a fuel is expressed in kilojoule per kg (kJ/kg).

Page No 88:

Question 13:

The lower the ignition temperature the better the fuel. True or false?

Answer:

False.

Fuels should have a moderately low ignition temperature. A moderately low ignition temperature makes a fuel useful.

Page No 88:

Question 14:

What are the fine particles released on burning carbon-containing fuels called?

Answer:

The fine particles released on burning carbon-containing fuels are called SPM (Suspended Particulate Matter).

Page No 88:

Question 1:

What is combustion?

Answer:

Combustion is a chemical process wherein a substance reacts with oxygen to produce energy in the form of heat or light or both. It is an oxidation process because it involves the burning of a fuel, completely or partially in the presence of air. However, the presence of oxygen is not must for oxidation to take place. Elements such as magnesium also burn in an atmosphere of nitrogen or chlorine to form magnesium nitride or magnesium chloride, respectively.

Page No 88:

Question 2:

Distinguish between combustible and non-combustible substances, giving three examples of each.

Answer:

 

Combustible Substances  Non-combustible Substances
  1. The substances that are capable of catching fire and burning are known as combustible substances.
  1. Examples: Coal, petrol, diesel etc.
  1. The substances that are not capable of catching fire and burning are known as non-combustible substances.
  1. Examples: Stone, water etc.

Page No 88:

Question 3:

Why is it more difficult to burn some combustible substances than others?

Answer:

It is more difficult to burn some combustible substances than others because different substances catch fire at different temperatures. Some substances have a low ignition temperature; therefore, they can catch fire more easily than others. Ignition temperature plays a crucial role in determining the combustibility of different substances.

Page No 88:

Question 4:

State the conditions under which combustion occurs.

Answer:

Combustion of a fuel requires:

  • Presence of a combustible substance such as coal, natural gas, wood, etc
  • Presence of air to supply oxygen
  • Availability of heat to raise the temperature of the fuel beyond its ignition temperature

Page No 88:

Question 5:

Why is more care needed to store kerosene than coal, though both are fuels?

Answer:

Both kerosene and coal are fuels, but we need extra care to store kerosene oil because the ignition temperature of kerosene oil is lower than that of coal. If kerosene oil is heated a little, it will catch fire easily. On the other hand, coal needs more heat to catch fire.

Page No 88:

Question 6:

What problem will arise if a fuel has an ignition temperature lower than the average room temperature? Name the type of combustion that will occur in such a case.

Answer:

If a fuel has an ignition temperature lower than the average room temperature, combustion will occur suddenly on its own. Such a combustion is known as spontaneous combustion.

Page No 88:

Question 7:

What is an explosion?

Answer:

Combustion that occurs very rapidly with the evolution of large amount of heat and light accompanied by loud sound is called explosion. Large amount of gas is also released during explosion. Hydrogen gas is an example of one of the explosive gases.

Page No 88:

Question 8:

Why is water not suitable for extinguishing a fire in an electrical appliance?

Answer:

Water is not suitable for extinguishing a fire in an electrical appliance. If water is poured on an electrical fire, it can conduct electricity and cause electric shocks to the person trying to extinguish the fire.

Page No 88:

Question 1:

Name and give chemical equations to show the products of the combustion reaction in
(a) limited supply of air.
(b) sufficient supply of air.

Answer:

(a) limited supply of air
If the supply of air is limited, it can result in incomplete combustion, which, in turn, produces carbon monoxide gas instead of carbon dioxide gas.
 

       2CH4+3O2 2CO+4H2O        

(b) Sufficient supply of air

If the supply of air is sufficient, it can result in complete combustion. The reactants burn in oxygen and produce a limited number of products. When hydrocarbons burn in oxygen, the reaction primarily yields carbon dioxide and water.

      CH4+2O2 CO2+2H2O

Page No 88:

Question 2:

List the different methods by which a fire can be extinguished.

Answer:

Different methods can be used to extinguish fires. They are as follows:

  • Cooling off the burning material.
  • Excluding oxygen from fire
  • Removing fuel from fire
  • Using a flame inhibitor

Page No 88:

Question 3:

What kind of fires can effectively be put out by water? What kind of fires is it not suitable for? Give reasons for both cases.

Answer:

Water cools the combustible material and the temperature is brought below the ignition temperature of the combustible material . This prevents fire from spreading. Water vapours also surround the combustible material and help in cutting off the air supply.
Water can be used when solid materials like paper, wood etc. are on fire. However, it is not suitable for extinguishing fire in electrical appliances. If water is poured on an electrical fire, it can conduct electricity and cause electric shock​s to the person trying to extinguish the fire. Moreover, water should not be used in fires involving hot cooking oil or fat because it can spread such fires.

