Combustion and Flame - Class 8 - Science
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Back Questions - Combustion and Flame | NCERT | Science | Class 8
List conditions under which combustion can take place.
Combustion can take place under the following conditions:
1. Presence of a Fuel: The substance that undergoes combustion is known as fuel. This can be solid, liquid, or gas. Examples are wood, petrol, coal, etc.
2. Supply of Air/Oxygen: For combustion to occur, oxygen is necessary. In most cases, the oxygen is obtained from the air.
3. Ignition Temperature: This is the lowest temperature at which a substance catches fire. The fuel must be heated to its ignition temperature for combustion to begin.
4. Heat Source: At least one source of heat is needed to maintain the reaction. This source can be a lit match, a hot plate, sunlight, or another source of heat that has the capability to raise the temperature of the fuel to its ignition temperature.
5. Sustained chemical chain reaction: Once the necessary temperature is reached and combustion begins, a chain reaction takes place that allows the process to continue as long as fuel and oxygen are available.
When all these conditions are met, combustion can take place.
Fill in the blanks.
(a) Burning of wood and coal causes _________ of air.
(b) A liquid fuel, used in homes is _________ .
(c) Fuel must be heated to its _________ _________ before it starts burning.
(d) Fire produced by oil cannot be controlled by _________ .
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Sign up nowExplain how the use of CNG in automobiles has reduced pollution in our cities.
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Sign up nowCompare LPG and wood as fuels.
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Sign up nowGive 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.
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Sign up nowMake a labelled diagram of a candle flame.
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Sign up nowName the unit in which the calorific value of a fuel is expressed.
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Sign up nowExplain how CO2 is able to control fires.
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Sign up nowIt is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.
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Sign up nowWhich zone of a flame does a goldsmith use for melting gold and silver, and why?
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Sign up nowIn an experiment, 4.5 kg of fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
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Sign up nowCan the process of rusting be called combustion? Discuss.
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Sign up nowAbida 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?
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Sign up nowExtra Questions - Combustion and Flame | NCERT | Science | Class 8
Industrially, hydrogen is prepared by passing hydrogen gas over red-hot coke at high temperature.
In industrial processes, hydrogen is produced by passing steam (not hydrogen gas, as mentioned in the question) over red-hot coke at a high temperature. This process results in the formation of carbon monoxide (CO) and hydrogen gas (H₂) as the primary products. The chemical reaction involved is represented by:
$$ \mathrm{C} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + \mathrm{H}_2 $$
This reaction is also exothermic, releasing heat during the process.
Standard molar enthalpy of formation of $\mathrm{CO}_2$ is equal to:
A. Zero
B. The standard molar enthalpy of combustion of gaseous carbon
C. The sum of standard molar enthalpies of formation of $\mathrm{CO}$ and $\mathrm{O}_2$
D. The standard molar enthalpy of combustion of carbon (graphite)
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Sign up nowThe following statements regarding the characteristics of carbon dioxide $\left(\mathrm{CO}_{2}\right)$ are true except that it is:
A. It is a colorless gas.
B. It is lighter than air.
C. It is an odorless gas.
D. It does not support combustion.
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Sign up nowWhich of the following is not combustible?
A. oxygen
B. hydrogen
C. butane
D. methane
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Sign up nowLong Questions - Combustion and Flame | NCERT | Science | Class 8
Can you explain the role of air in the process of combustion? Why is it essential?
In the process of combustion, air plays a pivotal role as it supplies the necessary oxygen for the process to occur. Combustible substances burn in air and produce heat and light. Without air, the flame from combustion processes will flicker and smokes, and eventually the flame will go off, indicating that combustion cannot occur without air. Hence, air is necessary for combustion. Air supplies the necessary oxygen which reacts with the fuel or combustible materials to produce flames, heat, and light. Consequently, the role of air is essential and has a significant effect on the burning process.
What is the role of ignition temperature in the process of combustion? Can you explain with examples?
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Sign up nowExplain how a fire can be controlled. What are the three essential requirements for causing a fire and how are they used to control it?
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Sign up nowDescribe how acid rain is formed and what impact it has on the environment.
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Sign up nowShort Questions - Combustion and Flame | NCERT | Science | Class 8
Why does water in a paper cup not burn when heated with a flame?
Water in a paper cup does not burn when heated with a flame because the heat supplied to the paper cup is transferred to the water through conduction. This keeps the temperature of the paper below its ignition point (the temperature at which it would burn). Therefore, the paper doesn't reach the necessary high temperature required to burn because the water inside the cup absorbs all the heat.
What is the difference between combustible and non-combustible substances?
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Sign up nowWhy is it harmful to sleep in a room with burning or smouldering coal fire in it?
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Sign up nowWhy is the ignition temperature important in the process of burning a substance?
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