Chapter 4 -Combustion and Flame

1. List Conditions Under Which Combustion Can Take Place.

  1. Fuel: Combustible material that can undergo oxidation. This could be gases (like methane), liquids (such as gasoline), solids (wood, paper), or even metals under certain conditions.
  2. Oxygen: The presence of oxygen, or another oxidizing agent, which is necessary to sustain the chemical reaction. Air, which contains about 21% oxygen, is the most common source.
  3. Heat: Sufficient heat to raise the temperature of the fuel to its ignition point. This can be from a spark, flame, friction, or any other heat source capable of initiating the reaction.

2. Fill in the blanks

a) Burning of wood and coal causes ______ of air.

Ans- Burning of wood and coal causes pollution of air.

b) A liquid fuel used in homes is _____.

Ans– A liquid fuel used in homes is Liquified Petroleum Gas (LPG).

c) Fuel must be heated to its ____ before it starts burning.

Ans- Fuel must be heated to its ignition temperature before it starts burning.

d) The fire produced by oil cannot be controlled by _____.

Ans- Fire produced by oil cannot be controlled by water.

Que 3–Explain How the Use of CNG in Automobiles Has Reduced Pollution in Our Cities.

AnsThe use of Compressed Natural Gas (CNG) in automobiles has contributed significantly to reducing pollution in cities due to several key factors:

  1. Cleaner Emissions: CNG produces significantly fewer pollutants compared to traditional gasoline or diesel engines. When burned, it emits lower levels of carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), and sulfur dioxide (SO2).
  2. Lower Greenhouse Gas Emissions: CNG combustion releases less carbon dioxide (CO2), which is a major greenhouse gas contributing to climate change.
  3. Reduced Smog Formation: The lower emissions of NOx and volatile organic compounds (VOCs) from CNG engines contribute to decreased smog formation.
  4. Improved Air Quality: With fewer pollutants emitted into the atmosphere, the overall air quality in cities where CNG vehicles are prevalent tends to be better.
  5. Incentives for Adoption: Governments often provide incentives and subsidies to encourage the adoption of CNG vehicles.

4. Compare LPG and Wood as Fuels.

Certainly, here’s a comparison between Liquefied Petroleum Gas (LPG) and wood as fuels:

  1. Composition:
  • LPG (Liquefied Petroleum Gas): LPG is a mixture of hydrocarbon gases, primarily propane and butane. It’s a fossil fuel extracted during the refining of crude oil and natural gas processing.
  • Wood: Wood is a renewable biomass fuel primarily composed of carbon, hydrogen, and oxygen obtained from trees and plants. It’s a natural and renewable resource.

2.Energy Content:

  1. LPG: LPG has a higher energy content per unit of volume compared to wood. It produces more energy when burned, making it more efficient in terms of energy output.
  • Wood: Wood has a lower energy content compared to LPG. It releases less energy when burned, requiring a larger volume to produce the same amount of heat as LPG.

3.Convenience:

  1. LPG: LPG is convenient as it’s available in compressed form in tanks or cylinders, making it easy to transport and store. It can be used instantly and doesn’t require preparation before burning.
  • Wood: Wood needs to be chopped, dried, and stored properly before use. It requires more effort in terms of preparation, handling, and storage compared to LPG.

4.Emissions and Pollution:

  1. LPG: LPG burns more cleanly than wood, producing fewer emissions of pollutants such as particulate matter (PM), carbon monoxide (CO), and volatile organic compounds (VOCs). It contributes less to indoor and outdoor air pollution.
  • Wood: Burning wood releases higher levels of pollutants, including PM, CO, VOCs, and potentially harmful compounds like creosote. Improperly burned or wet wood can produce more pollutants, contributing to health and environmental concerns.

5.Renewability and Sustainability:

  1. LPG: LPG is a fossil fuel and is not renewable. Its extraction and use contribute to carbon emissions and environmental concerns.
  • Wood: Wood is renewable and considered carbon-neutral when sourced sustainably. If harvested responsibly and replaced through afforestation, the carbon released during burning is offset by the carbon absorbed during growth, making it a more sustainable option.

Both LPG and wood have their advantages and disadvantages, and the choice between them often depends on factors like availability, convenience, environmental impact, and specific heating or cooking needs.

5. Give Reasons

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

Ans-Water is not used to control fires involving electrical equipment because water is a good conductor of electricity due to which the person may be electro-conducted and it may damage the equipment

b) LPG is a better domestic fuel than wood.

Ans– LPG is a better domestic fuel than wood because wood produces a lot of smoke on combustion and causes respiratory problems. Also, its efficiency is lower than LPG.

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

Ans– Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not because aluminum is a good conductor of heat which absorbs the heat from the paper and the paper does not catch fire. Whereas, the paper by itself catches fire.

6. Make a Labeled Diagram of a Candle Flame.

Ans: A labeled diagram of a candle flame is

7.Name the Unit in Which the Calorific Value of a Fuel is Expressed.

Ans-The calorific value of a fuel is typically expressed in units of energy per unit mass or volume. The two common units used to express calorific value are:

  1. Joules per kilogram (J/kg): This unit measures the energy produced per kilogram of the fuel when it’s completely burned.
  2. Kilocalories per kilogram (kcal/kg): It represents the amount of energy produced per kilogram of the fuel. 1 kilocalorie is equivalent to 4.184 kilojoules.

