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1. The use of CNG in automobiles has reduced pollution in our cities because it emits less harmful pollutants such as carbon monoxide, nitrogen oxides, and particulate matter compared to petrol and diesel. CNG is also cheaper than petrol and diesel, which makes it a more economical and environmentally friendly option.

2. LPG is a better fuel than wood because it burns more cleanly and produces less pollution. LPG also has a higher calorific value, which means it produces more heat per unit of fuel compared to wood. LPG is also more convenient to use because it is easy to store and transport.

3. (a) Water is not used to control fires involving electrical equipment because water is a good conductor of electricity and can cause electrocution. Instead, carbon dioxide or dry powder extinguishers are used to put out electrical fires.

(b) LPG is a better domestic fuel than wood because it produces less smoke and soot, which can cause respiratory problems. LPG is also easier to use and requires less maintenance compared to wood stoves.

(c) A piece of paper wrapped around an aluminum pipe does not catch fire easily because the aluminum pipe conducts heat away from the paper, which prevents it from reaching its ignition temperature.

4. The calorific value of a fuel is expressed in joules per gram (J/g) or kilojoules per kilogram (kJ/kg).

1. The use of Compressed Natural Gas (CNG) as a fuel in automobiles has reduced pollution in our cities due to the following reasons:

- CNG produces less harmful emissions compared to other fossil fuels like petrol or diesel. It contains a higher amount of methane, which burns more efficiently, resulting in lower levels of carbon monoxide, nitrogen oxides, and particulate matter.

- CNG combustion produces almost negligible amounts of sulfur dioxide and very low levels of harmful hydrocarbons and volatile organic compounds (VOCs), which contribute to air pollution and smog formation.

- CNG combustion releases fewer greenhouse gases, particularly carbon dioxide (CO2), which is a major contributor to global warming and climate change.

2. Comparison between LPG and wood as fuels:

- LPG (liquefied petroleum gas) is a cleaner and more efficient fuel compared to wood.

- LPG burns more efficiently, resulting in a higher heat output for the same amount of fuel compared to wood.

- LPG combustion produces a lower amount of harmful emissions like carbon monoxide, sulfur dioxide, nitrogen oxides, and particulate matter, compared to the burning of wood.

- LPG is a convenient and readily available fuel that can be easily stored and used in various appliances, whereas wood requires collection, storage, and manual handling.

- Wood burning releases harmful smoke, ash, and pollutants that can cause respiratory problems and air pollution, while LPG combustion is cleaner and does not release as many pollutants.

3. Reasons for the following statements:

(a) Water is not used to control fires involving electrical equipment:

- Water is a good conductor of electricity and can increase the risk of electric shock when used on electrical fires.

- Water can cause short circuits, damage electrical equipment further, or spread the fire if it comes in contact with live electrical sources.

- Suitable fire extinguishers like carbon dioxide (CO2) or dry powder extinguishers are used to control electrical fires as they do not conduct electricity.

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

- LPG burns more efficiently and produces a higher heat output compared to wood, making it more suitable for cooking and heating purposes.

- LPG combustion is cleaner and produces fewer harmful emissions like smoke, ash, and pollutants, making it better for indoor air quality and reducing health risks.

- LPG is easier to handle, store, and use compared to wood, which requires cutting, drying, and storing, and can be affected by moisture and insects.

(c) Paper by itself catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not:

- The piece of paper wrapped around an aluminium pipe does not catch fire easily because the aluminium acts as a conductor and draws away the heat from the paper, preventing it from reaching the ignition temperature.

- Heat is conducted away from the paper, and the rate of heat transfer is faster, making it difficult for the paper to sustain combustion.

4. The unit in which the calorific value of a fuel is expressed is kilojoules per gram (kJ/g) or kilojoules per kilogram (kJ/kg).

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