Combustions and flames, Science, Class 8
1. What is combustion?
Combustion is a chemical process in which a substance reacts with oxygen to give off heat[cite: 240].
2. What are combustible substances?
Substances that undergo combustion are called combustible substances or fuels[cite: 241].
3. Name the three essential requirements for producing fire.
The three requirements are fuel, air (to supply oxygen), and heat (to reach the ignition temperature)[cite: 348].
4. What is ignition temperature?
The lowest temperature at which a substance catches fire is called its ignition temperature[cite: 292].
5. Why is water not used to control electrical fires?
Water may conduct electricity and harm those trying to douse the fire[cite: 355].
6. What are inflammable substances?
Substances which have very low ignition temperature and can easily catch fire with a flame, such as petrol and LPG[cite: 331, 334].
7. Define rapid combustion.
A type of combustion where a gas burns rapidly and produces heat and light[cite: 374].
8. What is spontaneous combustion?
A type of combustion in which a material suddenly bursts into flames without any apparent cause[cite: 376].
9. What is an explosion?
A sudden reaction involving the evolution of heat, light, sound, and a large amount of gas[cite: 115, 116].
10. Name the hottest part of a candle flame.
The outermost zone of complete combustion (blue color) is the hottest part.
11. Why do goldsmiths use the outermost zone of the flame?
They use the outermost zone because it is the hottest part of the flame, which helps in melting gold and silver[cite: 141, 142].
12. What is calorific value?
The amount of heat energy produced on complete combustion of 1 kg of a fuel[cite: 154].
13. What is the unit for calorific value?
It is expressed in kilojoule per kg (kJ/kg)[cite: 155].
14. Why is CO2 considered an excellent fire extinguisher?
It is heavier than oxygen, covers the fire like a blanket to cut off oxygen, and cools down the fuel[cite: 360, 365].
15. What are the harmful effects of burning wood?
It produces smoke that causes respiratory problems and leads to deforestation[cite: 162, 164].
16. What is global warming?
The rise in temperature of the Earth's atmosphere, often caused by increased carbon dioxide concentrations[cite: 171, 172].
17. What causes acid rain?
Oxides of sulphur and nitrogen produced by burning coal, diesel, and petrol dissolve in rainwater to form acids[cite: 176, 177].
18. Why is CNG called a cleaner fuel?
Because it produces harmful products in very small amounts compared to diesel and petrol[cite: 178, 179].
19. Why does a paper cup filled with water not burn when heated?
The heat supplied to the cup is transferred to the water by conduction, preventing the paper from reaching its ignition temperature[cite: 328, 329].
20. What is the poisonous gas produced by incomplete combustion of carbon fuels?
Incomplete combustion produces carbon monoxide (CO) gas, which is very poisonous and can be fatal[cite: 166, 168].
