VIDYA VIHAR CLASS 8 TERM 2 Science
CONTRIBUTOR :- Nidhi Khobragade
CHAPTER 6:- Combustion and Flame
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A) Multiple Choice Question:-
which of the following condition is not necessary for combustion
a) Presence of combustible substance
b) Presence of Oxygen
c) Presence of Nitrogen
d) attainment of Ignition Temperature
Answer:- c) Presence of Nitrogen
2) which of these methods will not extinguish a fire ?
a) removing the combustible substance
b) Reducing the temperature to below the ignition temperature
c) cutting of air supply
d) throwing petrol on the fire
Answer:- d) Throwing petrol on the fire.
3) Which fuel need not burn with a flame?
a) Solid fuel
b) Liquid fuel
c) gaseous fuel
d) all fuels
Answer:- a) Solid Fuel
4) The zone that produces bright light in a candle flame is the _______ zone
a) non-luminous
b) luminous
c) blue
d ) dark
Answer:- b) Luminous
5) the gas formed by incomplete combustion of carbon is
a) Carbon monoxide
b) Carbon dioxide
c) soot
d) water vapour
Answer:- a) Carbon Monoxide
6) the hottest zone in a candle flame is the _______ zone
a) non-luminous
b) luminous
c) blue
d) dark
Answer:- a) Non- Luminous
7) The harmful by-product of a candle flame is _______
a) molten wax
b) carbon monoxide
c) water vapour
d) light
Answer:- b) Carbon Monoxide.
8) Which fuel has the highest calorific value?
a) petrol
b) LPG
c) CNG
d) hydrogen
Answer:- d) Hydrogen
9) which of the following fuels is expected to leave a residue on burning ?
a) Coal
b) Petrol
c)LPG
d) CNG
Answer:- a) Coal
10) which of the following fuels contribute most towards the formation of acid rain
a) hydrogen
b) LPG
c) CNG
d) Coal
Answer:- d) Coal
Objective type questions
B) Fill in the blanks
1) a substanct that can burn in air is called Combustible Substance
2) An inflammable substance has low ignition temperature.
3) if a combustible substance with an ignition temperature of100 C is heated to 20 0 C it wiil undergo Spontaneous Combustion
4) A Dry Chemical Fire Extinguisher are extinguisher uses chemical such as sodium bicarbonate in powder form to output fires Firers
5) Unburned Carbon particles in the luminous zone of a candle flame glow with a yellow colour
6) a substance that is burnt to obtain heat energy is called a Fuel
7) Gaseous fuels have to be stored in sealed containers under high pressure
8) a fuel that has a very Low ignition temperature is dangerous to store
9) Soot is an example of suspended Particulate matter.
10) Fuels like coal and diesel produce Sulphur di oxides and oxides of Nitrogen as undesirable by products of combustion.
C) Say whether the statements are true of false
1) The ignition temperature of coal is high ;hence it is easy to ignite False
2) During an explosion, a large amount of gas is evolved. True
3) Water should be used to extinguish a fire caused by electricity. False
4) Pouring water on a flame not only cools the burning substance but also cuts off the air supply. True
5) In a candle , wax melts results in the evolution of heat is the melting of wax False
6) In a candle flame , the innermost part is the hottest and the outermost part is the coolest zone. False
7) Every combustible substance is a fuel False
8) Hydrogen has the highest calorific value among all fuels and it has no emissions other than water vapour True
9) Burning of coal in thermal power plants can result in acid rain True
10) Soot, a by-product of combustion, is not harmful for us as it consists of only carbon False
Que Short Answer Question
1) what is combustion ?
Answer:- The process by which substances burn in air with the release of heat and light is known as combustion.
2) what are the conditions necessary for combustion to take place?
Answer:- The conditions necessary for combustion to take place are as follows
- A combustible substance is present
- Oxygen ( air)is available
- Ignition temperature is attained.
Que:- What is the difference between combustible and non combustible substances?
Answer:-
Combustible | Non-Combustible |
1) A substance that burns in air to produce heat and light is called a combustible substance. | 1) A substance that does not burns in air is called a non – combustible substances |
2) Wood paper kerosene and liquified petroleum gas (LPG) are example. | 2) Stone , Steel, and gas are example |
Que 4:- write the chemical equation of the reaction that occurs when natural gas is burnt.
Ans:- Natural gas and biogas contains methane which burns to produce carbon dioxide and water.
CH4 +2O2 ———> CO2 + 2H2O + Energy
5) kerosene has a lower ignition temperature than diesel . what does the statement mean?
Answer:- Kerosene is easier to burn because kerosene has a lower ignition temperature than diesel.
6) what kind of fire extinguishers are most often used for extinguishing an electrical fire
Answer:- foam fire extinguishers are most often used for extinguishing an electrical fire.
Que7:- Name the four zones of a candle flame
Answer:- 1) non luminous zone
2) Luminous zone
3 Blue zone
4) dark zone
8) What is the importance of the calorific value of fuels?
Answer:- The calorific value of a fuel is the amount of heat liberated when one kilogram of the fuel burns completely in sufficient supply of oxygen.
Question 9:- why is hydrogen considered a good fuels?
Answer:- Hydrogen has the highest calorific value. Therefore hydrogen considered a good fuel.
10) Name the harmful by-products formed by the burning of diesel and coal.
Answer:- Sulphur dioxide and oxide of nitrogen are the harmful by-product’s formed by the burning of diesel and coal.
Long Answer Questions
Question 1) what is ignition temperature ? how does the difference in ignition temperature of fuels affect their use ?
