Chapter 4: Combustion and Flame 
Class 8 Science Notes
1. What is Combustion?
Combustion
is a chemical process in which a substance reacts with oxygen to produce heat and light.
- **Example:** Burning of wood, paper, LPG, etc.
Conditions Necessary for Combustion
1. **Presence of a combustible substance (fuel)** – A material that can burn.
2. **Presence of oxygen (supporter of combustion)** – Required for burning.
3. **Ignition temperature (minimum temperature to burn)** – Different substances have different ignition temperatures.
2. Types of Combustion
|
Type |
Description |
Example |
|
Rapid Combustion |
A substance burns quickly with the release of heat and light. |
LPG |
|
Spontaneous Combustion |
A substance catches fire on its own without any flame or spark. |
Phosphorus, coal heaps |
|
Explosion |
Sudden reaction with release of heat, light, and sound. |
Firecrackers, bombs |
3. Flame
- A flame is the visible, gaseous part of a fire produced during combustion.
- **Substances that produce flame:**
- Burn with a flame (e.g., wax, kerosene).
- Burn without a flame (e.g., coal, charcoal).
Structure of a Flame
1. **Innermost Zone (Dark Zone)** – Unburnt vapours, least hot.
2. **Middle Zone (Luminous Zone)** – Partial burning, yellow flame, moderate heat.
3. **Outermost Zone (Non-Luminous Zone)** – Complete burning, blue flame, hottest part.
4. Fuels
- A fuel is a substance that is burned to produce heat energy.
Types of Fuels
| **Type** | **Examples** |
|----------|-------------|
| **Solid Fuels** | Wood, coal, cow dung. |
| **Liquid Fuels** | Petrol, diesel, kerosene. |
| **Gaseous Fuels** | CNG, LPG, biogas. |
Characteristics of a Good Fuel
- High calorific value (more heat per unit mass).
- Burns smoothly.
- Easily available & affordable.
- Does not produce harmful gases.
5. Calorific Value
- The amount of heat produced by burning **1 kg** of fuel completely.
- Measured in **kilojoules per kg (kJ/kg)**.
| **Fuel** | **Calorific Value (kJ/kg)** |
|----------|----------------------------|
| Wood | 17,000 - 22,000 |
| Petrol | 45,000 |
| CNG | 50,000 |
6. Harmful Effects of Burning Fuels
1. **Air Pollution** – Releases CO₂, CO, SO₂, and other harmful gases.
2. **Global Warming** – Excess CO₂ increases Earth’s temperature.
3. **Acid Rain** – SO₂ and NO₂ mix with rainwater, harming soil & water bodies.
4. **Respiratory Problems** – Smoke causes asthma, lung diseases.
7. Fire Control (How to Extinguish Fire?)
- **Remove any one condition of combustion:**
- **Cut off fuel supply** (e.g., turning off a gas cylinder).
- **Cut off oxygen supply** (e.g., using a fire blanket, sand, or CO₂ extinguisher).
- **Cool below ignition temperature** (e.g., using water).
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### **Key Points to Remember**
✔ Combustion requires fuel, oxygen, and heat.
✔ Different types of combustion: rapid, spontaneous, explosion.
✔ Flame has three zones: innermost (dark), middle (luminous), outermost (hottest).
✔ Calorific value measures fuel efficiency.
✔ Burning fuels cause pollution, global warming, and health issues.
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**End of Notes**