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Question:
Give reasons.
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer:
(a) We should not use water when electrical equipment is on fire as water may conduct electricity and give a shock to people dousing the fire. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. Hence, water is not used to control fires involving electrical equipment.
(b) LPG or Liquid petroleum gas is a liquid fuel that produces no smoke on burning. Its calorific value is 55000 kJ/kg and hence does not cause pollution on combustion. While wood is a solid fuel that causes smoke and ash on burning, It has a low calorific value (17000-22000 kJ/kg) and hence causes pollution on burning. LPG can be stored in containers and can easily be transported. Hence, LPG is a better fuel than wood.
(c) The lowest temperature at which a combustible substance catches fire when heated in the air is called its ignition temperature. Paper has a low ignition temperature so, it catches fire easily. When a piece of paper is wrapped around an aluminium, it doesn't catch fire easily as aluminium absorb heat and paper does not get its ignition temperature. Hence, paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
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