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: There must be a combustible substance (fuel) present, such as wood, coal, LPG, or paper.
2. Supporter of Combustion: An adequate supply of a supporter of combustion is necessary. For most common combustion processes, this is oxygen from the air.
3. Ignition Temperature: The fuel must be heated to its ignition temperature, which is the minimum temperature at which it starts to burn in the presence of the supporter of combustion.

Solution:

1. Burning of wood and coal causes pollution of air. These fuels release harmful gases and particulate matter when burned.

(b) A liquid fuel, used in homes is Liquefied Petroleum Gas (LPG). LPG is a common cooking fuel due to its convenience and efficiency.

(c) Fuel must be heated to its ignition temperature before it starts burning. This is the minimum temperature required for the fuel to ignite and sustain combustion.

(d) Fire produced by oil cannot be controlled by water. Water is denser than oil and sinks below it. When water is applied to an oil fire, the oil can spread over the water, continuing to burn and potentially worsening the fire. Also, if electrical equipment is involved, water can conduct electricity.

Solution: The use of Compressed Natural Gas (CNG) in automobiles has significantly reduced pollution in cities because CNG is a much cleaner fuel compared to traditional fuels like petrol and diesel. When CNG burns, it produces very small amounts of harmful byproducts. Specifically, it releases significantly less sulfur dioxide and nitrogen oxides, which are major contributors to air pollution, acid rain, and respiratory problems. Its combustion is more complete, leading to fewer particulate emissions as well. Therefore, switching to CNG helps improve air quality and public health in urban areas.

Solution: Comparing LPG (Liquefied Petroleum Gas) and wood as fuels reveals several key differences:

• Energy Content: LPG generally produces more energy per unit mass compared to wood.
• Pollution: LPG burns more cleanly, producing less smoke and fewer harmful pollutants than wood, which releases significant amounts of smoke, soot, and gases like carbon monoxide.
• Residue: LPG leaves no residue after burning. Wood, however, leaves behind ash, which needs to be disposed of.
• Ease of Use & Storage: LPG is convenient to store and use in cylinders, allowing for controlled burning. Wood requires more space for storage and can be less convenient to handle and burn efficiently.
• Ignition: LPG ignites easily and burns at a moderate rate, while wood may require more effort to ignite and its burning rate can be variable.

Solution:

1. Water is not used to control fires involving electrical equipment because water is a good conductor of electricity. If water is used on an electrical fire, it can conduct the electric current to the person attempting to extinguish the fire, leading to a severe electric shock or even fatality. Dry chemical or CO2 extinguishers are preferred for such fires.

(b) LPG is considered a better domestic fuel than wood primarily because:

• Efficiency: LPG burns more completely and efficiently, producing a large amount of heat with less fuel compared to wood.
• Cleanliness: It burns with a clean flame, producing very little smoke and harmful pollutants, thus reducing air pollution indoors and outdoors.
• Residue: LPG leaves no ash or soot residue, making the cooking area cleaner and reducing the need for frequent cleaning.
• Convenience: It is readily available in cylinders, easy to store, and provides a controlled flame for cooking.

1. A piece of paper wrapped around an aluminium pipe does not catch fire as easily as plain paper because aluminium is a good conductor of heat. When the paper is wrapped around the aluminium pipe, the aluminium rapidly absorbs the heat from the paper. This heat conduction by the aluminium prevents the paper from reaching its ignition temperature. In contrast, plain paper, being a poor conductor of heat, quickly reaches its ignition temperature when exposed to a flame, causing it to burn.

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

Diagram: (Imagine a diagram here showing a candle with its flame divided into three zones)

• Innermost Zone (Dark Zone): This is the region closest to the wick. It contains unburnt wax vapor. It is the coolest part of the flame and is non-luminous.
• Middle Zone (Luminous Zone): This zone surrounds the innermost zone. Here, the wax vapor undergoes partial combustion in the presence of limited oxygen. This incomplete combustion produces soot particles, which glow and make this part of the flame luminous and yellow. It is moderately hot.
• Outermost Zone (Non-luminous Zone): This is the hottest part of the flame. Here, the wax vapor undergoes complete combustion with sufficient oxygen from the air. This zone is blue and non-luminous. It is where most of the heat is generated.

The diagram would show these three zones clearly marked around the wick of the burning candle.