8. Explain how

able to control fires.

Ans: We know that combustion takes place under the following three conditions: 

(i) In the presence of oxygen (air).

(ii) In the presence of fuel.

(iii) In the presence of heat – the minimum temperature at which a substance catches fire, known as ignition temperature.

We can control the fire if any one of the three conditions is not met. 

(i) CO2–2 is heavier than oxygen and hence it acts as a protective blanket and prevents oxygen from reaching the fire.

(ii) Since CO2–2 is stored in a liquid state, therefore, when it is used on the fire, it expands and cools which lowers down the temperature. This prevents heat from reaching the fire.

9. It is Difficult To Burn a Heap of Green Leaves, but Dry Leaves Catch Fire Easily. Explain

Ans-Green leaves contain more moisture compared to dry leaves. The moisture content in green leaves makes them harder to burn because before the leaves can combust and sustain a fire, the heat from the flame must first evaporate the water within the leaves. This process consumes a significant amount of energy and heat, making it difficult for the leaves to reach the temperature required for combustion.

Dry leaves, on the other hand, have minimal moisture content. This absence of water makes it easier for the leaves to reach their ignition point. When exposed to a heat source like a spark or an open flame, dry leaves can quickly absorb the heat and reach the temperature needed to ignite and sustain combustion.

The high moisture content in green leaves acts as a barrier to ignition because the heat initially applied to the leaves is absorbed to evaporate the water rather than raising the temperature of the leaves to the point of ignition. Dry leaves, lacking this moisture barrier, readily absorb heat and ignite more easily.

This difference in moisture content between green and dry leaves illustrates why dry leaves are more susceptible to catching fire compared to their green counterparts.

10. Which Zone of a Flame Does a Goldsmith Use for Melting Gold and Silver and Why?

Ans-A goldsmith typically uses the innermost zone of a flame, known as the “reducing flame,” for melting gold and silver. The reducing flame is characterized by its low oxygen content and high fuel (such as gas) concentration. This zone appears as a bright blue cone at the base of a flame.

Here’s why the reducing flame is preferred for melting precious metals like gold and silver:

  1. Less Oxidation: The reducing flame has limited oxygen, minimizing the chance of oxidizing the metals. Oxidation can affect the quality of the molten metal by causing impurities or altering its properties, such as color or ductility. By using a reducing flame, the goldsmith can prevent or reduce the formation of oxides on the surface of the metal being melted.
  2. Higher Temperature: The reducing flame tends to have a higher temperature compared to other parts of the flame. The concentrated fuel allows for a more intense and focused heat, facilitating faster and more uniform melting of the metals.
  3. Precise Control: Goldsmiths require precise control over the melting process to ensure the metals reach their specific melting points without overheating or causing damage. The reducing flame’s concentrated heat and controlled oxygen content offer better control over the melting process.

By utilizing the reducing flame, goldsmiths can achieve clean, uniform, and high-quality melts of gold and silver while minimizing the risk of oxidation and maintaining the integrity of the metals’ properties.

11. In an Experiment

4.5 kg4.5 ��

of a fuel was completely burnt. The heat produced was measured to be

180,000 kJ180,000 ��

. Calculate the calorific value of the fuel.

Ans-The calorific value of a fuel is calculated using the formula:

(\text{Calorific Value} = \frac{\text{Heat Produced}}{\text{Mass of Fuel}})

Given:
Mass of the fuel = 4.5 kg
Heat produced = 180,000 kJ

Let’s plug the values into the formula:

(\text{Calorific Value} = \frac{180,000 \text{ kJ}}{4.5 \text{ kg}})

(\text{Calorific Value} = 40,000 \text{ kJ/kg})

Therefore, the calorific value of the fuel is (40,000 \text{ kJ/kg}).

12. Can the Process of Rusting Be Called Combustion? Discuss.

Ans-Rusting and combustion are both chemical processes involving the interaction of substances, but they differ significantly in their mechanisms and outcomes.

Combustion is a rapid chemical reaction between a fuel and an oxidizing agent (usually oxygen) that produces heat and light. It’s an exothermic reaction where the fuel combines with oxygen to release energy in the form of heat and often light. In combustion, the substances involved are typically gases, liquids, or solids that burn in the presence of oxygen.

Rusting (or corrosion) is a slow chemical process that occurs when metals, especially iron and steel, react with oxygen and moisture from the air to form iron oxide (rust). This reaction takes place over time and involves the oxidation of the metal. Unlike combustion, rusting is a gradual process and does not produce significant amounts of heat or light.

While both combustion and rusting involve the oxidation of materials, the key differences lie in the speed of the reaction, the involvement of heat and light production, and the nature of the substances involved:

  1. Speed: Combustion is a rapid process, while rusting occurs slowly over an extended period.
  2. Energy Release: Combustion releases energy in the form of heat and light, whereas rusting does not produce significant energy in the form of heat or light.
  3. Substances Involved: Combustion typically involves fuels reacting with oxygen, whereas rusting involves the oxidation of metals by oxygen and moisture.

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