Solution:

Conditions Under Which Combustion Can Take Place

Combustion is a chemical process where a substance reacts with oxygen, releasing heat and light. For combustion to take place, the following conditions must be met:

• Presence of a Fuel: Combustion requires a substance that can burn, like wood, paper, or fossil fuels.
• Availability of Oxygen: Oxygen is essential for combustion to occur. It combines with the fuel to release energy.
• Ignition Temperature: The fuel must reach a certain temperature, called the ignition temperature, to start burning.
• Heat: Heat is necessary to start the process of combustion. This is usually provided by an external source, like a match or spark.

In summary, combustion occurs when there is a fuel, oxygen, the right temperature, and enough heat to start the reaction.

Solution:

Fill in the blanks:

Solution:

Explanation of How the Use of CNG in Automobiles Has Reduced Pollution

The use of Compressed Natural Gas (CNG) as a fuel for automobiles has led to a significant reduction in pollution levels in cities. CNG is a cleaner alternative to traditional fuels like petrol and diesel. Below are some key points explaining how it helps reduce pollution:

• Lower Emissions: CNG burns more efficiently than petrol and diesel, producing fewer harmful pollutants such as carbon monoxide ($CO$), nitrogen oxides ($NO_x$), and particulate matter. This leads to cleaner air and less environmental damage.
• Reduced Carbon Dioxide ($CO_2$) Emissions: CNG produces less $CO_2$ compared to petrol and diesel. Since $CO_2$ is a major greenhouse gas, its reduction helps in mitigating the effects of climate change.
• Lower Levels of Sulphur and Lead: CNG contains very little sulphur and no lead, which are common pollutants found in petrol and diesel. These substances are harmful to both human health and the environment.
• Less Noise Pollution: Vehicles running on CNG generally produce less noise compared to those running on petrol or diesel, contributing to lower noise pollution in urban areas.
• Renewable and Abundant: CNG is a relatively abundant fuel and can be produced from natural gas resources. Its use reduces dependency on petroleum-based fuels, promoting a shift towards more sustainable energy sources.

Solution:

Comparison Between LPG and Wood as Fuels

LPG (Liquefied Petroleum Gas):

• Composition: LPG is a mixture of hydrocarbons, primarily propane (C₃H₈) and butane (C₄H₁₀).
• State: It is stored in liquid form under pressure and used as a gas when released.
• Energy Content: LPG has a high calorific value, producing more heat per unit volume compared to wood.
• Cleanliness: LPG burns with a clean blue flame, producing less smoke and pollutants.
• Storage and Handling: It is easy to store in cylinders and can be transported efficiently.
• Usage: LPG is commonly used for cooking, heating, and as a fuel in vehicles.
• Environmental Impact: LPG is a fossil fuel and contributes to greenhouse gas emissions when burned.

Wood:

• Composition: Wood is primarily composed of cellulose, hemicellulose, and lignin, which are organic compounds.
• State: Wood is a solid fuel that requires combustion to produce heat.
• Energy Content: Wood has a lower calorific value than LPG, producing less heat per unit volume.
• Cleanliness: Wood burns with a yellow or reddish flame and produces smoke, soot, and other pollutants.
• Storage and Handling: Wood is bulky and requires space for storage. It is more difficult to transport compared to LPG.
• Usage: Wood is traditionally used for cooking and heating, especially in rural areas.
• Environmental Impact: Burning wood releases carbon dioxide (CO₂), but it is considered carbon-neutral because the carbon released is offset by the carbon absorbed by trees during growth.

Solution:

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

Water is a good conductor of electricity. When used to control fires involving electrical equipment, it can cause electrical shocks or electrocution, posing a greater danger to humans. Additionally, water can damage electrical components, worsening the situation.

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

LPG (Liquefied Petroleum Gas) is a cleaner and more efficient fuel compared to wood. It burns with a higher calorific value, producing more heat and fewer pollutants. Unlike wood, which releases smoke and harmful gases, LPG burns with a clean flame, causing less air pollution and reducing health risks.

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

Paper catches fire easily because it is made up of cellulose, which is highly flammable. However, when wrapped around an aluminium pipe, the aluminium conducts heat away from the paper, preventing it from reaching the ignition point. The pipe absorbs the heat, making it harder for the paper to catch fire.

