COMBUSTION AND FLAME
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| Forest Fires - Why do forests catch fire in hot summers? |
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.
Q5: Your LPG gas stove at home is giving a yellow flame. What can this mean?
Answer: If fuel and air mixture is correct, gas stove gives blue flame. When air supply (oxygen) is insufficient, fuel does not burn completely thus giving a yellow flame. Gas burner requires cleaning and air shutter adjustment to provide sufficient air.
Q6: A flame always points upwards. Why do you think this is so?
(HINT: Gases produced in a flame are hot and hence lighter).
Answer: Hot air is less dense and lighter as compared to cold air. Flame heats the air around it. Hot air rises up and cold air tries to take that space from bottom area of the flame. This makes a circulation effect thus making the flame points upwards.
Q7: When a candle burns, is it possible to get the wax back
a) when the wax melts.
b) after the wax burns.
(HINT: Are these physical changes or chemical changes.)
Answer: Physical changes are reversible while chemical changes are irreversible.
a) The melting of wax is a physical change. The melted wax can be solidified on cooling.
b) Burning of wax is a chemical change in which new products carbon dioxide and water is formed. Wax cannot be restored from the new products formed.
Q8: Why do we wrap a blanket around a person whose clothes have caught fire?
Answer: Burning requires constant supply of oxygen. Wrapping a blanket cuts the supply of oxygen and extinguishes the fire.
Q9: Why do forest fires occur during hot summers?
Answer: There are many reasons for forest catching fire during hot summers:
1. Dry leaves and wood are flammable. Hot air and friction on dried wood is sufficient to raise temperature to its ignition level.
2. Long concentration of sunlight also raises the temperature causing fire.
3. A small fire from a left cigarrette butt or camp fire can cause burning of dried leaves and wood.
Q10: Why is it easier to burn dry leaves but not green leaves?
Answer: Green leaves contains water (moisture) which absorbs heat and raises the ignition temperature. While dry leaves have low ignition temperature and burn easily.
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