Chapter 6. COMBUSTION AND FLAME

Exercises :

1. List conditions under which combustion can take place.

Answer : The conditions required for combustion to take place are:

(i) Presence of a combustible material (fuel)

(ii) Availability of oxygen (oxidizer)

(iii) Ignition source (heat or spark)

(iv) Proper mixing of fuel and oxygen (in the right proportion)

(v) Removal of any inhibiting factors or fire suppressants.

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 in cities due to its cleaner burning properties. CNG produces lower levels of harmful emissions such as carbon monoxide, nitrogen oxides, and particulate matter compared to traditional fuels like gasoline and diesel, leading to improved air quality and reduced environmental impact.

4. Compare LPG and wood as fuels.

Answer :   LPG (liquefied petroleum gas) and wood are both used as fuels, but they have some differences:

Availability: LPG is a type of gas that is produced during the refining of crude oil or natural gas extraction. It is readily available in cylinders or piped to homes. Wood, on the other hand, is a renewable resource obtained from trees and is available in the form of logs or chopped wood.

Convenience: LPG is a convenient fuel as it is stored in cylinders and can be easily transported and used for various purposes like cooking, heating, and powering appliances. Wood requires more effort to obtain, chop, and store. It is commonly used for heating and cooking in traditional settings or areas where LPG may not be easily accessible.

Cleanliness: LPG is considered a cleaner fuel compared to wood. When burned, LPG produces fewer emissions and pollutants, making it better for the environment and indoor air quality. Wood, when burned, can release smoke, particulate matter, and other pollutants, which can contribute to air pollution.

Efficiency: LPG is a more efficient fuel than wood. It has a higher energy content, which means it can produce more heat per unit of fuel compared to wood. Wood, although widely used, can be less efficient and may require more fuel to achieve the same level of heating.

Cost: The cost of LPG can vary depending on factors like location and market prices. It is generally more expensive than wood. Wood, on the other hand, is often more affordable or even available for free in areas where trees are abundant.

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 conductor of electricity. It can lead to electric shocks or short circuits, posing a risk to firefighters and individuals. Water can also cause the fire to spread by splashing and carrying electrical currents.

(b) LPG is a better domestic fuel than wood as it burns cleaner with fewer emissions and particulate matter. It provides better heat control, convenience, and eliminates the need for storage and handling of bulky wood. LPG also reduces indoor air pollution and health risks associated with wood smoke.

(c) Paper by itself catches fire easily due to its flammable nature. However, when wrapped around an aluminium pipe, the aluminium acts as a heat conductor and a heat sink, dissipating the heat quickly and preventing the paper from reaching its ignition temperature. The aluminium pipe acts as a barrier, reducing the direct exposure of the paper to heat and flames.

6. Make a labelled diagram of a candle flame.

Answer :

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 called "joules per kilogram" (J/kg) or "kilojoules per kilogram" (kJ/kg).

8. Explain how CO2 is able to control fires.

Answer :  (carbon dioxide) is able to control fires by displacing oxygen. When  is released onto a fire, it forms a blanket of gas that prevents oxygen from reaching the fuel source, thereby inhibiting the combustion process and extinguishing the fire.

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

Answer : Green leaves contain a higher moisture content compared to dry leaves. The moisture in green leaves acts as a natural fire retardant, making it difficult for them to burn. In contrast, dry leaves have low moisture content, making them more susceptible to ignition and rapid combustion.

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 or blue zone, for melting gold and silver. This zone has the highest temperature and is fuel-rich, providing a clean and intense heat that is ideal for melting precious metals efficiently and with minimal impurities.

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 : We know that,

Calorific value = Heat produced / Mass of fuel burned

Given, the heat produced is 180,000 kJ and the mass of the fuel burned is 4.5 kg.

Calorific value

Calorific value = 40,000 kJ/kg

Therefore, the calorific value of the fuel is 40,000 kJ/kg.

12. Can the process of rusting be called combustion? Discuss.

Answer : The process of rusting cannot be called combustion. Combustion is a rapid chemical reaction that occurs between a fuel and an oxidizing agent, typically producing heat and light. Rusting, on the other hand, is a slow oxidation process that occurs specifically with iron or steel in the presence of moisture and oxygen. While both processes involve oxidation, they differ in terms of speed and the nature of the reaction.

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 compared to Abida's. The outermost part of the flame, known as the blue zone, has the highest temperature. Placing the beaker in this region allows for more efficient heat transfer, resulting in faster heating of the water. The yellow part of the candle flame, where Abida placed the beaker, has a lower temperature compared to the blue zone, so the heat transfer to the water will be slower.