Basic Science Solutions for Class 8 Science Chapter 7 Combustion And Fuels are provided here with simple step-by-step explanations. These solutions for Combustion And Fuels are extremely popular among Class 8 students for Science Combustion And Fuels Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Basic Science Book of Class 8 Science Chapter 7 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Basic Science Solutions. All Basic Science Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

Page No 75:

Question A.1:

Answer:

(a) A phenomenon in which a substance combines with oxygen and gives out heat and light is called combustion.
(b) A substance that is essential for the combustion of another substance is called a supporter of combustion.
(c) A substance that is burnt with a view to obtain heat (and light) is called fuel.



Page No 76:

Question A.2:

(a) A phenomenon in which a substance combines with oxygen and gives out heat and light is called combustion.
(b) A substance that is essential for the combustion of another substance is called a supporter of combustion.
(c) A substance that is burnt with a view to obtain heat (and light) is called fuel.

Answer:

Ignition temperature of a substance refers to a temperature below which the substance does not undergo combustion.

Page No 76:

Question A.3:

Ignition temperature of a substance refers to a temperature below which the substance does not undergo combustion.

Answer:

A paper cup containing water does not burn when it is placed over a flame because the cold water inside the cup does not allow it to reach its ignition temperature by taking away the heat.

Page No 76:

Question A.4:

A paper cup containing water does not burn when it is placed over a flame because the cold water inside the cup does not allow it to reach its ignition temperature by taking away the heat.

Answer:

The calorific value of a fuel is defined as the amount of heat released by a unit mass of the fuel on its complete combustion.

Page No 76:

Question A.5:

The calorific value of a fuel is defined as the amount of heat released by a unit mass of the fuel on its complete combustion.

Answer:

The order of petrol, diesel and kerosene in increasing boiling range is:
petrol < kerosene < diesel

Page No 76:

Question A.6:

The order of petrol, diesel and kerosene in increasing boiling range is:
petrol < kerosene < diesel

Answer:

Three gaseous fuels are:

(i) Petroleum gas (LPG): It is obtained from petroleum.
(ii) Natural gas: It is extracted from petroleum wells.
(iii) Biogas: It is obtained from the anaerobic fermentation of domestic sewage.

Page No 76:

Question A.7:

Three gaseous fuels are:

(i) Petroleum gas (LPG): It is obtained from petroleum.
(ii) Natural gas: It is extracted from petroleum wells.
(iii) Biogas: It is obtained from the anaerobic fermentation of domestic sewage.

Answer:

The ignition temperature of a fuel should not be below room temperature because it will undergo combustion at room temperature and can catch fire.

Page No 76:

Question B.1:

The ignition temperature of a fuel should not be below room temperature because it will undergo combustion at room temperature and can catch fire.

Answer:

Three disadvantages of using coal as a fuel are:

(a) It releases a lot of soot on burning.
(b) It release pollutants such as oxides of sulphur and nitrogen in the atmosphere.
(c) It releases dust that causes health problems for miners.

Page No 76:

Question B.2:

Three disadvantages of using coal as a fuel are:

(a) It releases a lot of soot on burning.
(b) It release pollutants such as oxides of sulphur and nitrogen in the atmosphere.
(c) It releases dust that causes health problems for miners.

Answer:

Biogas is a fuel that is obtained by the anaerobic fermentation of domestic sewage and animal dung. Methane is the main component of biogas. When it is burnt, carbon dioxide and water vapours are released without forming soot and carbon monoxide. Following reaction occurs:

CH4 + 2O2 → CO2 + 2H2O + Heat

Page No 76:

Question B.3:

Biogas is a fuel that is obtained by the anaerobic fermentation of domestic sewage and animal dung. Methane is the main component of biogas. When it is burnt, carbon dioxide and water vapours are released without forming soot and carbon monoxide. Following reaction occurs:

CH4 + 2O2 → CO2 + 2H2O + Heat

Answer:

Page No 76:

Question C.1:

Answer:

Take a candle and light it. Place a glass tube (with one short and one long arm) bent at a right angle such that the shorter arm is just above the flame. Now, test different gases that are released by the flame.

(a) Take anhydrous copper (II) sulphate (white) in a watch glass and pass the gases over it. The copper sulphate turns blue indicating the presence of water vapours in the gases.
(b) Set up an apparatus such that the longer arm of the tube is inside a test tube containing lime water. Close the test tube with a cork. Now take another bent glass tube B and place one of its end in the test tube. Now suck through tube B. It would be observed that the lime water turns milky as gases from the flame are drawn from the other tube, indicating the presence of carbon dioxide in the gases.
(c) Take a stainless steel dish and place it over the flame for a while. It would be observed that a black solid, called soot, deposits on the underside of the dish.

Page No 76:

Question C.2:

Take a candle and light it. Place a glass tube (with one short and one long arm) bent at a right angle such that the shorter arm is just above the flame. Now, test different gases that are released by the flame.

