COMBUSTION AND FLAME
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HOTS Q & A (Class 8 Living Science Textbook Chapter Solutions)
Q1: Only gases burn with a flame. When you burn wood it initially burns with a flame. Later it only glows without a flame. What do you think the answer is?
Answer: Wood contains many chemicals like resin, lignin. When heated, these chemicals broke up and release combustible gases. That's why wood burns initially with a flame. When these gases are exhausted, wood burns without flame.
Q2: Would you consider coal as a good fuel? Give reasons?
Answer: Coal is not considered as a good fuel because of the following reasons:
1. It has low calorific value as compared to Petroleum and LPG.
2. On burning it releases many polluting gases like CO2 which is a green house gas and it causes global warming.
3. Coal contains traces of Sulphur and Nitrogen which on burning gives harmful gases like Nitrogen Oxide and Sulphur Oxide. These gases when mixed with rain water cause acid rain.
4. Unburnt traces of carbon also causes respiratory problems.
Q3: When a fuel is burnt, carbon dioxide (or carbon monoxide) and water vapour are given out. Can you name one fuel which burns without giving off water vapour? Why does it not give out water?
Answer: Charcoal is pure form of carbon. When it burns with oxygen, it forms carbon dioxide and no waters vapours are given off.
Q4: Why does a matchstick burn on rubbing it on the side of the match box?
Answer: Matchstick uses chemicals like phosphorus which has low ignition temperature (35°C). When the stick is rubbed, due to friction heat is generated which is sufficient to raise temperature to above 35°C and ignites the stick.