Combustion Chapter 7 D.A.V School Science book

 A. Fill in the blanks.

1.A good fuel should have its ignition temperature above the room temperature.

2. Burning of cowdung cakes is an example of slow combustion.

3. The inner zone of a candle flame, is the coldest.

4. The non-luminous zone of a candle flame is the outer part of the flame.

5. During incomplete combustion of a fuel, CO2 gas is formed.

B. State true or false for the following statements.

1. Coal is an inflammable fuel.

Ans- False

2.The outermost part of a candle flame contains unburnt carbon particles.

Ans- False

3. The heat produced, by burning completely one kilogram of a given fuel, is known as the efficiency of that fuel.

Ans- True

4. All solid fuels have a higher calorific value than liquid and gaseous fuels.

Ans- False

5. Sulphur dioxide gas, produced during combustion of fossil fuels, causes acid rain.

Ans- True

C. Tick the correct option.

1. An example of a non-combustible substance is----

Ans- Iron

2. A fuel starts burning only when it is heated to----

Ans- Its ignition temperature

3. The highest temperature zone of a candle flame, is its----

Ans- Blue zone

4. The amount of heat produced, on complete burning of 10 kg of a given fuel, in pure oxygen, equals H kilojoule. The calorific fuel, of that fuel, would equal----

Ans- (0.1 H) kj/kg

5. The gas produced, due to incomplete combustion of carbon fuels, is----

Ans- Carbon dioxide

D. Answer the following questions in brief.

1. Define the term 'combustible material'? Give two examples of combustible material.

Ans- Those substances that burn in the presence of air to produce heat and light are known as combustible material. 

Example- Coal, wood

2. State the three conditions that are needed for combustion to take place.

Ans- The three conditions that are needed for combustion to take place are:-

i) There must be a combustible substance.

ii) Continuous supply of oxygen.

iii) The temperature should be above the ignition temperature of the combustible material.

3. State the condition under which 'slow combustion'  of a material takes place.

Ans- When there is an incomplete supply of oxygen, slow combustion of the material takes place.

4. Define 'calorific value of a fuel'.

Ans- The amount of heat energy produced when 1 kg of any fuel is burnt in the presence of air.

5. State any two characteristics of an ideal fuel.

Ans- Two characteristics of an ideal fuel are:- 

i) High calorific value.

ii) Easy to transport.

6. Why is water is not used in controlling electric fires.

Ans- We can't use water when an electric fire is there because the water is a good conductor of electricity. So, it may cause dangerous shocks.

E. Answer the following questions.

1. Draw the diagram showing the different zones of candle flame. Label the zones which contains:

(a) Unburnt vapours of wax.

(b) Unburnt carbon particles.

 Ans- 

2. Calorific value of wood is 18,000 kj/kg. How much of wood is required to produce 360,000 kg of heat energy?

Ans- Calorific value of wood= 18,000 kj/kg

          Therefore, the amount of wood required to produce 360,000 kg of heat energy = 360,000/18,000 = 20 kg

3. Why are gaseous fuels considered better than liquid or solid fuels.

Ans- Gaseous fuels considered better than liquid or solid fuels because:-

i) Gaseous fuels have higher calorific value.

ii) These produce the fewer amount of pollutants.

iii) It is easier to transport and store gaseous fuels.

4. Why do we say that 'burning of fuels' can cause 'health hazards'?

Ans- Burning of fuels causes air pollution and it also produces smog, carbon monoxide gas results into respiratory problems. Hence, it is said that the burning of fuels can cause health hazards.