CBSE Notes Class 8 Science Chapter 4 - Combustion and Flames

Combustion is a chemical process where a substance reacts with oxygen, releasing heat and light, often in the form of a flame. Flames are visible, gaseous parts of the combustion reaction. Combustion can be complete (with enough oxygen, producing CO₂ and water) or incomplete (with limited oxygen, producing CO or soot). The color of the flame depends on the material burning and the temperature of the combustion.

1.0What is Combustion?

Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light. This process is commonly known as burning. The substance that undergoes combustion is called a fuel, and it usually requires heat to initiate the reaction.

Examples of Combustion:

• Lighting a candle
• Burning wood in a fireplace
• Using petrol in car engines

2.0Conditions for Combustion

For combustion to take place, three things are required:

1. Fuel: A substance that burns to produce energy.
2. Oxygen: Present in the air and necessary for combustion.
3. Heat: Required to ignite the fuel.

This combination of fuel, oxygen, and heat is often referred to as the fire triangle. If any one of these elements is missing, combustion will not occur.

3.0Types of Combustion

In this chapter, three types of combustion are discussed:

1. Rapid Combustion:

This type of combustion occurs when a substance burns rapidly, releasing a large amount of heat and light in a short time.

Example: Burning of LPG in gas stoves.

2. Spontaneous Combustion:

In spontaneous combustion, the substance ignites without any external heat source due to internal chemical reactions.

Example: Fire in a heap of coal or dry leaves.

2. Explosive Combustion:

This occurs when combustion happens with the release of a large amount of gas and heat, producing a loud sound.

Example: Firecrackers and dynamite explosions.

4.0Flame and its Structure

A flame is the visible part of the fire. Different substances produce flames with different colors. For example, wood and paper produce yellow flames, while some gases like methane produce a blue flame.

The structure of a flame consists of three main zones: 

Understanding the flame's structure helps in recognizing the process of combustion in different settings.

1. Innermost Zone (Dark Zone):

• This is the part of the flame closest to the wick or fuel source.
• It is dark and contains unburnt wax or fuel particles.
• It is the least hot part of the flame.

2. Middle Zone (Luminous Zone):

• The middle part of the flame is yellow in color due to incomplete combustion.
• This zone is moderately hot and emits light.

3. Outermost Zone (Non-luminous Zone):

• The outer zone is the hottest and burns with complete combustion, resulting in a blue flame.
• It is the hottest part of the flame and is usually non-luminous (does not glow brightly).

5.0Fuels and Their Efficiency

A fuel is a substance that produces heat and light when burnt. Fuels can be classified into three categories:

• Solid Fuels: Wood, coal, charcoal
• Liquid Fuels: Petrol, diesel, kerosene
• Gaseous Fuels: LPG (liquefied petroleum gas), natural gas, biogas

Fuel Efficiency:

The efficiency of a fuel is determined by its calorific value, which is the amount of heat energy produced when 1 kilogram of fuel is completely burnt. It is expressed in kilojoules per kilogram (kJ/kg).

Calorific Values of Different Fuels:

Some fuels, like LPG and natural gas, are considered more efficient because they have higher calorific values, meaning they produce more energy with less fuel.

6.0Harmful Effects of Burning Fuels

While burning fuels is essential for our day-to-day activities, it can also have harmful effects on the environment and health. Some of the key negative impacts are:

1. Air Pollution: Burning of fuels releases harmful gases like carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen oxides (NOₓ) into the atmosphere, leading to air pollution.
2. Global Warming: Excessive burning of fossil fuels increases the amount of CO₂ in the atmosphere, contributing to the greenhouse effect and global warming.
3. Acid Rain: Pollutants like sulfur dioxide and nitrogen oxides react with water vapor in the atmosphere to form acids, which fall back to the earth as acid rain. This can damage crops, buildings, and aquatic life.
4. Respiratory Problems: Inhalation of polluted air can cause respiratory issues like asthma, bronchitis, and other lung diseases.

7.0Fire Extinguishers

Fire extinguishers are devices used to put out small fires by cutting off the fire's supply of oxygen or by lowering its temperature.

Common types of fire extinguishers include:

• Water extinguishers: Used for fires involving paper, wood, or cloth.
• CO₂ extinguishers: Effective for electrical fires or fires involving flammable liquids.
• Foam extinguishers: Used for fires involving flammable liquids like petrol.

8.0Solved Questions

Q. Name the unit in which the calorific value of a fuel is expressed.

Ans. The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

Q. Explain how CO₂ is able to control fires.

Ans. CO₂ is heavier than air and forms a blanket around the fire, cutting off the supply of oxygen and thereby stopping the combustion process.

Q. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Ans. Green leaves contain moisture, which absorbs heat and makes them difficult to ignite. In contrast, dry leaves have no moisture and easily catch fire as they burn faster.

Q. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Ans. A goldsmith uses the outermost (non-luminous) zone of the flame because it is the hottest zone and ensures complete combustion, making it ideal for melting gold and silver.