Extra 30 short questions and answers from the chapter:4 "Combustion and Flame" for Class 8 Science (CBSE):-
1. What is combustion?
Ans: Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. Example: Burning of wood or LPG.
2. What are combustible substances? Give two examples.
Ans: Substances that catch fire and burn to produce heat and light are called combustible substances.
Examples: Wood, Petrol, LPG.
3. What are non-combustible substances? Give two examples.
Ans: Substances that do not catch fire and do not burn are called non-combustible substances.
Examples: Glass, Sand, Iron.
4. What is ignition temperature?
Ans: The minimum temperature at which a substance catches fire is called its ignition temperature.
Example: Wood has a high ignition temperature, whereas petrol has a low ignition temperature.
5. Why does a matchstick catch fire when rubbed against a rough surface?
Ans: A matchstick contains phosphorus on its tip, which has a low ignition temperature. When rubbed against a rough surface, it produces heat due to friction, igniting the matchstick.
6. What are inflammable substances? Give two examples.
Ans: Substances that catch fire very easily and have a low ignition temperature are called inflammable substances.
Examples: Petrol, LPG, Alcohol.
7. Why is water not used to extinguish an oil fire?
Ans: Oil is lighter than water, so it floats on water and continues to burn. Instead, sand or foam is used to cut off the oxygen supply.
8. What are the three necessary conditions for combustion?
Ans:
Fuel
(e.g., wood, petrol, paper)
Oxygen
(from air)
Heat to reach ignition temperature
9. What is rapid combustion? Give an example.
Ans: When a substance burns quickly and produces a large amount of heat and light, it is called rapid combustion.
Example: Burning of LPG in a gas stove.
10. What is spontaneous combustion? Give an example.
Ans: When a substance catches fire on its own without any external heat source, it is called spontaneous combustion.
Example: Burning of phosphorus in air.
11. What is an explosion?
Ans: An explosion is a sudden reaction that releases a huge amount of heat, light, and gases with a loud sound.
Example: Firecrackers bursting.
12. Why does a burning candle go out when covered with a glass jar?
Ans: A candle requires oxygen for burning. When covered with a glass jar, oxygen supply gets cut off, and the flame goes out.
13. What is the function of a fire extinguisher?
Ans: A fire extinguisher removes oxygen or lowers the temperature to stop the fire. Most fire extinguishers release carbon dioxide (CO₂), which does not support combustion.
14. Why is carbon dioxide used in fire extinguishers?
Ans: Carbon dioxide is heavier than air and does not support combustion. It covers the fire and cuts off the oxygen supply, putting out the fire.
15. What is the difference between luminous and non-luminous flames?
Ans:
Luminous flame
: Yellow, bright, and produces
more smoke
(e.g., burning of wood).
Non-luminous flame
: Blue, hotter, and
smokeless
(e.g., LPG stove flame).
16. Why is an LPG stove flame blue?
Ans: LPG burns completely with sufficient oxygen, producing a hot, blue flame that does not produce smoke.
17. What are the different zones of a candle flame?
Ans:
Dark inner zone
– Unburnt wax
vapors
.
Luminous middle zone
– Partial combustion, yellow in
color
.
Non-luminous outer zone
– Complete combustion, hottest part, blue in
color
.
18. Why is the outermost zone of a flame the hottest?
Ans: In the outermost zone, complete combustion occurs due to ample oxygen supply, making it the hottest part of the flame.
19. What is a calorific value?
Ans: The amount of heat energy produced by burning 1 kg of fuel is called its calorific value, measured in kilojoules per kilogram (kJ/kg).
20. Why is wood not considered a good fuel?
Ans:
Produces
smoke
that causes air pollution.
Has a
low calorific value
.
Leads to
deforestation
when used in large amounts.
21. Why is LPG a better fuel than wood?
Ans:
Higher calorific value
– Produces more heat.
Smokeless burning
– Does not cause air pollution.
Easier storage and transport
–
Comes
in cylinders.
22. What are the harmful effects of burning fuels?
Ans:
Produces
carbon dioxide
, leading to global warming.
Incomplete combustion
releases
carbon monoxide
, a poisonous gas.
Causes
air pollution
, leading to health issues.
23. What happens when fossil fuels are burned incompletely?
Ans: Incomplete combustion produces carbon monoxide (CO), which is a poisonous gas that can cause breathing problems and even death.
24. How does burning fossil fuels contribute to global warming?
Ans: Burning fossil fuels releases carbon dioxide (CO₂), which traps heat in the atmosphere, leading to climate change and rising temperatures.
25. Why do goldsmiths use the outermost part of a flame for melting gold and silver?
Ans: The outermost part of a flame is the hottest because it undergoes complete combustion, making it suitable for melting metals.
26. Name two solid fuels, two liquid fuels, and two gaseous fuels.
Ans:
Solid fuels
: Coal, Wood
Liquid fuels
: Petrol, Kerosene
Gaseous fuels
: LPG, CNG
27. Why does kerosene burn with a flame while charcoal does not?
Ans:
Kerosene is a
liquid fuel
, which
vapourizes
and burns with a flame.
Charcoal is a
solid fuel
, which
does not vaporize
and burns without a flame.
28. Why does paper burn faster than wood?
Ans: Paper has a low ignition temperature, so it catches fire quickly, while wood has a higher ignition temperature and takes time to burn.
29. How is CNG better than petrol and diesel?
Ans:
Less pollution
– Produces fewer harmful gases.
Higher calorific value
– Gives more energy.
Cheaper and easily available
.
30. How can we prevent fires at home?
Ans:
Do not store inflammable materials
like petrol near fire sources.
Use fire extinguishers
in kitchens and offices.
Turn off gas stoves
when not in use.
