4.10 - Exercises

For combustion to occur, three essential conditions must be met: there must be fuel, oxygen (typically from the air), and heat. Fuel refers to any combustible material like wood, kerosene, or coal. Oxygen is necessary for the reaction to happen, as it supports the combustion process. Heat is needed to raise the fuel to its ignition temperature, allowing it to start burning. If any one of these conditions is absent, combustion cannot take place.

This exercise emphasizes understanding key concepts related to combustion. For part (a), burning wood and coal leads to the depletion of oxygen in the air. Part (b) refers to kerosene as a common liquid fuel used in households. For part (c), the temperature to which a fuel must be heated to ignite is known as its ignition temperature. Finally, part (d) states that water is not effective for extinguishing oil fires because oil floats on top of water and continues to burn.

Compressed Natural Gas (CNG) has been adopted in many vehicles because it burns cleaner than gasoline or diesel. It produces fewer harmful emissions, such as carbon monoxide and carbon dioxide, which are responsible for air pollution and contribute to global warming. By switching to CNG, cities have experienced improvements in air quality and a reduction in smog and respiratory issues among residents.

Liquefied Petroleum Gas (LPG) is a cleaner, more efficient fuel compared to wood. LPG burns at a higher temperature, produces less smoke, and leaves no residue, making it more environmentally friendly. In contrast, burning wood can produce significant amounts of smoke and unburnt carbon particles, which are harmful to health. LPG is more convenient as it is easy to store and transport, while wood requires chopping and can be less readily available.

For part (a), water can conduct electricity, making it dangerous to use on electrical fires, which could lead to electric shocks. Part (b) explains that LPG burns more cleanly and efficiently than wood, producing less harmful smoke. For part (c), the aluminum pipe acts as a heat sink, meaning the paper does not reach its ignition temperature, preventing it from burning, whereas exposed paper has direct access to heat.

Creating a diagram of a candle flame involves identifying and labeling its three distinct zones: the dark zone (where wax vaporizes), the luminous zone (where the flame glows due to combustion of wax vapors), and the non-luminous zone (which is significantly hotter and blue). Each of these zones plays a crucial role in understanding how a flame works.

The calorific value, which measures the energy produced by the complete combustion of a fuel, is expressed in kilojoules per kilogram (kJ/kg). This unit helps us compare the energy content of various fuels, allowing us to choose the most efficient one for our needs.

Carbon dioxide (CO2) extinguishes fire by displacing oxygen around the fire. Since fire requires oxygen to sustain combustion, reducing the oxygen supply effectively extinguishes the flame. CO2 is also cooler than burning materials, further preventing combustion from continuing.

Green leaves contain moisture, which makes it hard for them to reach their ignition temperature. When leaves are dry, they lack this moisture and can reach combustion temperatures much easier; hence they burn quickly. Dry leaves have lower water content and more combustibles, making them efficient fuel.

Goldsmiths usually use the outer zone of the flame because it is the hottest zone and provides sufficient heat to melt metals like gold and silver. This area of the flame has a higher temperature due to complete combustion, making it efficient for such tasks.

To calculate the calorific value, divide the total heat produced by the mass of the fuel burned. The formula is: Calorific Value = Total Heat Produced / Mass of Fuel. Here, you divide 180,000 kJ by 4.5 kg, which gives you the calorific value in kJ/kg.

Rusting is a slow oxidation process involving iron reacting with oxygen and moisture, which results in iron oxide (rust). Unlike combustion, which is a rapid exothermic reaction releasing energy, rusting is a slow, generally non-exothermic process. Thus, while both involve oxygen, the nature and speed of the reactions differ significantly.

Ramesh's water will get heated in a shorter time because the outermost part of the flame is the hottest zone. The yellow part of the candle flame (where Abida placed her beaker) is less hot compared to the blue flame (the outer zone) which burns at a higher temperature, allowing for quicker heat transfer and faster heating of the water.

These activities aim to engage students in real-world applications of concepts learned in class. Surveys on fuel availability and pollution help students understand fuel sources and their implications on the environment. Understanding fire safety through extinguisher locations establishes awareness of safety measures. Making a fire extinguisher model creatively illustrates principles explored in lessons about combustion and fire control.