CBSE Class 8 Science Chapter 6: Combustion and Flame NCERT Solutions

Solution: Combustion, which is the process of burning, requires specific conditions to occur. These are:

1. Fuel: A combustible substance (like wood, paper, LPG, etc.) must be present.
2. Supporter of Combustion: An oxidant, typically oxygen from the air, must be available to react with the fuel.
3. Ignition Temperature: The fuel must be heated to a temperature at or above its specific ignition temperature, which is the minimum temperature at which it starts burning.

Solution:

1. Burning of wood and coal causes pollution of air. These fuels release harmful gases and particulate matter when burned.
2. A liquid fuel, used in homes is Liquefied Petroleum Gas (LPG).
3. Fuel must be heated to its ignition temperature before it starts burning. This is the minimum temperature required for combustion.
4. Fire produced by oil cannot be controlled by water. Since oil is less dense than water and does not mix with it, applying water can spread the oil and the fire.

Solution: Compressed Natural Gas (CNG) has significantly reduced pollution in cities because it is a much cleaner fuel compared to petrol and diesel. When CNG burns, it produces very small amounts of harmful by-products. Unlike other fuels, it releases minimal sulfur dioxide and nitrogen oxides, which are major contributors to air pollution, acid rain, and respiratory problems. Therefore, its widespread use in vehicles leads to cleaner air in urban areas.

Solution: Here is a comparison between LPG and wood as fuels:

Feature	LPG	Wood
Energy content	Produces high energy per unit mass.	Produces less energy per unit mass compared to LPG.
Pollution	Causes less pollution; burns relatively cleanly.	Causes more pollution; releases smoke, soot, and harmful gases.
Residue	Leaves no residue after burning.	Leaves ash as residue.
Storage and Transport	Easily stored and transported in cylinders.	Requires more space for storage and can be bulky to transport.
Ease of Use	Convenient for cooking and heating.	Requires more effort for lighting and maintaining the fire.

Solution:

1. Water is not used to control fires involving electrical equipment because water is a conductor of electricity. If water is used, it can complete an electrical circuit, potentially causing electric shock to the person attempting to douse the fire and damaging the equipment further. Dry chemical or CO2 extinguishers are preferred for such fires.
2. LPG is a better domestic fuel than wood because it has a higher calorific value (produces more heat per unit mass), burns without producing smoke or leaving ash, and can be easily stored and transported. It also ignites easily and burns at a controlled rate, making it more convenient and efficient for cooking and heating purposes.
3. A piece of paper wrapped around an aluminium pipe does not catch fire easily because aluminium is a good conductor of heat. When the paper is heated, the aluminium pipe absorbs the heat rapidly and conducts it away from the paper. This prevents the paper from reaching its ignition temperature. In contrast, paper by itself has a low ignition temperature and can easily reach it when heated, leading to combustion.

Solution: A candle flame has three distinct zones, each with different characteristics:

1. Innermost Zone (Dark Zone): This is the unburnt wax vapour zone. It is the coolest part of the flame and is dark because there is no combustion happening here.
2. Middle Zone (Luminous Zone): This zone is where partial combustion of wax vapour occurs. It is yellowish and produces light because of the presence of unburnt carbon particles that glow. This zone is moderately hot.
3. Outermost Zone (Non-luminous Zone): This is the hottest part of the flame. Here, complete combustion of wax vapour takes place with sufficient oxygen. This zone is blue and produces the most heat, but less visible light compared to the middle zone.

(Note: A diagram would typically be drawn here showing these three zones with labels.)