4.10. Exercises

1. List conditions under which combustion can take place.

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.

Imagine trying to start a campfire. You need dry wood (fuel), a lighter or matches (heat), and a place where air can flow (oxygen). If you have wet wood, no matches, or if the campfire is in a closed box, you won’t be able to start the fire.

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 ___.

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.

Think of a firepit. As you burn wood, it uses up the oxygen in the air around it. If you’ve ever used kerosene to start a fire, you know it makes things go up in flames quickly! Just like how you can't put out an oil fire with water, you can't put a swimming pool on top of a water fountain; it just wouldn't work!

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

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.

Consider a bustling city where cars once puffed out black smoke. By using CNG, it’s like replacing a smoky barbecue with a clean electric grill; the air is fresher, and people can breathe easier without the haze of pollution.

4. Compare LPG and wood as fuels.

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.

If you think about cooking, using a gas stove (LPG) is like using a microwave oven—fast, clean, and efficient—whereas cooking over an open fire with wood is similar to using an old-fashioned campfire, a lot of effort and cleanup afterward!

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.

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.

Picture pouring water on an electrical appliance that’s on fire—it could not only fail to extinguish the fire but could also hurt you! Cooking with LPG is like using the easiest tool in your toolbox instead of a hammer and a wedge (wood): it just works better without the mess and effort.

6. Make a labelled diagram of a candle flame.

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.

Think of a candle flame like an ice cream cone: the bottom (dark zone) is what holds it up, the middle (luminous zone) is the flavor everyone sees, and the top (non-luminous zone) is where it melts away and gets hot. Each part has its job in making the whole candle function.

7. Name the unit in which the calorific value of a fuel is expressed.

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.

Think of calorific value like comparing the power of different batteries: just like you’d want to buy a battery that lasts longer, you want fuels that give you more energy. The higher the number in kJ/kg, the better the fuel!

8. Explain how CO2 is able to control fires.

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.

Imagine blowing out a birthday cake candle. Just like your breath pushes away the oxygen around the flame, CO2 smothers the fire, making it impossible for it to keep burning. It’s like putting a blanket over a flame to quench it!

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

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.

Try lighting fresh spinach versus dried herbs for cooking. The spinach (green leaves) is too wet to catch fire as easily as the dried herbs are; it’s a bit like trying to start a campfire with wet matchsticks versus dry ones!