Page No 88:

Question 4:

How does carbon dioxide extinguish a fire?

Answer:

Carbon dioxide is heavier than air and does not support combustion. Carbon dioxide extinguishes fire by displacing oxygen or by taking away oxygen from the fire triangle. Carbon dioxide is also very cold when it comes out of the extinguisher. Thus, it reduces the temperature of the fuel as well.

Page No 88:

Question 5:

What is calorific value of a fuel? Why is hydrogen not used as a fuel though it has the highest calorific value?

Answer:

The amount of heat liberated, when one kg of a fuel is completely burnt in sufficient supply of oxygen, is called the calorific value of the fuel.
 
Hydrogen is not used as fuel despite the highest calorific value because:
 

  • It is lighter than air; thus, it is not easily available.
  • It cannot be used in liquid form because its melting point is -253⁰C.
  • Hydrogen gas in external air may spontaneously ignite.
  • Hydrogen fire, while being extremely hot, is almost invisible; thus, it can lead to accidental burns.
  • Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition and wide range of combustible fuel-air mixtures.

Page No 88:

Question 6:

How are fuels classified on the basis of their physical state? Give two examples of each.

Answer:

We know that matter can exist in three states - solid, liquid and gas.
On this basis, fuels can be classified as follows:
Solid fuels: Examples: Wood, coal, cattle-dung cakes, bagasse (sugarcane from which juice has been extracted), etc.
Liquid fuels: Examples: Kerosene, liquefied petroleum gas (LPG), fuel oil, petrol, diesel, etc.
Gaseous fuels: Examples: Natural gas, coal gas, water gas, producer gas, biogas, acetylene, etc.

Page No 88:

Question 7:

How is burning of fossil fuels thought to be related to global warming?

Answer:

Combustion of most of the fuels releases carbon dioxide and nitrogen dioxide in the environment. Increase in their concentration in the air causes global warming.



Page No 89:

Question 1:

Explain, with the help of a labelled diagram, how a soda-acid type fire extinguisher works.

Answer:




A soda-acid type fire extinguisher contains concentrated solution of sodium hydrogen carbonate (NaHCO3) in a small cylinder fitted with a glass bottle filled with sulphuric acid. When fire breaks out, the cylinder is hit on a hard surface to break the bottle. Once the bottle breaks, the acid reacts with sodium hydrogen carbonate to evolve carbon dioxide and water in the form of vapours. Carbon dioxide, being a non-supporter of combustion, forms a blanket over the fire and extinguishes it. Water, because of its cooling effect, brings down the temperature below the ignition temperature to check burning.

Page No 89:

Question 2:

Name the different zones of a candle flame. Give details of the conditions in each zone.

Answer:

Different zones of a candle flame



As shown in the above diagram, a candle flame has four zones. They are as follows:
 
Innermost zone: The flame starting from the wick has a small blue colour zone. Here, carbon monoxide produced by the incomplete combustion of carbon particles burns to give blue color.
 
Central zone: It lies above the innermost zone, where no combustion takes place. It is not a hot zone. It consists of unburnt vaporised particles of the wax.
 
Luminous zone:  It is luminous and consists a pale yellow glow above the central zone. It is formed by incomplete combustion of wax vapors in limited supply of oxygenated air.
 
Non-luminous zone:  It is the outermost and the hottest zone of the flame above the luminous flame. It is formed by complete combustion of wax vapours with plenty of oxygen available from the surrounding atmospheric air.

Page No 89:

Question 3:

What are the characteristics of a good fuel?

Answer:

Characteristics of a good fuel are as follows:
1. High calorific value: In order to have high fuel efficiency, a good fuel must have high calorific value.
2. Moderate ignition temperature: A good fuel should have moderate ignition temperature.
3. Burns easily at a moderate rate: A good fuel should burn easily in sufficient air at a moderate rate.
4. Low content of non-combustible material: A good fuel should have low content of non-combustible material.
5. Easy to store and handle: A good fuel should be easy to transport and store at a low cost and without causing any harm.
6. Lower moisture content: A good fuel should have less moisture content.
7. Fuels on combustion should not leave behind any harmful product such as CO, SO2, etc.
8. A good fuel should have low volatile matter content.

Page No 89:

Question 4:

Discuss the impact of burning fuels on air.

Answer:

Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants, which cause respiratory diseases such as asthma.
Incomplete combustion of these fuels gives carbon monoxide gas, which is a very poisonous gas.
Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air causes global warming.
Use of fossil fuels damages the environment with the realease of toxic gases like oxides of nitrogen and sulphur. Such gases are responsible for acid rain, which, in turn, is very harmful for crops, buildings and soil. Moreover, nitric oxide and CFCs are also responsible for depletion of ozone layer.

