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CBSE Questions for Class 8 Chemistry Combustion And Flame Quiz 4 - MCQExams.com
CBSE
Class 8 Chemistry
Combustion And Flame
Quiz 4
Which of the following chemical starts the combustion process in the matchstick?
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antimony sulphide
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potassium chlorate
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phosphorous
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starch and wool
Explanation
Phosphorous which is applied on the head of a matchstick has a very low ignition temperature of $$35^{\circ}$$ C. During rubbing, the heat produced due to friction is sufficient to attain this temperature and combustion takes place.
Option $$C$$ is correct.
The fuels which leave more smoke and ash on burning is/are_____.
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solid fuels
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liquid fuels
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gaseous fuels
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none of these
Explanation
Solid fuels leave more smoke and ash on burning because they contain a large number of impurities.
Which of the following is an ideal fuel?
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Coal
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LPG
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Petrol
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None of these
Explanation
An ideal fuel must possess high calorific value, must have a moderate rate of combustion and must be easily available, there is no emission of harmful substances which means it is an eco-friendly fuel
Coal and petrol on burning emit a high amount of harmful substances in the environment and hence cannot be considered ideal fuels.
LPG is close to being an ideal fuel but still cannot be considered as an ideal fuel as it doesn't have all the characteristics of an ideal fuel.
Hence, the correct option is $$\text{D}$$
Which of the following is a non-combustible substance?
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Coke
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Diamond
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Coal
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Wood
Explanation
Combustible substances can easily catch up the fire and burn rapidly while non-combustible substances cannot be easily ignited.
We know that coke, coal, and wood are fossil fuels and are used for cooking purposes, therefore, these are combustible substances. So the substance which is non-combustible is diamond.
A burning substance will ___________, if the temperature falls below its ignition temperature.
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extinguish
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burn brightly
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dimly burn
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burn with emission of smoke
Explanation
The ignition temperature of a substance is the lowest temperature at which a substance catches fire and burns in the air.
Below this temperature, a substance will stop burning and get extinguished.
The lowest temperature at which a substance starts burning is called _________.
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vaporization temperature
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ignition
temperature
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boiling temperature
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none of these
Explanation
The ignition temperature of a substance is the lowest temperature at which the substance starts burning.
The blue zone in an LPG flame indicates:
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no combustion
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partial combustion
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complete combustion
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none of the above
Explanation
Flame colour helps in determining the changes taking place in the particular region of the flame. In the blue zone, complete combustion of fuel takes place. So, the amount of soot decreases, and the temperature is highest.
Hence, the correct option is C.
Which of the following has the characteristics of a good fuel?
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Coke
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Butane
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Coal
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Kerosene
Explanation
A good fuel must be readily available, a good conductor of heat energy, easy to store, inexpensive and must not produce any toxic gases. Butane is a good fuel as compared to others as it does not leave any residue on burning or does not gives out any smoke.
In soda-acid fire extinguisher, _______ helps in putting out the fire.
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$$CO_2$$
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$$CO_2 + H_2O$$
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$$H_2O$$
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$$H_2O + H_2$$
Explanation
Fire extinguishing is done by the carbon dioxide that is produced when the acid reacts with baking soda in the extinguisher.
$$2NaHCO_3 + H_2SO_4 \rightarrow Na_2SO_4 + 2CO_2 + 2H_2O$$
Water cannot be used as fire extinguisher to put out_________.
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burning wood
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burning oil
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burning cloth
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burning charcoal
Explanation
Water cannot be used as fire extinguisher to put out burning oils like petrol, diesel, etc. Oil is lighter than water and so it floats over it and continues to burn. Moreover, the water can spread and carry the oil with it, thus extending the fire over more area.
The conditions necessary for combustion are ___________.
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presence of combustible substance
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presence of supporter of combustion
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attainment of ignition temperature
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all of these
Explanation
Combustion is a chemical reaction in which a combustible or flammable substance reacts with a gas (which is a supporter of combustion, for example oxygen), releasing heat. The reaction is exothermic in nature. Necessary factors required for a combustion reaction to take place are:
1. A combustible or flammable substance. For example wood, petroleum, etc.
2. Ignition temperature. The fuel must be at or above this 'ignition temperature'.
3. A gas that is a supporter of combustion (oxygen). An abundant supply of such a gas is required.
The type of combustion in which heat, light and sound are produced is:
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rapid
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explosive
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spontaneous
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none of these
Explanation
A very fast combustion reaction in which a large amount of heat, light, and sound is produced is called explosive combustion.
Which of the following statement is not true?
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Flame can be seen over burning solids and liquids
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Type of flame depends upon the amount of oxygen available
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Invisible zone of a flame is the least hot zone
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Blue zone indicates complete combustion of fuel
Explanation
The invisible zone of the flame is also called a non-luminous zone which is the zone of complete combustion. Due to complete combustion, the zone becomes the hottest zone of the flame. Hence option C is correct.
Type of flame produced by a fuel depends upon _________.
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calorific value
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amount of oxygen
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type of fuel and its chemical composition
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all of the above
Explanation
The type of flame produced when fuels burn is based on the amount of oxygen available to the burning fuel.
Hence, option $$B$$ is correct.
Incomplete combustion of a fuel gives _____________.
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$$CO$$
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$$CO_2$$
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$$SO_2$$
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oxides of nitrogen
Explanation
Incomplete combustion represent the process in which fuel is partially burned due to which it causes removal of carbon monoxide gas.
Apurva covered a burning candle with a glass. What'll happen to the burning candle?
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It will extinguish immediately.
