NCERT Solutions for Class 8 Science Chapter 6 – Combustion and Flame

The use of CNG in automobiles has reduced pollution in our cities because it is a clean fuel and the amount of unburnt fuel in CNG is very less compared to petroleum. It does not produce smoke and harmful substances that are produced in petroleum.

 

a) Burning of wood and coal causes pollution of air.

 

b)  A liquid fuel used in homes is Liquified Petroleum Gas (LPG).

 

c)  Fuel must be heated to its ignition temperature before it starts burning.

 

d) Fire produced by oil cannot be controlled by water.

 Combustion takes place under the following three conditions: 

(i) In the presence of fuel.

(ii) In the presence of oxygen (air).

(iii) In the presence of heat - the minimum temperature at which a substance catches fire, known as ignition temperature.

 

The difference between LPG and wood as fuels is: 

 

a) Water is not used to control fires involving electrical equipment because water is a good conductor of electricity due to which the person may be electro-conducted and it may damage the equipment.

 

b) LPG is a better domestic fuel than wood because wood produces a lot of smoke on combustion and causes respiratory problems. Also, its efficiency is lower than LPG.

 

c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not because aluminum is a good conductor of heat which absorbs the heat from the paper and the paper does not catch fire. Whereas, the paper by itself catches fire.

 

 A labeled diagram of a candle flame is: 

“Kilo Joule per Kilogram

(kJ/kg)

(��/��)” is used to express the calorific value of a fuel.

 

 We know that combustion takes place under the following three conditions: 

(i) In the presence of oxygen (air).

(ii) In the presence of fuel.

(iii) In the presence of heat - the minimum temperature at which a substance catches fire, known as ignition temperature.

We can control the fire if any one of the three conditions is not met. 

(i)

C

O

2

��2 is heavier than oxygen and hence it acts as a protective blanket and prevents oxygen from reaching the fire.

(ii) Since

C

O

2

��2 is stored in a liquid state, therefore, when it is used on the fire, it expands and cools which lowers down the temperature. This prevents heat from reaching the fire.

 

It is difficult to burn a heap of green leaves, but dry leaves catch fire easily because combustion takes place in the presence of heat - the minimum temperature at which a substance catches fire, known as ignition temperature. Green leaves have high moisture than dry leaves and hence, it takes more time for green leaves to reach ignition temperature than dry leaves.

 

Goldsmiths use the outermost zone of the flame to melt gold and silver because gold and silver have a high melting point and the outermost part undergoes complete combustion and it is the hottest part of the flame which supply the adequate amount of heat required for melting. 

 

The amount of heat released during combustion of

1 kg

1 �� fuel is known as calorific value i.e., 

Calorific Value=

Heat produced during combustion

Quantity of fuel

��������� �����=���� produced during combustion�������� of fuel  

⇒Calorific Value=

180,000

4.5

 kJ/kg

⇒��������� �����=180,0004.5 ��/��

∴Calorific Value=40,000 kJ/kg

∴��������� �����=40,000 ��/��

 

A chemical process in which a substance reacts with oxygen to give off heat is called combustion. When an iron metal combines with the oxygen in the presence of water to form a compound iron oxide is known as rusting. 

Therefore, the process of rusting can be called combustion because in both processes, heat, and light are produced. Rusting is known as slow combustion. 

 

A labeled diagram of a candle flame is: 

Observe that the innermost zone of the candle which is near to the wick is the least hot whereas the outer zone of the candle is the hottest part of the candle flame.

Since Ramesh kept the beaker in the outermost part of the flame, therefore his beaker will be heated in a shorter time as compared to Abida.