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Living Science Solutions for Class 8 Science Chapter 6 - Combustion And Flame

Living Science Solutions for Class 8 Science Chapter 6 Combustion And Flame are provided here with simple step-by-step explanations. These solutions for Combustion And Flame are extremely popular among class 8 students for Science Combustion And Flame Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science Book of class 8 Science Chapter 6 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science Solutions. All Living Science Solutions for class 8 Science are prepared by experts and are 100% accurate.

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 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 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 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 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:

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 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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