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It will burn for some time before being cut off.
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It will continue burning.
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Candle will melt.
Explanation
Fire is a chemical reaction that creates light and heat from oxygen and fuel. A lit candle needs to draw oxygen from the air in order to continue burning. If you limit the amount of air available, the candle's flame eventually goes out once it uses up all the oxygen and candle will burn for some time before being cut off.
Hence, the correct option is $$\text{B}$$
The disadvantages of incomplete combustion is/are:
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unburnt carbon gets released
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air is polluted
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formation of carbon monoxide gas
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all of the above
Explanation
Incomplete combustion leads to the:
Formation of soot which is nothing but unburnt carbon which pollutes the atmosphere.
Formation of carbon monoxide which is a poisonous gas.
From the above points, it is clear that the correct answer is option $$D$$.
Which of the following is a necessary condition for spontaneous combustion to take place?
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Flame is required.
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Spark is replied.
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The temperature of the substance should be increased to ignition temperature.
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Nothing is required.
Explanation
No flame or spark is required to start spontaneous combustion. It is only the rise in temperature to the ignition temperature which starts combustion of material producing heat and light energy.
When wood is used as fuel in chulhahs, gaps are left in between the logs of wood. This is done for _________.
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movement of air
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less consumption of wood
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to control temperature
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None of the above
Explanation
The gaps are left in between the logs of wood because t
he gaps allow air (supporter of combustion) to freely come in contact with wood and hence combustion continues.
When water is poured over burning wood, the fire goes off. This is because _________.
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Temperature of wood decreases
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Temperature of water goes up
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Molecules of water reacts with oxygen present in the flame
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Flame vaporizes
Explanation
Water, when poured over fire absorbs a large amount of heat and hence the temperature of wood falls below the ignition temperature. Thus the fire goes off.
So, the correct answer is option A.
Inflammable substances have a __________ ignition temperature.
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high
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low
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moderate
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none of the above
Explanation
The lowest temperature at which a substance catches fire and undergoes combustion liberating heat and light is called the ignition temperature. Inflammable substances have low ignition temperatures and, so they easily catch fire.
Fire involving electrical equipment can be extinguished by using __________.
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water
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$$CO_2$$
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blanket
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naphthalene
Explanation
In case of fires that involve electrical equipment and inflammable materials, carbon dioxide ($$CO_2$$) is used as extinguisher. It is heavier than oxygen and covers the fire like a blanket. This way, the contact between the fuel and oxygen is cut off, and the fire is controlled. Moreover, carbon dioxide does not harm the electrical equipment.
Which of the following is an excellent fuel?
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Solid fuel
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Liquid fuel
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Gaseous fuel
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None of the above
Explanation
Out of three different types of fuels, gaseous fuel is excellent fuel because it is are easier to handle than solid and liquid fuels.
It is easily transported through pipelines. They do not leave a residue after burning.
Gaseous fuels produce low smoke and have high calorific value,
they have relatively low ignition temperature and produce more heat.
Hence, option C is correct.
In which zone of a candle flame does complete combustion take place?
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Inner
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Outer
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Middle
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All three zones
Explanation
Outer Zone: This zone is blue in colour. It is the hottest part of the flame, because complete combustion takes place in this zone
The head of a safety match contains:
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Antimony trisulphide and potassium chlorate.
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Antimony chloride and potassium sulphide.
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Antimony carbonate
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Potassium sulphide.
Explanation
The heads of safety match contains Antimony trisulphide, potassium chlorate,sulphur ,powdered glass,inert filters and animal glue.
Antimony trisulphide cannot be ignited directly by the heat of friction,even in the presence of oxidizing agent like potassium chlorate and it requires another source of ignition to start the combustion.
That source of ignition comes from the striking surface,which is deposited on the side of the matchbox or on the back cover of the matchbox.
The striking surface contains red phosphorus, powder glass and glue.
When a safety match is rubbed against the striking surface, the friction generates enough to convert a trace of the red phosphorus into white phosphorus.
This is immediately react with potassium chlorate in the match head to produce enough heat to ignite the Antimony tri sulphide and start the combustion.
The process of burning a substance in the presence of air with the evolution of heat is called as __________.
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distillation
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carbonisation
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combustion
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refining
Explanation
The process of burning a substance in the presence of air with the evolution of heat is called combustion.
Option $$\text{C}$$ is correct.
A material when burnt gives out flame because of _________.
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vaporization
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Melting
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Combustibility
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Conductivity
A circular blackish ring on the glass plate kept over the candle flame indicates ___________.
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Complete combustion
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Dark zone of the flame
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Non-luminous zone
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Luminous zone
Explanation
A circular blackish ring on the glass plate kept over the candle flame indicates the luminous zone. The candle flame consists of mainly 3 zones depending on the temperature i.e the outer zone ( hottest among all, blue in colour, complete combustion occurs here and is the non-luminous part of the flame), middle zone ( moderately hot, yellow in colour, partial combution), inner zone (least hot and black in colour, and is the luminous part).
The colour of the flame in luminous zone is:
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Blue
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Red
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Yellow
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Black
Fire is extinguished by __________.
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removing all combustible substances
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cutting off the supply of air
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cooling the burning substance
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all of the above
Explanation
Fire is extinguished by :
(A) Removing all combustible substances.
Fuel is one of the requirements for producing fire. In the absence of fuel, combustion will not occur.
(B) Cutting off the supply of air.
Air (oxygen) is essential for combustion. In the absence of air, combustion will not occur.
(C) Cooling the burning substance.
This reduces the temperature below its ignition temperature. So, the fire will not spread.
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