Combustion and flame are important concepts in NCERT Class 8 Science. These topics help us understand how substances burn the types of flames produced and the science behind fire. This knowledge is crucial for fire safety energy production and everyday applications like cooking and heating.
In this topic we will explore combustion its types the structure of a flame and related concepts in a simple and easy-to-understand manner.
What is Combustion?
Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. This reaction is commonly called burning. The substances that undergo combustion are known as combustible substances or fuels.
Examples of Combustion
- Burning of wood
- Combustion of petrol in car engines
- Candle burning
- LPG (Liquefied Petroleum Gas) burning in kitchens
Conditions Necessary for Combustion
For combustion to take place the following three conditions must be met:
- Presence of a Fuel – A substance that can burn (e.g. wood coal petrol).
- Oxygen (Air Supply) – Oxygen is necessary for burning to occur.
- Heat (Ignition Temperature) – The minimum temperature required to start combustion.
This combination of fuel oxygen and heat is called the “Fire Triangle”. If any one of these elements is removed combustion will stop.
Types of Combustion
1. Rapid Combustion
- In this type a substance burns quickly and produces heat and light.
- Example: Burning of LPG in a gas stove.
2. Spontaneous Combustion
- This occurs when a substance catches fire on its own without an external heat source.
- Example: Dry grass in hot summer catching fire.
3. Explosion
- A sudden and fast combustion reaction that produces heat light and sound.
- Example: Firecrackers bursting.
Flame and Its Structure
A flame is the visible part of a fire. Not all substances produce a flame when they burn. For example kerosene and wax produce flames while charcoal burns without a flame.
Structure of a Flame
A flame has three distinct zones:
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Outer Zone (Blue Zone) – Complete Combustion
- This is the hottest part of the flame.
- It undergoes complete combustion and produces a blue flame.
- Example: The blue part of an LPG flame.
-
Middle Zone (Yellow Zone) – Partial Combustion
- It is moderately hot and produces a yellow flame.
- This happens due to partial combustion of fuel.
- Example: The yellow flame of a candle.
-
Inner Zone (Dark Zone) – No Combustion
- This is the least hot part of the flame.
- It contains unburnt carbon ptopics and does not undergo combustion.
Why Does LPG Burn with a Blue Flame?
LPG (Liquefied Petroleum Gas) undergoes complete combustion producing a blue flame with high heat efficiency. This is why LPG stoves are designed to give a blue flame instead of yellow.
What are Fuels?
Fuels are substances that burn to produce heat and energy. They can be classified into three types:
1. Solid Fuels
- Example: Wood coal charcoal
- Used in traditional cooking and heating.
2. Liquid Fuels
- Example: Petrol diesel kerosene
- Used in vehicles lamps and industries.
3. Gaseous Fuels
- Example: LPG CNG (Compressed Natural Gas)
- Used in kitchens and vehicles because they burn cleanly.
What is a Good Fuel?
A good fuel should have the following properties:
- High Calorific Value – Produces more heat per unit.
- Burns Easily – Should ignite easily but not too quickly.
- Does Not Produce Harmful Gases – Should be environmentally friendly.
- Economical and Easily Available – Should be affordable and widely available.
LPG and CNG are considered good fuels because they are efficient clean-burning and safe compared to solid fuels like wood and coal.
Fire Control and Safety
Fire can be controlled by removing one of the elements from the fire triangle (fuel oxygen heat).
How to Extinguish Fire?
- Removing Fuel – Shutting off the gas supply stops combustion.
- Cutting Oxygen Supply – Using a fire extinguisher or sand to cut off oxygen can stop burning.
- Lowering Temperature – Pouring water reduces the heat and stops combustion.
Types of Fire Extinguishers
- Water Extinguisher – Used for paper wood and textile fires.
- Foam Extinguisher – Used for oil and petrol fires.
- CO₂ Extinguisher – Used for electrical fires.
Why Shouldn’t You Use Water on Oil Fires?
Water is heavier than oil so it sinks and spreads the burning oil making the fire worse. Instead a fire blanket or foam extinguisher should be used.
Harmful Effects of Combustion
Although combustion is useful it also has some negative effects:
1. Air Pollution
- Incomplete combustion of fuels produces carbon monoxide which is harmful to health.
- Burning fuels release sulfur dioxide and nitrogen oxides leading to acid rain.
2. Global Warming
- Combustion of fossil fuels releases carbon dioxide which traps heat and causes global warming.
3. Health Hazards
- Smoke from combustion can cause breathing problems lung diseases and irritation.
Alternative Fuels and Clean Energy
To reduce pollution and dependency on fossil fuels alternative fuels are being developed.
1. Biofuels
- Derived from plants and organic matter.
- Example: Ethanol biodiesel.
2. Solar Energy
- Uses sunlight to generate electricity and heat.
- Example: Solar panels for cooking and heating water.
3. Hydrogen Fuel
- A clean fuel that produces only water as a byproduct.
- Used in hydrogen fuel cell vehicles.
4. Wind and Hydropower
- Use wind and water to generate electricity.
Combustion and flame play an important role in our daily lives. From cooking and transportation to electricity production understanding the principles of combustion helps us use fuels efficiently and safely.
By using clean energy sources and alternative fuels we can reduce pollution protect the environment and ensure a sustainable future. Learning about combustion not only helps in academics but also in making responsible choices about energy consumption.
