CBSE Class 8 Science • Chapter 7 • Detailed Master Notes
Chapter Overview
In this chapter, we explore the chemical science behind fire. We will study the process of combustion, how fires start and how they are extinguished, the physical structure of a flame, and the efficiency and environmental impact of burning different fuels.
Definition: A chemical process in which a substance reacts rapidly with oxygen to give off heat and light is called combustion. This is the scientific term for burning.
Based on their ability to burn, substances are classified into two categories:
For a fire to start and continue burning, three essential conditions must be met simultaneously. If even one of these conditions is removed, the fire will extinguish. These three conditions form the Fire Triangle.
AI Image Prompt: A vibrant, clean vector graphic of the "Fire Triangle" on a white background. An equilateral red triangle with three sides distinctly labeled "Fuel", "Heat", and "Oxygen". In the center of the triangle sits a glowing orange campfire illustration radiating heat.
Q1. Why doesn't a piece of wood catch fire on its own at room temperature?
Ans: A piece of wood does not catch fire on its own because room temperature is much lower than the ignition temperature of wood. Heat must be supplied to reach the ignition temperature.
Since fire requires all three conditions of the fire triangle to survive, firefighters extinguish fires by safely removing one or more of these requirements.
| Type of Combustion | Description | Example |
|---|---|---|
| Rapid Combustion | The substance burns very fast, quickly producing large amounts of heat and light. | Turning on a gas stove (LPG burner) with a lighter. |
| Spontaneous Combustion | The material bursts into flames suddenly on its own, without any external heat applied. | White phosphorus catches fire instantly when simply exposed to room temperature air. |
| Explosion | A sudden, violent reaction resulting in the massive evolution of heat, light, sound, and a large volume of gas. | Igniting a firecracker or dynamite. |
Not all burning substances produce a flame. Only those substances which vaporise (turn into gas) during burning produce flames. Kerosene oil and molten wax vaporise during burning and form distinct flames. Solid charcoal does not vaporise and does not produce a flame; it simply glows red hot.
AI Image Prompt: A detailed vector scientific diagram of a burning candle against a navy blue background. Show three distinct glowing zones of the flame: a faint blue, hot outer halo; a bright, luminous yellow middle zone; and a dark, black inner core right over the wick. Include white label lines for each zone indicating their temperature levels.
If you were asked to boil a pot of water using cow dung cakes, coal, and LPG, you would likely choose LPG because it generates much more heat much faster with less mass.
Calorific Value: The precise amount of heat energy produced on the complete combustion of exactly 1 kilogram (1 kg) of a specific fuel is called its calorific value.
It is measured and expressed in the unit of kilojoule per kilogram (kJ/kg).
Hydrogen gas has the highest calorific value (150,000 kJ/kg) of all common fuels, followed by LPG (55,000 kJ/kg) and Methane (50,000 kJ/kg). Wood offers a much lower value (17,000 kJ/kg).
Q1. Why is LPG considered a better fuel than wood?
Ans: LPG has a much higher calorific value than wood, meaning it produces far more heat per kilogram. Furthermore, it undergoes complete combustion without producing harmful smoke or toxic ash, making it cleaner and more efficient.
The increasing global consumption and burning of fossil fuels directly threatens the health of humans and the environment.