Combustion and Flame

Q22. What are the three zones of flame?

Ans. There are three different zones of a flame – innermost zone (dark zone), middle zone (luminous zone) and outer zone non-luminous zone.

Q23. What is combustion? Give example.

Ans. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. Example: Burning of charcoal.

Q24. Why food is called fuel for our body?

Ans. Food is called fuel for our body because in our body food is broken down by reaction with oxygen and heat is produced.

Q25. Why is the innermost zone of a flame black in colour?

Ans. The innermost zone of a flame is black in colour due to presence of unburnt vapours of the combustible material.

Q26. Explain how CO₂ is able to control fires.

Ans. CO₂, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled.

Q27. Which is the best fire extinguisher for fires involving electrical equipment and inflammable materials like petrol?

Ans. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO₂) is the best extinguisher.

Q28. Why is water not used to control fires involving electrical equipment?
Or
Explain why fire caused by electricity should not be extinguished by pouring water?
Or
Why water is not suitable for extinguishing fire caused due to electrical appliances?
Or
Why can we not use water to extinguish fire caused due to electrical appliances?

Ans. Water is not used to control fires involving electrical equipment because water may conduct electricity and harm those trying to douse the fire.

Q29. What are the harmful products released by the burning of fuels?

Ans. The harmful products released by the burning of fuels are unburnt carbon particles, carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxides etc.

Q30. What is rapid combustion? Give one example.

Ans. The combustion that takes place rapidly and produces heat and light is called rapid combustion. Example: Gas burns rapidly and produces heat and light.

Q31. Why are fires produced by burning oil not extinguished by pouring water?
Or
Water is not suitable for fires involving oil and petrol. Explain.
Or
The fire produced by petrol cannot be extinguished by using water. Explain why?

Ans. Water is heavier than oil.  So, it sinks below the oil, and oil keeps burning on top. Thus, water is also not suitable for fires involving oil and petrol.

Q32. What causes global warming?

Ans. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.

Q33. What are the effects of global warming?

Ans. Global warming results in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coastal areas. Low lying coastal areas may even be permanently submerged under water.

Q34. What is combustible substance? Give some examples.

Ans. The substance that undergoes combustion is said to be combustible substance. Some of the combustible substances are wood, coal, charcoal, paper, dry leaves, petrol etc.

Q35. What is non-combustible substance? Give some examples.

Ans. The substance that does not burn is said to be non-combustible substance. Some of the non-combustible substances are soil, stone, glass, water etc.

Q36. Can you burn a piece of wood by bringing a lighted matchstick near it? Explain.

Ans. The ignition temperature of a piece of wood is high which cannot be reached by the small heat produced by a burning matchstick. So, a matchstick cannot light (or burn) a piece of wood directly.

Q37. How forest fires occur during the hottest summer days?
Or
Why do forest fires occur during hot summers?

Ans. During hot summers, sometimes the ignition temperature of dry grass in the forest is reached, which makes the dry grass catch fire. From grasses, it spreads to trees, and very soon the whole forest is on fire.