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SEBA Class 8 Science Chapter 5 Combustion And Flame
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Combustion And Flame
Chapter: 5
PART – I
EXERCISE
1. List conditions under which combustion can take place.
Ans: Conditions necessary for combustion:
(i) Presence of Fuel: A material that can burn must be available.
(ii) Presence of Oxygen: Enough oxygen must be present to react with the fuel.
(iii) Sufficient Heat: The fuel must be heated to its kindling or ignition temperature.
(iv) Continuous Supply: The fuel and oxygen must continue to be supplied for combustion to keep going.
2. Fill in the blanks.
(a) Burning of wood and coal causes _____ of air.
Ans: Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is ______.
Ans: A liquid fuel, used in homes is kerosene.
(c) Fuel must be heated to its ________ before it starts burning.
Ans: Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by _______.
Ans: Fire produced by oil cannot be controlled by water.
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans: CNG (Compressed Natural Gas) burns completely in air and produces air pollutants( Carbon dioxide , carbon monoxide , carbon particles , sulphur oxide , nitrogenn oxide etc) , in very small amounts. Because of this cleaner combustion, CNG is considered a cleaner fuel. The use of CNG in automobiles has helped reduce air pollution in cities by lowering the emission of these harmful gases, thus improving air quality and reducing health risks caused by pollution.
4. Compare LPG and wood as fuels.
Ans:
| Basic of difference | Compare LPG and wood as fuels | Wood |
| Energy Efficiency | Burns more efficiently and produces more heat per unit mass | Less efficient; produces less heat per unit mass |
| Cleanliness | Burns cleanly with very little smoke and ash | Produces smoke, ash, and other pollutants |
| Environmental Impact | Produces less pollution compared to wood | Produces more smoke and pollutants; can contribute to deforestation if not managed sustainably |
| Storage | Stored in cylinders under pressure | Needs dry storage, bulky |
5. Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Ans: Water is a good conductor of electricity. Using water on an electrical fire can cause electric shock to the person trying to extinguish it and can also cause the fire to spread due to short circuits. Therefore, water is unsafe for fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
Ans: LPG burns more cleanly and efficiently than wood. It produces more heat and very little smoke or ash, which makes it healthier and more convenient for indoor cooking. Wood produces smoke, ash, and pollutants, which can cause respiratory problems and makes cooking less efficient.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Ans: Paper by itself catches fire easily because it is combustible and exposed directly to heat and oxygen. When wrapped around an aluminium pipe, the pipe conducts heat away from the paper, lowering its temperature and preventing it from reaching ignition temperature easily. Also, the aluminium acts as a barrier, reducing direct exposure to heat.
6. Make a labelled diagram of a candle flame.
Ans:
7. Name the unit in which the calorific value of a fuel is expressed.
Ans: The calorific value of a fuel is expressed in kilojoules per gram (kJ/g).
8. Explain how CO2, is able to control fires.
Ans: Carbon dioxide is effective in controlling fires because it does not support combustion. When CO₂ is released on a fire, it displaces the oxygen around the burning material. Since fire needs oxygen to keep burning, reducing the oxygen concentration causes the fire to die out. Additionally, CO₂ is a cold gas when released from fire extinguishers, which helps to cool down the burning material. Because of these properties, CO₂ is widely used in fire extinguishers, especially for electrical and liquid fuel fires where water is unsafe.
9. It is difficult to bum a heap of green leaves but dry leaves catch fire easily. Explain.
Ans: Green leaves contain a high amount of moisture, which makes them wet and cool. When you try to burn green leaves, the heat energy first goes into evaporating the water present in them. This process requires a lot of energy and delays the leaves from reaching their ignition temperature. As a result, green leaves do not catch fire easily. On the other hand, dry leaves have very little or no moisture content. Without water to evaporate, dry leaves heat up quickly and reach their ignition temperature faster. This makes dry leaves catch fire and burn easily compared to green leaves.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans: A goldsmith uses the outermost blue zone of the flame for melting gold and silver. Because the outermost blue zone of a flame is the hottest part because complete combustion occurs there. It produces maximum heat, which is necessary to melt metals like gold and silver efficiently. Using this hottest zone ensures quick and effective melting.
11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Ans: Formula:
Calorific Value = Heat produced / Mass of fuel burnt
Given:
Heat produced = 180,000 kJ
Mass of fuel burnt = 4.5 kg
Calorific value = 180,000 kJ / 4.5 kg = 40,000 kJ/kg
The calorific value of the fuel is 40,000 kJ/kg.
12. Can the process of rusting be called combustion? Discuss.
Ans: Rusting is a chemical process in which iron reacts slowly with oxygen and moisture to form iron oxide. Unlike combustion, rusting occurs at room temperature and does not produce heat or light. Combustion is a rapid reaction that releases heat and light energy, commonly called burning. Rusting is a slow, gradual oxidation, whereas combustion is fast and energetic. Therefore, rusting cannot be called combustion because it lacks the rapid energy release, flames, and heat that characterize combustion. Rusting is a type of slow oxidation, not burning.
13. Abida 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?
Ans: In a candle flame, different zones have different temperatures. The outermost part of the flame, which is blue, is the hottest because complete combustion occurs there. This zone produces maximum heat and is ideal for heating. The yellow part of the flame, where incomplete combustion happens, is cooler and produces soot. When Abida kept the beaker near the yellow part, the water received less heat. Ramesh kept the beaker near the outermost blue zone, where more heat is available. Therefore, Ramesh’s water will heat up faster than Abida’s.
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