icse class-8 biology copy chapter-6 long question and answer
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Question 1.
Why isn’t hydrogen gas used as a domestic or industrial fuel, although it has a very high calorific value? State three reasons for the answer.
Answer:
Although hydrogen gas has a very high calorific value, it is not used as a domestic or industrial fuel due to the following reasons:
It is expensive.
It burns with an explosion.
It is extremely inflammable, so it is risky to store and transport hydrogen.
Q.uestion 2.
Explain how water gets boiled in paper cup without burning it.
Answer:
When we heat the paper cup containing water, the heat given to it is rapidly transferred to water from the paper cup. The temperature of water goes on increasing until it attains its boiling point, and starts boiling. As, during this process, the heat is continuously being transferred to water; the paper cup does not attain its ignition temperature. Hence, it does not burn.
Question 3.
Why does a piece of paper burn with yellow flame? Give a reason.
Answer:
The inadequate supply of oxygen during the combustion of the piece of paper produces solid carbon particles that rise up in the flame. They become hot and glow to give off yellowish light. This makes the piece of paper burn with a yellowish flame.
Question 4.
It is observed at petrol pumps and airports, that hydrocarbon fire extinguishers are used, instead of soda-acid fire extinguisher. Give reasons why.
Answer:
At petrol pumps and airports, there is more probability of fire break out due to oil. In such situations, soda-acid fire extinguisher does not work as it contains water or uses water to take off fire by cooling down the place. Water being heavier than oil sinks to bottom and hence, fire does not get controlled. In such a case, hydrocarbon fire extinguisher is very useful, as it contains turkey red oil, which causes the foaming of carbon dioxide gas under pressure. The foam covers the surface of the burning substances and dispels the supply of air to control fire.
Question 5.
Explain complete combustion.
Answer:
This type of combustion involves complete burning of the combustible substance. No residue is left behind. Ash or smoke is not given off during or after this type of combustion. Mostly gases, such as hy¬drocarbons go through this form of combustion. On combustion, hydrocarbon produces carbon dioxide, water and heat