questions in details [4 marks ]

Question 14 Marks

Compare $LPG$ and wood as fuels.

Answer

Wood has been a traditional fuel for both domestic and industrial use. However, it produces a lot of smoke that can cause respiratory problems. Also, wood is obtained from trees. Thus, using wood as a fuel causes deforestation. Therefore, slowly wood is replaced by $LPG$, which is a liquefied form of petroleum gas. It does not give out smoke and other pollutants and is a cleaner fuel. Again, fuel efficiency of $LPG$ is more than that of wood.
The calorific value of $LPG$ is 55000 kJ/kg, while that of wood is between $17000$ to $22000 kJ/kg$.
Hence, $LPG$ is favoured over wood.

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Question 24 Marks

Draw a diagram showing zone containing very hot carbon particles.

Answer

A diagram showing zone containing very hot carbon particles is:

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Question 34 Marks

Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable. If yes, why? If not, why not? In such a condition what should Manu have done?

Answer

Pouring water to extinguish the fire due to oil was not suitable action. It is because oil is lighter than water. So, water will settle down below the oil. The oil floats on water and continues to burn.In such condition, sand or soil should be used. They cut off the supply of air to the fire. Thus, fire can be controlled. But sand or soil will not be available at that place.
So, Manu should have switched off the flame of the burner and put a lid on the frying pan. By doing this, the contact between fuel and oxygen is cut off and the flame will go off,

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Question 44 Marks

What type of combustion is represented by:
$a.$ Burning of white phosphorus in air at room temperature?
$b.$ Burning of $\text{LPG}$ in a gas stove?
$c.$ Ignition of a cracker?
$d.$ Burning of coal dust in a coal mine?

Answer

$a.$ Burning of white phosphorus in air at room temperature is a spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.
$b.$ Burning of $\text{LPG}$ is combustion in which a substance burns rapidly and produces heat and light with the help of external heat.
$c.$ Ignition of a cracker is a combustion in which a substance burns suddenly and produces heat, light and sound with the help of heat or pressure.
$d.$ Burning of coal dust in a coalmine is a spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.

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Question 54 Marks

What makes the middle zone of a candle flame luminous (or light-giving)?

Answer

In the middle zone, partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame.
It is given in the following picture:

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Question 64 Marks

Complete the crossword Fig. with the help of the clues:

Across
$1.$ Non$-$metal which catches fire if exposed to air. $(10)$
$3.$ The lowest temperature at which a substance catches fire is called its $………$ temperature. $(8)$
$5.$ The most common fire extinguisher. $(5)$
Down
$2.$ A chemical process in which a substance reacts with oxygen to give off heat. $(10)$
$4.$ Petrol is used as a $……..$ in automobiles. $(4)$
$6.$ It is as hard as stone and black in colour. $(4)$

Answer

Across.
$1.$ Phosphorus.
$3.$ Ignition.
$5.$ Water.
Down.
$2.$ Combustion.
$4.$ Fuel.
$6.$ Coal.

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Question 74 Marks

What is global warming? Name the gas whose increasing percentage in air is leading to global warming. State a harmful effect which can be caused by global warming.

Answer

Global warming is the rise in temperature of earth atmosphere caused by the excessive amount of carbon dioxide in the air. Due to rise in the temperature of atmosphere, the ice in polar regions will melt very fast, producing a lot of water. Burning of fuels releases carbon dioxide in air in the environment. Increased percentage of carbon dioxide in air is causing global warming. Carbon dioxide gas in the air trap sun’s heat rays by producing greenhouse effect. Rise in water may cause a rise in the sea level leading to floods in coastal areas. The low-lying areas may be completely submerged under water leading to loss of life and property.

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Question 84 Marks

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.

Answer

Three essential requirements to produce fire are as follows:
$i.$ Fuel $($combustible substance$).$
$ii.$ Air $($or oxygen$).$
$iii.$ Heat to acquire the ignition temperature.
The job of fire extinguisher is to cut off the supply of air or to bring down the temperature of fuel or both.
The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. Water extinguishes fire by cooling the burning substance.
For fires involving electrical equipment and inflammable materials like petrol, then carbon dioxide $(Co_2)$ is the best extinguisher.
$Co_2$ being heavier than oxygen, covers the fire like a blanket. Since, the contact between the fuel and oxygen is cut off, so the fire can be controlled. The added advantage of $Co_2$ is that in most cases, it does not harm the electrical equipment.

