Understanding Combustion and Ignition Temperature

Combustion Defined: Combustion is a chemical process where a substance reacts with oxygen to produce heat and light. In the examples provided, magnesium and charcoal were demonstrated as combustible substances. Magnesium burned to form magnesium oxide, while charcoal burned in air, producing carbon dioxide, heat, and light.

Conditions for Combustion: A critical observation is that for combustion to occur, air is essential. This is exemplified by the candle experiment, where the flame requires a continuous supply of air for sustained burning. The concept of ignition temperature is introduced — the lowest temperature at which a substance catches fire.

Importance of Ignition Temperature: Understanding ignition temperature helps explain various experiences, such as why a matchstick doesn’t catch fire at room temperature but ignites when rubbed against the matchbox. Different substances have different ignition temperatures, and the material must reach this temperature to combust. The activity with a paper cup and water illustrates this principle: the presence of water prevents the paper cup from reaching its ignition temperature, showcasing the role of conduction in heat transfer.

Inflammable Substances: Substances with very low ignition temperatures that easily catch fire with a flame are termed inflammable. Examples include petrol, alcohol, Liquified Petroleum Gas (LPG), and more. Recognizing and handling inflammable substances with care is crucial for safety.

Fire Control and Fire Extinguishers

Fire Incidents: Fire accidents are unfortunately common in homes, shops, and factories. If you have witnessed such an incident, sharing your experience with classmates can be both informative and a way to raise awareness about fire safety. Additionally, it’s essential to check if your city or town has a fire brigade station, as these play a crucial role in responding to fire emergencies.

Role of Fire Brigade: When a fire brigade arrives at the scene, one of its primary actions is to pour water on the fire. Water serves multiple purposes in fire control. It cools the combustible material, bringing its temperature below the ignition temperature, thereby preventing the fire from spreading. Water vapors also help cut off the air supply, contributing to the extinguishing of the fire.

Three Essential Requirements for Fire: You’ve learned that three essential requirements are necessary for fire: fuel, air (to supply oxygen), and heat (to raise the temperature of the fuel beyond the ignition temperature). To control or extinguish a fire, it’s essential to eliminate or reduce one or more of these requirements.

Fire Extinguishers: Fire extinguishers play a vital role in controlling fires. They work by cutting off the supply of air, lowering the temperature of the fuel, or a combination of both. There are different types of fire extinguishers designed for specific types of fires, such as those caused by wood, paper, flammable liquids, or electrical equipment.

Types of Combustion

1. Rapid Combustion:

• Observation: When a burning matchstick or a gas lighter is brought near a gas stove in the kitchen, and the gas stove is turned on, the gas burns rapidly, producing heat and light.
• Explanation: This type of combustion is known as rapid combustion. It occurs when a substance burns quickly, releasing a significant amount of heat and light.

2. Spontaneous Combustion:

• Example: Some substances, like phosphorus, can burn in air at room temperature without any external ignition.
• Definition: Spontaneous combustion is the type of combustion in which a material suddenly bursts into flames without the application of any apparent cause.

3. Explosion:

• Fireworks Example: Fireworks on festival days involve a sudden reaction that produces heat, light, and sound. A large amount of gas is liberated during this reaction.
• Definition: Explosion is a type of combustion characterized by a sudden release of energy, often accompanied by the liberation of gases. It can occur when pressure is applied to a substance, as seen in the case of ignited crackers.

Flame Structure and Observations

Vapourized Substances and Flames:

• Explanation: Flames are formed by substances that vaporize during burning. For example, in the case of candles, kerosene oil and molten wax rise through the wick, vaporize during burning, and create a visible flame.

Observations with Glass Plate/Slide:

• Procedure: Introducing a clean glass plate/slide into the luminous zone of the flame for about 10 seconds.
• Observation: A circular blackish ring is formed on the glass plate/slide, indicating the deposition of unburnt carbon particles present in the luminous zone of the flame.

Observations with Copper Wire:

• Procedure: Holding a thin, long copper wire just inside the non-luminous zone of the flame for about 30 seconds.
• Observation: The portion of the copper wire just outside the flame gets red-hot, suggesting that the non-luminous zone of the flame has a high temperature.

Goldsmiths and Flame Zones:

• Explanation: Goldsmiths use the outermost zone of a flame with a metallic blow-pipe for melting gold and silver.
• Reason: The outermost zone of the flame is the hottest part and provides the ideal conditions for melting metals like gold and silver efficiently.

Classification of Fuels

Solid Fuels:

1. Wood
2. Charcoal
3. Coal

Liquid Fuels:

1. Petrol
2. Kerosene
3. Diesel
4. Fuel Oil

Gaseous Fuels:

1. Natural Gas (CNG – Compressed Natural Gas)
2. LPG (Liquefied Petroleum Gas)
3. Biogas

Note:

• Solid fuels are typically in a solid state, such as wood, charcoal, and coal.
• Liquid fuels exist in a liquid state and include substances like petrol, kerosene, diesel, and fuel oil.
• Gaseous fuels are in a gaseous state and include natural gas, LPG, and biogas.

Fuel Efficiency

The calorific values of the mentioned fuels are as follows:

1. Cow Dung: The calorific value of cow dung is relatively low, as it contains a significant amount of water and organic matter. It is often used as a fuel in rural areas for cooking and heating, but its efficiency is lower compared to other fuels.
2. Coal: Coal has a higher calorific value compared to cow dung. It is a widely used solid fuel for various applications, including electricity generation, industrial processes, and domestic heating.
3. LPG (Liquefied Petroleum Gas): LPG has a high calorific value, making it a very efficient and clean-burning fuel. It is commonly used for cooking, heating, and in some industrial processes.

Burning of Fuels Leads to Harmful Products

1. Unburnt Carbon Particles:
   • Source: Wood, coal, petroleum.
   • Effect: Release of fine carbon particles leading to respiratory diseases, such as asthma.
2. Carbon Monoxide (CO) Gas:
   • Source: Incomplete combustion of fuels.
   • Effect: Extremely poisonous gas; can be lethal in closed spaces.
3. Carbon Dioxide (CO₂) Emission:
   • Source: Combustion of most fuels.
   • Effect: Increased concentration contributes to global warming.
4. Sulfur Dioxide (SO₂) and Oxides of Nitrogen (NOx) Emission:
   • Source: Burning coal and diesel; petrol engines.
   • Effect: Formation of sulfur dioxide and nitrogen oxides leads to acid rain, harmful to crops, buildings, and soil.