NCERT 8th Class (CBSE) Science: Combustion and Flame

Question:  A flame always points upwards. Why do you think this is so? (ENT: Gases produced in a flame are hot, and hence lighter.)

Answer: Gases produced in a flame are hot, hence lighter. Therefore they rise up. That is why the flame always points upwards.

Question: When a candle burns, is it possible to get the wax back a. after the wax melts? b. after the wax burns? (HINT: Are these physical changes or chemical changes?)

Answer:

1. After the wax melts, it is possible to get the wax back because it is a physical change.
2. After the wax burns, it is not possible to get the wax back because it is a chemical change.

Question: Why do we wrap a blanket around a person whose clothes have caught fire?

Answer: We wrap a blanket around a person whose clothes have caught fire to cut off the supply of oxygen which eventually extinguishes the fire. However, the blanket must be removed immediately after the fire is extinguished so that the body can cool. Otherwise it will lead to severe burns.

Question: Why do forest fires occur during hot summers?

Answer: During hot summers, the temperature is high, the air has less water vapour, and there is lots of dry vegetation in the forest. All these conditions are favourable for a fire, started by friction between trees, becoming a major forest fire.

Question: Why is it easier to burn dry leaves but not green leaves?

Answer: Green leaves contain moisture and as a result, the ignition temperature of green leaves is much higher than that of dry leaves. Due to this, it is difficult to burn a heap of green leaves but dry leaves catch fire easily.

Question: Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer: CNG is a cleaner fuel; compared to petrol and diesel. CNG produces lesser amount of harmful gases on burning. Due to this, CNG is now being used in automobiles in many cities; which has helped in reducing pollution in our cities.