(a) Take anhydrous copper (II) sulphate (white) in a watch glass and pass the gases over it. The copper sulphate turns blue indicating the presence of water vapours in the gases.
(b) Set up an apparatus such that the longer arm of the tube is inside a test tube containing lime water. Close the test tube with a cork. Now take another bent glass tube B and place one of its end in the test tube. Now suck through tube B. It would be observed that the lime water turns milky as gases from the flame are drawn from the other tube, indicating the presence of carbon dioxide in the gases.
(c) Take a stainless steel dish and place it over the flame for a while. It would be observed that a black solid, called soot, deposits on the underside of the dish.

Answer:

A candle flame consist of the following regions:

(a) Innermost zone: The flame starting from the wick has a small blue colour zone. Here, carbon monoxide, produced by the incomplete combustion of carbon particles, burns to give the characteristic blue colour.
(b) Central zone: It lies above the innermost zone. Here, no combustion takes place. It is not a hot zone. It consists of unburnt vapourised particles of the wax.
(c) Luminous zone: This zone is luminous and consists of pale yellow glow above the central zone. It is formed by the incomplete combustion of wax vapours in limited supply of oxygenated air.
(d) Non-luminous zone: It is the outermost and the hottest zone of the flame. It lies above the luminous flame. It is formed by complete combustion of wax vapours in the presence of plenty of oxygen available from surrounding atmosphere.

Page No 76:

Question C.3:

A candle flame consist of the following regions:

(a) Innermost zone: The flame starting from the wick has a small blue colour zone. Here, carbon monoxide, produced by the incomplete combustion of carbon particles, burns to give the characteristic blue colour.
(b) Central zone: It lies above the innermost zone. Here, no combustion takes place. It is not a hot zone. It consists of unburnt vapourised particles of the wax.
(c) Luminous zone: This zone is luminous and consists of pale yellow glow above the central zone. It is formed by the incomplete combustion of wax vapours in limited supply of oxygenated air.
(d) Non-luminous zone: It is the outermost and the hottest zone of the flame. It lies above the luminous flame. It is formed by complete combustion of wax vapours in the presence of plenty of oxygen available from surrounding atmosphere.

Answer:

Following five factors should be considered while choosing a fuel:

(a) It should have a high calorific value.
(b) It should be cheap.
(c) It should be safe; i.e., one can store, handle and transport it safely.
(d) It should not leave any residue on combustion.
(e) It should have a low ignition point. The ignition point should not be below the room temperature else it will catch fire at room temperature.

Page No 76:

Question D:

Following five factors should be considered while choosing a fuel:

(a) It should have a high calorific value.
(b) It should be cheap.
(c) It should be safe; i.e., one can store, handle and transport it safely.
(d) It should not leave any residue on combustion.
(e) It should have a low ignition point. The ignition point should not be below the room temperature else it will catch fire at room temperature.

Answer:

1. A log of wood attains the ignition temperature slower than wood shavings do.
2. In the liquid state, hydrogen is used as a fuel in spacecrafts.
3.

Fuel Calorific value
(in KJ/g)
Ignition temperature
(in degree celcius)
Remark
A 100 5 Unsuitable
B 80 70 Best
C 70 80 Better
D 60 90 Good

4. A good fuel burns neither too fast nor too slowly.
5. Gaseous fuels are better than solid and liquid fuels.

Page No 76:

Question E.1:

1. A log of wood attains the ignition temperature slower than wood shavings do.
2. In the liquid state, hydrogen is used as a fuel in spacecrafts.
3.

Fuel Calorific value
(in KJ/g)
Ignition temperature
(in degree celcius)
Remark
A 100 5 Unsuitable
B 80 70 Best
C 70 80 Better
D 60 90 Good

4. A good fuel burns neither too fast nor too slowly.
5. Gaseous fuels are better than solid and liquid fuels.

Answer:

(c) cools the burning substance to a temperature below its ignition temperature

Water is used to extinguish fire because it cools the burning substance to a temperature below its ignition temperature.

Page No 76:

Question E.2:

(c) cools the burning substance to a temperature below its ignition temperature

Water is used to extinguish fire because it cools the burning substance to a temperature below its ignition temperature.

Answer:

(d) Hydrogen

In the given options, hydrogen has the highest calorific value.

Page No 76:

Question E.3:

(d) Hydrogen

In the given options, hydrogen has the highest calorific value.

Answer:

(d) Biogas

Biogas is not derived from a fossil fuel.



Page No 77:

Question E.4:

(d) Biogas

Biogas is not derived from a fossil fuel.

Answer:

(a) Carbon

Carbon is the main constituent of coal.

Page No 77:

Question E.5:

(a) Carbon

Carbon is the main constituent of coal.

Answer:

(a) high calorific value

A good fuel should have a high calorific value.

Page No 77:

Question F:

(a) high calorific value

A good fuel should have a high calorific value.

Answer:

A B
(a) Biogas (iv) Anaerobic fermentation of cattle dung
(b) Natural gas (iii) Accumulated in petroleum wells
(c) Petroleum gas (ii) The lowest boiling fraction in petroleum refining
(d) Kerosene (i) A liquid fraction from the refining of petroleum



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