Answer:- Ignition temperature :- The minimum temperature at which a substance starts running on its own is called its ignition temperature
1) a substance with a very low ignition temperature can catch fire easily, means it is hazardous
2) however, if the ignition temperature of a substance is very high, it becomes difficult to use as a fuel.
3) Substance like petrol, kerosene , alcohol and LPG catch fire at low temperature . ‘They are called inflammable substances.
4) LPG and petrol can catch fire even if there is a small spark nearby therefore, they need to be stored carefully.
Que 2 :- Differentiate between spontaneous combustion and explosion
Answer:- Spontaneous Combustion:- Occurs when a substance starts burning suddenly even without being set on fire. For ex. The ignition temperature of white phosporus is 35 0C . If the temperature in a room rises above 35 0, C white phosphorus will burst into flames spontaneously.
Explosion:- is a combustion reaction that occurs suddenly when a material is heated liberating a large amount of heat, light and sound. Fire crackers going off are a good example of explosion.
Que3:- How can be fire extinguished
Answer:- A fire gets extinguished is any of the conditions necessary for combustion is not met . the following methods can be used to extinguish fire .
- i) Removing the combustible material :- this may not be possible at all times, especially if the combustible material is already burning. However , other combustible material nearby can be removed to prevent the fire from spreading .
- ii) Reducing the temperature to blow the ignition temperature i) This can be done by pouring or spraying water on the fire. Iii) as it evaporations , the water takes heat away from the combustible substances, there by reducing its temperature .
iii) Cutting off air supply:- i) this can be done by pouring a sand or spraying water on the fire. Ii) Sand covers the combustible substance and cuts off the air supply.
iii) water is often sprayed on a fire to form a mist offline droplets.
- iv) The water evaporates to form water vapor and cuts off the supply of air to the combustible substances
- iv) Air supply can also be cut off by directing carbon dioxide on foam from a fire extinguisher into the fire. This method is used for extinguishing electrical and petrol fires
Que4:- Draw a labelled diagram of soda-acid fire extinguisher . explain how it works.
Answer:-
It contains of a small glass bottle filled with suphuric acid that is surrounded by a strong metal vessel containing sodium bicarbonate solution on striking the knob , the bottle containing the acid breaks . the acid reacts with sodium bicarbonate and carbon dioxide is liberated . the carbon dioxide helps extinguish the fire.
Que 5) draw a labelled diagram of a candle flame and explain what happens in each zone.
Answer:-
A flame has four different zone
Zone of non-combustion: – 1) The dark zone: – this is the dark zone that is seen around the wick. Ii) it is the innermost zone and contains unburnt wax vapors iii) Combustion does not take place here as there is no air supply. It is the coolest part of flame.
Zone of incomplete Combustion:- The luminous zone i) In this zone , the wax vapors do not enough for complete combustion.
- ii) Waz is a mixture of hydrocarbon, made up of carbon and hydrogen.
iii) Some of the carbon undergoes incomplete combustion and forms carbon monoxide
- iv) The unburnt carbon particles glow and five the flame its pale yellow color
- v) this is why this zone is called the luminous zone
- vi) This is the middle part of the flame and is moderately hot.
vii) If an object is placed in this zone unburnt carbon settles on it as a black powdery substances called soot.
Zone of complete combustion:- the non luminous zone 1) this is the outermost and hottest region of the flame
- ii) since inadequate air is available there is complete combustion of carbon and hydrogen to form carbon dioxide and water vapor.
Blue Zone. 1) This zone is at the base of the flame ii) the blue color is due to the burning of carbon monoxide produced by the incompetent combustion of carbon particles.
Que 6:- Compare the usefulness of solid , liquid and gaseous fuels.
Answer:- Solid fuels are easy to store , but they have high ignition temperature and they leave behind solid residues and produce smoke when burnt.
Gaseous fuels are difficult to store, they have to be compressed at high pressure and stored in sealed cylinder or tankers. If a leak develop , the fuel can spread very fast . as such a fuel has a low ignition temperature a leak could thus lead to an explosion.
Liquid fuels are easier to store than gaseous fuels and have higher ignition temperatures.
Q.7:- what are the characteristics of a good fuel ? which fuel comes closet to an ideal fuel ?
Answer:- Followings are the characterizes of good fuel
1) It should have good calorific value.
2) Its ignition temperature should be neither too low nor too high
3) It should be cheap and readily available
4) it should be easy and safe to transport, handle and store
5) it should not cause air pollution on burning
6) it should not leave behind a solid residue on burning
Question 8:- write a short note on the harmful by-products of combustion.
Answer:- Combustion can lead to the production of smoke, which contains soot and other small particles. Such small particles are a part of suspended particulate matter (SPM) which can enter the lungs and cause respiratory diseases. It also contributes to smog.
The carbon dioxide released during combustion can increase the percentage of the gas in air. An atom sphere with excess carbon dioxide traps more heat ( greenhouse effect). This can result in an increase in the average temperature of the earth’s atmosphere. This phenomenon is called global warming.
Carbon monoxide is also produced during the combustion of fuels. It is a highly toxic gas, and can even lead to death if inhaled in large quantities.
Some fuels such as coal and diesel , contains sulphur which changes to sulphur dioxide ( SO2) on combustion. Automobile engine gives out oxides of nitrogen. These gases are released with the exhaust of automobile that use diesel and petrol to run. Beside being poisonous, these oxides dissolve in rain water to form acids. This falls to the ground as acid rain.