Page No 89:

Question 1:

Only gases burn with a flame. But when you burn wood, it initially burns with a flame. Later, it only glows without a flame. What do you think is the reason for this?

Answer:

Gases burn with a flame because they vaporise on burning. Wood initially burns with a flame because of the presence volatile gases in it. When all the gases escape, it glows without a flame.

Page No 89:

Question 2:

Would you consider coal as a good fuel? Give reasons.

Answer:

Coal meets nearly all the conditions of being a good fuel. It is economically cheap, easily available and produce no harmful odour on burning. It can be transported easily and its combustion can be controlled. Therefore, coal can be considered as a good fuel.

Page No 89:

Question 3:

When a fuel is burnt, carbon dioxide (or carbon monoxide) and water vapour are given out. Can you name one fuel which burns without giving off water vapour? Why does it not give out water?

Answer:

The fuel that burns without giving out water vapour is coke, which is a pure form of carbon. It has high carbon content and low amount of hydrocarbon.
Water vapour is released from a fuel if it contains hydrogen. However, coke has very low hydrogen content and hence, water vapour is not given out. It simply burns giving carbon dioxide gas.

    C+O2CO2

Page No 89:

Question 4:

Why does a matchstick burn on rubbing it on the side of the matchbox?

Answer:

The head of a safety match contains antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorous. When the match is struck against the rubbing surface, some red phosphorous gets converted into white phosphorous. This immediately reacts with potassium chlorate in the matchstick head to produce heat to ignite antimony trisulphide and start the combustion.

Page No 89:

Question 5:

Your LPG gas stove at home is giving a yellow flame. What can this mean?

Answer:

Yellow flame indicates that the flame tube is starved of oxygen.
There can be various reasons for less oxygen like air holes of the burner may be clogged causing the fuel to burn insufficiently due to insufficient supply of air, or less amount of fuel is available; i.e., the cylinder is about to get empty.

Page No 89:

Question 6:

A flame always points upwards. Why do you think this is so?
(HINT: Gases produced in a flame are hot, and hence lighter.)

Answer:

Hot gas is generally less dense than cool gas. Flame is a very hot gas. As the flame burns, it takes the oxygen from the atmospheric air and heats the surrounding atmospheric gases. Therefore, the hot air around the flame is pushed up because it is less dense. As the air around the flame is pushed up, the air around the flame is drawn towards the flame to take the space of the lighter gases, which are pushed up. Meanwhile, the surrounding cold air is pulled down by gravity and the flame is pressurised all around by the atmospheric gases. This elongates the flame upward. 

Page No 89:

Question 7:

When a candle burns, is it possible to get the wax back
(a) after the wax melts?
(b) after the wax burns?
(HINT: Are these phyusical changes or chemical changes?)

Answer:

Candle is a solid cylindrical block of wax with a wick embedded in it.
a) A candle starts melting when it comes in contact with the heat of the flame. The change in shape and size of the candle takes place. This indicates a physical change from solid state to liquid state. It is possible to get the wax back after it melts.
b) A wax candle acts as a fuel when we burn the candle. It is basically carbon. The wax oxidises (burns) in the flame to yield water and carbon dioxide, which dissipate in the air around the candle in a reaction, which also yields light and heat.  The unburnt carbon is deposited as a black substance called soot. This signifies a chemical change. It is not possible to get the wax back after it burns.

Page No 89:

Question 8:

Why do we wrap a blanket around a person whose clothes have caught fire?

Answer:

For a fire to burn, all three elements - heat, fuel and oxygen - of the fire triangle must be present. If any of these is cut off, fire will be put out. When a fire blanket completely surrounds the person whose clothes caught fire, it seals the person’s body surface around the fire and cuts off the oxygen supply to the fire; thereby, putting out the fire . Thus, fire blanket acts as a fire extinguisher and stops the fire from spreading.

Page No 89:

Question 9:

Why do forest fires occur during hot summers?

Answer:

During hot summers, due to hot environment, the speed of hot air is high. This causes the dry leaves and trees to collide each other. This collision results in friction, which, in turn, produces a spark. Dry leaves easily catch those sparks and start burning. The burned leaves further spread due to air and cause the forest fire to expand.

Page No 89:

Question 10:

Why is it easier to burn dry leaves but not green leaves?

Answer:

The ignition temperature of green leaves is higher than that of the dry leaves because they contain moisture in them. Therefore, its easier to burn dry leaves but not green leaves.



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