Combustion and Flame Class 8 Science Notes - Chapter 4 NCERT

Chapter 4: Combustion and Flame

What is Combustion?

Combustion is a chemical process in which a substance reacts with oxygen to give off heat and, usually, light. The substance that undergoes combustion is called a combustible substance or a fuel.

Conditions Necessary for Combustion:

1. Presence of a combustible substance (fuel).
2. Presence of a supporter of combustion (like oxygen).
3. Attainment of ignition temperature.

The ignition temperature is the lowest temperature at which a substance catches fire. A substance will not catch fire if its temperature is lower than its ignition temperature.

How Do We Control Fire?

Fire can be controlled by removing any one of the three requirements for combustion. The job of a fire extinguisher is to cut off the supply of air, bring down the temperature of the fuel, or both.

• Water: The most common fire extinguisher, but it only works for fires involving materials like wood and paper. It cools the combustible material so its temperature is brought below its ignition temperature. Water cannot be used for electrical fires or oil fires.
• Carbon Dioxide (CO₂): The best extinguisher for fires involving electrical equipment and flammable liquids like petrol. Being heavier than oxygen, CO₂ covers the fire like a blanket, cutting off the contact between the fuel and oxygen.

Types of Combustion

• Rapid Combustion: Combustion that occurs rapidly, producing heat and light. Example: Burning of LPG.
• Spontaneous Combustion: The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause. Example: Forest fires, combustion of phosphorus.
• Explosion: A combustion reaction where a large amount of gas is evolved suddenly, producing heat, light, and sound. Example: Ignition of a firecracker.

Flame

A flame is the visible, gaseous part of a fire. Substances which vaporise during burning give flames. For example, kerosene oil and molten wax rise through the wick, get vaporised during burning and form flames. Charcoal, on the other hand, does not vaporise and so does not produce a flame.

Structure of a Flame

A candle flame has three main zones:

• Innermost zone (Dark zone): Contains unburnt wax vapours. It is the least hot part.
• Middle zone (Luminous zone): Zone of incomplete combustion. The particles of carbon burn and glow, producing a yellow flame. It is moderately hot.
• Outermost zone (Non-luminous zone): Zone of complete combustion. It is blue in colour and is the hottest part of the flame. Goldsmiths use this zone for melting gold and silver.

What is a Fuel?

A fuel is any material that can be made to react with other substances so that it releases energy as heat energy or to be used for work. Fuels can be solid (coal, wood), liquid (petrol, kerosene), or gas (CNG, LPG).

Fuel Efficiency

The efficiency of a fuel is expressed in terms of its calorific value. The calorific value is the amount of heat energy produced on complete combustion of 1 kg of a fuel. Its unit is kilojoule per kg (kJ/kg).

Characteristics of an Ideal Fuel:

• High calorific value.
• Readily available and cheap.
• Easy to store and transport.
• Burns at a moderate rate.
• Does not leave behind undesirable substances.

Harmful Effects of Burning Fuels

• Air Pollution: Burning fuels like wood, coal, and petroleum release unburnt carbon particles, which can cause respiratory diseases.
• Carbon Monoxide Poisoning: Incomplete combustion of fuels produces carbon monoxide, a very poisonous gas that can be fatal.
• Global Warming: Increased concentration of carbon dioxide in the atmosphere from burning fuels traps heat and leads to a rise in the Earth's temperature.
• Acid Rain: Burning of coal and diesel releases sulphur dioxide and nitrogen oxides. These dissolve in rainwater to form acids, resulting in acid rain, which is harmful to crops, buildings, and soil.

Frequently Asked Questions (FAQ)

What is ignition temperature?

It is the minimum temperature that a combustible substance must reach before it can catch fire and sustain combustion.

Why is CO₂ an effective fire extinguisher?

CO₂ is effective because it is heavier than air (oxygen) and non-combustible. It displaces the oxygen surrounding the fire, cutting off its supply and extinguishing the flames.

Which zone of a flame is the hottest?

The outermost, non-luminous (blue) zone of a flame is the hottest because complete combustion of the fuel takes place there.