Chapter 6: Combustion and Flame

Extra Internal Question and Answer:

Question: What is Combustion? Write the types of combustion.

Answer: A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

Types of combustion are:

(i) Rapid combustion – Burns quickly with heat and light.

(ii) Spontaneous combustion – Starts burning on its own without external flame.

(iii) Explosive combustion – Burns suddenly with a loud sound and large amount of heat and light.

Question: What is ignition temperature?

Answer: The lowest temperature at which a substance catches fire is called its ignition temperature.
Question: What are inflammable substances? Give examples.

Answer: The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol and Liquified Petroleum Gas (LPG) .
Question: How Do We Control Fire?

Answer: Fire can be controlled by stopping any one of the three requirements of burning fuel, oxygen, or heat. Water is used to cool the fire and bring down temperature. Sand or fire extinguisher is used to cut off oxygen supply. Fire blankets can also be used. This breaks the fire triangle and stops the fire from spreading.
Question: What is a Fuel?

Answer: A fuel is a substance that burns in air to produce heat energy. It is used for cooking, heating, and running vehicles and machines. Common examples include wood, coal, petrol, and LPG.
Question: What is calorific value?

Answer: The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question: Write the formula used to calculate calorific value of a fuel.

Answer: The formula used to calculate calorific value of a fuel is:

 

It is expressed in kJ/kg (kilojoules per kilogram).

Question:  If 5 kg of a fuel releases 250,000 kJ of heat, calculate its calorific value.

Solution: Given, heat produced = 250,000 kJ and mass of fuel = 5 kg

We have,

Therefore, the calorific value of the fuel is 50,000 kJ/kg
Question: A fuel of mass 2 kg produces 96,000 kJ of heat on complete burning. Find its calorific value.

Solution: Given, heat produced = 96,000 kJ and mass of fuel = 20 kg

We have,

Therefore, the calorific value of the fuel is 4800 kJ/kg
Question: A 2 kg sample of wood produces 28,000 kJ of heat on burning. Calculate its calorific value.

Solution: Given, heat produced = 28,000 kJ and mass of wood = 2 kg

We have,

 

Therefore, the calorific value of the fuel is 14000 kJ/kg .
Question: A fuel has a calorific value of 45,000 kJ/kg. How much heat will be produced by burning 5 kg of it?

Solution: Given, calorific value = 45,000 kJ/kg and mass of fuel = 5 kg

We have, Heat produced = Calorific value × Mass
Heat = 45,000 × 5 = 225,000 kJ

Therefore,the heat produced is 2,25,000 kJ.

Question: Which fuel is better: one with 30,000 kJ/kg or 50,000 kJ/kg? Why?

Answer: A fuel with 50,000 kJ/kg is better. This is because a fuel with higher calorific value produces more heat energy on burning the same amount of fuel. So, 50,000 kJ/kg gives more energy than 30,000 kJ/kg, making it more efficient.

Question: A fuel has calorific value 40,000 kJ/kg. How much fuel is needed to produce 8,000 kJ of heat?

Solution: Here, calorific value = 40,000 kJ/kg and heat produced = 8,000 kJ

We have,

Thus, 0.2 kg (or 200 g) of fuel is needed to produce 8,000 kJ of heat.

Question: If 1 kg of biogas gives 35,000 kJ energy, how much energy will 200 g of biogas produce?

Solution: Here, energy from 1 kg biogas = 35,000 kJ and mass given = 200 g = 0.2 kg

We have,

Energy produced = 35,000 × 0.2 = 7,000 kJ

Thus, 200 g of biogas will produce 7,000 kJ of energy.

Question: How does burning of fuels contribute to global warming?

Answer: Burning of fuels like coal, petrol, and diesel releases carbon dioxide into the air. Carbon dioxide is a greenhouse gas that traps heat in the atmosphere. This increases the Earth’s temperature slowly, which leads to global warming.

Question: What is acid rain and how is it formed?

Answer: Acid rain is a type of rain that contains harmful acids. It is formed when burning of coal and diesel releases sulphur dioxide gas, and petrol engines release oxides of nitrogen into the air. These gases dissolve in rainwater and form acids. Acid rain is very harmful for crops, soil, and buildings.

Exercises :

1. List conditions under which combustion can take place.

Answer: The conditions required for combustion:

(i) Presence of air (oxygen): Air is necessary for burning.

(ii) Fuel: A substance that can burn must be present.

(iii) Sufficient heat (ignition temperature): The fuel must be heated to its ignition temperature.
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) Burning of wood and coal causes pollution of air.

(b) A liquid fuel, used in homes is kerosene.

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

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

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 because it produces harmful products in very small amounts. Therefore, CNG is a cleaner fuel compared to petrol and diesel, and its use helps in reducing air pollution in cities.
4. Compare LPG and wood as fuels.

Answer: the comparison of LPG and Wood as Fuels:

LPG	Wood
(i) It is a clean fuel and produces very little smoke.	(i) It produces a lot of smoke and harmful gases.
(ii) It has high calorific value and burns easily.	(ii) It has lower calorific value.
(iii) It does not leave ash after burning.	(iii) It leaves ash after burning.
(iv) It causes less air pollution.	(iv) It causes more air pollution.

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 aluminium pipe does not.

Answer: (a) Water is not used to control fires involving electrical equipment because water is a good conductor of electricity. It can cause electric shock to the person trying to put out the fire.

(b) LPG is a better domestic fuel than wood because it burns easily, has high calorific value, and produces very little smoke, so it causes less air pollution.

(c) Paper alone reaches its ignition temperature quickly and catches fire. When wrapped around an aluminium pipe, the aluminium conducts away the heat, so the paper does not reach its ignition temperature.
6. Make a labelled diagram of a candle flame.

Answer: The diagram of a candle flame :

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

Answer: The calorific value of a fuel is expressed in the unit kilojoule per kilogram (kJ/kg).
8. Explain how CO₂ is able to control fires.

Answer: Carbon dioxide (CO₂) controls fire because it does not support combustion. Being heavier than air, it forms a blanket over the fire and cuts off the supply of oxygen. Since oxygen is not available, the fire gets extinguished.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer: It is difficult to burn a heap of green leaves because they contain a lot of water. The heat first gets used to evaporate the water, so they do not reach the ignition temperature easily. Dry leaves have very little moisture, so they reach ignition temperature quickly and catch fire easily.

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

Answer: A goldsmith uses the non-luminous (inner blue) zone of a flame for melting gold and silver because it is the hottest part of the flame and provides maximum heat for melting metals quickly.

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: Given, Heat produced = 180,000 kJ and Mass of fuel = 4.5 kg

We know that, 

  

Therefore, the calorific value of the fuel is 40,000 kJ/kg.
12. Can the process of rusting be called combustion? Discuss.

Answer: No, rusting cannot be called combustion. Combustion is a rapid reaction with oxygen that produces heat and light. Rusting, on the other hand, is a slow chemical reaction of iron with oxygen and moisture, forming rust (iron oxide), and it does not release heat or light.

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 kept the beaker in the outermost (blue) part of the flame, which is the hottest zone. Abida’s beaker near the yellow (luminous) part is in a cooler region, so it heats up more slowly.