NCERT CLASS 8 SCIENCE CHAPTER 4 COMBUSTION AND FLAME

 NCERT CLASS 8 SCIENCE CHAPTER 4 COMBUSTION AND FLAME

Question 1.

List conditions under which combustion can take place.

Answer:

Combustion can take place when:

(a) there is a combustible substance.

(b) there is oxygen, which supports combustion.

(c) the substance reaches its ignition temperature.

 

Question 2.

Fill in the blanks.

(a) Burning of wood and coal causes _____ of air.

(b) A liquid fuel used in homes is ______.

(c) Fuel must be heated to its ______ before it starts burning.

(d) Fire produced by oil cannot be controlled by ______.

Answer:

(a) pollution

(b) LPG (liquefied petroleum gas)

(c) ignition temperature

(d) water

 

Question 3.

Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer:

The use of CNG (compressed natural gas) in automobiles has reduced pollution in our cities due to the following reasons:

(a) CNG emits fewer harmful gases like carbon dioxide, carbon monoxide, sulphur dioxide, and nitrogen dioxide, which contribute to global warming and acid rain.

(b) CNG combustion leaves behind no residue or particulate matter after burning, reducing air pollution.

 

Question 4.

Compare LPG and wood as fuels.

Answer:

 

LPG:

 

Does not cause pollution on combustion.

Does not produce smoke.

It is a liquid fuel.

Has higher calorific value (55000 kJ/kg).

Can be easily transported, stored in cylinders.

Wood:

 

Pollutes air on combustion.

Produces smoke.

It is a solid fuel.

Has lower calorific value (17000 kJ/kg).

Not easily transported like LPG fuels.

Question 5.

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 aluminum pipe does not.

Answer:

(a) Water is not used to control fires involving electrical equipment because water is a good conductor of electricity and could result in electric shocks to the person trying to extinguish the fire.

 

(b) LPG is a better domestic fuel than wood because it produces no gases or smoke on combustion, leaves no residue, and has a higher calorific value compared to wood.

 

(c) Paper by itself catches fire easily because it has a low ignition temperature. However, when a piece of paper is wrapped around an aluminum pipe, the heat generated gets absorbed by the aluminum pipe, preventing the paper from reaching its ignition temperature and catching fire easily.

 

 

Question 7.

Name the unit in which the calorific value of a fuel is expressed.

Answer:

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

 

Question 8.

Explain how CO2 is able to control fires.

Answer:

CO2 (carbon dioxide) is able to control fires through the following mechanisms:

 

CO2 is heavier than oxygen and forms a blanket around the fire, depriving it of oxygen and suppressing the flames.

By displacing oxygen, CO2 reduces the concentration of oxygen required for combustion.

CO2 absorbs heat and lowers the temperature of the burning material, further aiding in fire control.

Question 9.

It is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.

Answer:

Green leaves contain moisture, which increases their ignition temperature, making it difficult to burn them. On the other hand, dry leaves have no moisture content, resulting in a lower ignition temperature. Therefore, dry leaves catch fire easily as they reach their ignition temperature more quickly compared to green leaves.

 

Question 10.

Which zone of a flame does a goldsmith use for melting gold and silver, and why?

Answer:

A goldsmith uses the outermost zone of a flame, known as the non-luminous zone, to melt gold and silver. This zone is the hottest part of the flame, with the highest temperature. By utilizing the non-luminous zone, the goldsmith can achieve the high temperatures necessary for melting gold and silver efficiently.

 

Question 11.

In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Answer:

Calorific value of a fuel = Heat produced / Amount of fuel

Calorific value = 180,000 kJ / 4.5 kg

 = 40,000 kJ/kg

 

Question 12.

Can the process of rusting be called combustion? Discuss.

Answer:

The process of rusting is not considered combustion. Combustion is a rapid chemical reaction that occurs between a fuel and oxygen, resulting in the release of heat and light. Rusting, on the other hand, is a slow oxidation process that occurs when iron or steel reacts with oxygen and moisture in the presence of air. While rusting does release some heat during the formation of iron oxide, it is a gradual process that does not involve the rapid release of energy and light characteristic of combustion.

 

Question 13.

Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Answer:

Ramesh's water will get heated in a shorter time because he placed the beaker in the outermost part of the flame, which is the hottest zone. The outermost part of the flame has the highest temperature, while the yellow part of the flame, where Abida placed the beaker, has a lower temperature. Therefore, Ramesh's water will receive more heat energy and heat up faster compared to Abida's water.