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Question 94 Marks

Water can boil in a paper cup. How?

Answer

Water can be boiled in a paper cup the method is as described below:
$i.$ Make a paper cup as shown in the figure.
$ii.$ Pour about 50ml of water in it.
$iii.$ Heat it on a flame as shown in the figure.
$iv.$ After a few minutes of heating you will find that the water in the cup is boiling.

In this process the water is boiled in the paper cup without the cup being burning because when we add water to the cup the ignition temperature of the paper is increased and hence it keeps on transferring the heat that it receives from the candle to the water by the process of conduction and causes the water to boil.

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Question 104 Marks

Match the items of Column A with the items of Column $B.$

	Column A		Column B
$(a)$	Oxides of sulphur and nitrogen	$(i)$	fire extinguisher
$(b)$	$CNG$	$(ii)$	incomplete combustion of coal
$(c)$	Oxygen	$(iii)$	very low ignition temperature
$(d)$	inflammable substance	$(iv)$	acid rain
$(e)$	carbon dioxide	$(v)$	necessary for combustion
$(f)$	carbon monoxide	$(vi)$	fuel for automobiles.

Answer

	Column A		Column B
$(a)$	Oxides of sulphur and nitrogen	$(i)$	Acid rain
$(b)$	$CNG$	$(ii)$	fuel for automobiles.
$(c)$	Oxygen	$(iii)$	necessary for combustion
$(d)$	inflammable substance	$(iv)$	very low ignition temperature
$(e)$	carbon dioxide	$(v)$	fire extinguisher
$(f)$	carbon monoxide	$(vi)$	incomplete combustion of coal

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Question 114 Marks

Fill in the blanks using the words given in the box.
$a.$ A chemical process in which a substance reacts with oxygen to give off heat is called $............$
$b.$ Wood, paper, $\text{CNG}$ are $............$ substances.
$c.$ The lowest temperature at which a substance catches fire is called its $............$ temperature.
$d.$ Ignition temperature of $............$ is lower than that of wood.
$e.$ The substances which have very low $............$ temperature and can easily catch fire with a flame are called $............$ substances.
$f.$ The amount of heat energy produced on complete combustion of $1\ kg$ of a fuel is called its $............$

Answer

$a.$ A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
$b.$ Wood, paper, $\text{CNG}$ are combustible substances.
$c.$ The lowest temperature at which a substance catches fire is called its ignition temperature.
$d.$ Ignition temperature of petrol is lower than that of wood.
$e.$ The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
$f.$ The amount of heat energy produced on complete combustion of $1\ kg$ of a fuel is called its calorific value.

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Question 124 Marks

Forest fire produces a lot of air pollution. Write in brief about them reasons of forest fires.

Answer

Reasons of forest fires are:
$i.$ At high temperature, sometimes dry grass catches fire which spreads throughout the forest.
$ii.$ Camp fire may also be a reason.
$iii.$ Due to the spark of lightning from the sky.
$iv.$ The use of fires by villagers to ward off wild animals.
$v.$ Fire lit intentionally by people living around forests for recreation.
$vi.$ Fires started accidentally by careless visitors to forests. When they throw away lighted cigarettes in the forest.
$vii.$ The friction of bamboos due to high wind velocity and rolling stones.

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Question 134 Marks

With the help of an experiment, show that air is necessary for burning of a candle.

Answer

An experiment to show that air is necessary for burning of a candle is as follows:
$i.$ At first, light a candle on a table.
$ii.$ A glass chimney is then put over the candle which is being rested on a few wooden blocks in such a way that air can enter the chimney from the bottom.
$iii.$ It will be seen that in this case, the candle burns freely in this case.
$iv.$ In the second set, we had removed the woods and had allowed the chimney to rest on the table.
$v.$ In this case, the flame of the candle flickers and produces smoke.
$vi.$ In the third arrangement, we had put a glass plate over the chimney of the second arrangement above.
$vii.$ In this case, the flame finally goes off.

From the above three experimental setups we can see that the candle burns freely when there was a continuous supply of air as in the first case. The candle started flickering when we had stopped the air supply from the bottom as in the second case. Finally in the third case when we had removed the supply of air from both the bottom and above the candle goes off.
The above observations clearly suggest that air is necessary for the burning of a candle.

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