scientific reasons[2 marks ]

Question 12 Marks

Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer

$-$The ignition temperature of paper is low, so it catches fire easily.
$-$When paper wrapped around an aluminium pipe is brought near a flame, it does not burn because the heal given to
$-$The Paper gets transferred to the aluminium Pipe and ignition temperature of the paper is not attained.

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Question 22 Marks

$\text{LPG}$ is a better domestic fuel than wood.

Answer

$-$Its calorific value is higher than wood.
$-$It neither produces smoke nor does not leave any residue after burning, While wood produces a lot of smoke on burning and leaves behind ashes on burning.
$-$There is no control on the combustion of wood but we can control combustion of $\text{LPG.}$
$-\text{LPG}$ has low ignition temperature while wood has high ignition temperature, so it takes more time to start burning than $\text{LPG}$
$-$So $\text{LPG}$ is better domestic fuel than wood.

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Question 32 Marks

Water is not used to control fires involving electrical equipment.

Answer

$-$Water is good conductor of electricity.
$-$It conducts electricity and may cause electric shock to the person who extinguish fire.
$-$So water is not used to control fires involving electrical equipment.

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Question 42 Marks

Sourav was heating oil to fry potato chips. The cooking oil all of a sudden caught fire. He took water to pour on the fire to extinguish it. But meanwhile his mother came and switched off the gas and covered the wok completely with a plate. $1.$ Do you think pouring water to the burning oil would have worked? Why$?$ $2.$ Do you think what Sourav’s mother had done is right$?$ Why$?$ $3.$ Can you suggest other ways in which we can stop fire due to burning oil$?$ $4.$ What values of Sourav and Sourav’s mother are shown here$?$

Answer

$1.$ No. As oil being lighter than water will come up and spill all around resulting in a major accident. $2.$ Sourav’s mother had done right because she cut off the heat and oxygen supply from the oil. The fire will ultimately extinguish if it doesn’t get the required supply of air. $3.\ (c)$ By using excess of baking soda. By using fire extinguisher. By putting a completely wet towel on the pan to cut off the air supply. $4.$ Sourav is immature in handling such situation but his mother is mature, wise with scientific temperament.

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Question 52 Marks

If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame?

Answer

We will observe that the iron wire will become red hot and start glowing but it will not produce a flame as it is non flammable object.

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Question 62 Marks

Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Answer

Wood pollutes air very much and using wood as a fuel would also lead to deforestation on a large scale. So we still discourage wood as a fuel.

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Question 72 Marks

A person is covered with a blanket when the clothes of that person catch fire.

Answer

$-$When the cloths of a person catch fire, the person is covered with a blanket to extinguish the fire.
$-$This is because the supply of air to the burning cloths is cut off by blanket, due to which the clothes stop burning and the fire gets extinguished.

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Question 82 Marks

Water should not be used to extinguish fire caused due to petrol.

Answer

$-$Petrol is lighter than water and hence it floats on water.
$-$When water is used to extinguish fire due to petrol, the petrol starts floating on water.
$-$Hence, the fire does not extinguish but the fire spreads immediately. Hence, water is not used to extinguish fire caused due to petrol.

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Question 92 Marks

When a paper cup filled with water is heated upon the flame of a candle the paper cup does not burn.

Answer

$-$When the paper cup is kept above the flame of a candle it gets heated.
$-$The heat absorbed by the cup is transferred to the water in the cup. Due to which the water filled in the cup gets heated.
$-$Hence, the paper cup does not reach its ignition point which is very essential for it to burn. Therefore the paper cup does not burn.

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Science scientific reasons[2 marks ]

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