Solution:

Labelled Diagram of a Candle Flame

The candle flame consists of different zones, each having different characteristics:

Explanation of the labelled parts:

• Blue Inner Zone: This is the region closest to the wick. It is the hottest part of the flame where combustion is complete due to the sufficient oxygen supply.
• Light Yellow Outer Zone: This part of the flame has partial combustion, and it produces light and heat.
• Dark Outer Zone: This is the zone of incomplete combustion, and the flame is cooler here.
• Wick: The wick is the central part that draws molten wax upward and supplies fuel to the flame.
• Smoke: When the flame is extinguished, the smoke contains unburnt carbon particles.

Solution:

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

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

Solution:

How CO$_2$ is Able to Control Fires

Carbon dioxide (CO$_2$) is used to control fires due to its unique properties. When CO$_2$ is released in large amounts, it displaces the oxygen in the surrounding air. Since fire requires oxygen to continue burning, the reduction in oxygen concentration helps to smother the fire, making it unable to sustain itself.

Additionally, CO$_2$ is heavier than air, so when it is released, it forms a layer over the fire, effectively cutting off the supply of oxygen from the surroundings. This lack of oxygen causes the combustion process to stop, thus controlling and eventually extinguishing the fire.

Another important factor is that CO$_2$ does not support combustion. Unlike other gases, CO$_2$ does not react with fire to produce heat or additional fuel, making it an effective agent for fire suppression in enclosed areas or places where other fire-extinguishing methods may not be safe or suitable.

Solution:

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

Green leaves contain a high amount of water. When we try to burn them, the water present in the leaves absorbs a lot of heat and prevents the leaves from reaching the temperature required for combustion. The heat is used to evaporate the water, and thus, it takes more time and energy to burn the green leaves.

On the other hand, dry leaves have less water content, so they heat up more quickly and reach the required temperature for combustion easily. This is why dry leaves catch fire faster than green leaves.

Solution:

Which zone of a flame does a goldsmith use for melting gold and silver and why?

A goldsmith uses the inner zone of the flame, also known as the blue inner core, for melting gold and silver. This zone is the hottest part of the flame, where the temperature is around 1500°C to 1600°C. The high temperature in this region ensures that the metals reach their melting point quickly and efficiently. The inner zone provides a steady, concentrated heat source, which is essential for the precise melting of gold and silver without causing oxidation or other impurities.

Solution:

The calorific value of a fuel is defined as the amount of heat produced when 1 kg of the fuel is completely burnt. It is given by the formula:

Calorific value = Heat produced / Mass of the fuel burnt

Given:

• Heat produced = 180,000 kJ
• Mass of the fuel burnt = 4.5 kg

Now, substituting the values in the formula:

Calorific value = $\dfrac{180,000 \text{ kJ}}{4.5 \text{ kg}}$

Calorific value = 40,000 kJ/kg

Solution:

Can the process of rusting be called combustion? Discuss.

The process of rusting and combustion both involve chemical reactions, but they are not the same. Let's discuss each process separately:

Rusting: Rusting is a slow chemical reaction between iron and oxygen in the presence of water or moisture. The iron reacts with oxygen from the air to form iron oxide, commonly known as rust. The chemical equation for rusting is:

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

Rusting is a gradual process that occurs at room temperature and does not release a large amount of energy. It is an example of an oxidation reaction where iron gains oxygen and forms iron oxide.

Combustion: Combustion is a rapid chemical reaction between a substance and oxygen, usually producing heat and light. It is an exothermic reaction, meaning it releases a significant amount of energy. For example, the combustion of iron in oxygen can be written as:

3Fe + 4O₂ → 2Fe₃O₄

In combustion, energy is released in the form of heat and light, which is not the case with rusting. Combustion occurs quickly and often results in a flame or heat, while rusting is a slow and energy-consuming process that does not produce light or heat.

Key Differences:

• Rusting is a slow process; combustion is rapid.
• Rusting does not release heat or light, while combustion does.
• Rusting occurs at room temperature, whereas combustion requires high temperatures.
• Rusting is an oxidation process; combustion is a redox reaction.

Hence, the process of rusting cannot be called combustion due to these fundamental differences in their nature and characteristics.

Solution:

Abida's water will get heated in a shorter time.

Explanation:

The flame of a candle has different temperature regions. The yellow part of the flame, near the wick, is the hottest part of the flame, where the combustion is not complete. This region provides more heat to the beaker. On the other hand, the outermost part of the flame is cooler than the yellow part. As Ramesh kept the beaker in this region, it will heat the water at a slower rate compared to Abida's beaker.