Combustion And Flames Class 8 Chapter Notes

Key Points: Must Remember

• Combustion is a chemical process in which a substance reacts with oxygen to give off heat and often light.
• The three essential requirements for combustion are Fuel, Oxygen (air), and Ignition Temperature (heat). This is often called the 'Fire Triangle'.
• Ignition temperature is the lowest temperature at which a substance catches fire.
• Substances with very low ignition temperatures are called inflammable substances (e.g., petrol, LPG, alcohol).
• There are three main types of combustion: Rapid Combustion, Spontaneous Combustion, and Explosion.
• Fire can be controlled by removing the fuel, cutting off the air supply, or lowering the temperature (below ignition temperature).
• Water is a common fire extinguisher but is ineffective for electrical fires and oil fires.
• Carbon dioxide (CO2) is excellent for electrical and oil fires as it is heavier than oxygen and non-conductive.
• A flame is the visible, gaseous part of a fire. Not all substances produce a flame upon burning.
• The calorific value of a fuel is the amount of heat energy produced on complete combustion of 1 kg of fuel (measured in kJ/kg).

Essential Terminology

Combustion

A chemical process in which a substance reacts rapidly with oxygen and produces heat and light.

Ignition Temperature

The minimum temperature at which a substance catches fire and begins to burn.

Inflammable Substances

Substances that have a very low ignition temperature and can easily catch fire with a flame.

Calorific Value

The amount of heat energy produced on complete combustion of 1 kilogram of a fuel, expressed in kilojoules per kilogram (kJ/kg).

Fuel

A substance that produces heat and light energy on burning, such as wood, coal, LPG, petrol.

Fire Extinguisher

A device used to put out small fires, typically by removing one or more elements of the fire triangle.

Luminous Zone

The middle, yellow, and bright part of a candle flame, which is rich in unburnt carbon particles and where incomplete combustion occurs.

Non-luminous Zone

The outermost and hottest part of a candle flame, where complete combustion takes place due to ample oxygen supply.

Understanding Combustion: The Basics

Combustion is a fundamental chemical process that plays a vital role in our daily lives, from generating electricity to cooking food. At its core, combustion is defined as a chemical process in which a substance reacts with oxygen to give off heat and often light. The substance undergoing combustion is called the combustible substance or fuel. For instance, when you burn a piece of wood, the wood is the fuel, and it reacts with oxygen from the air.

For combustion to occur, three essential conditions must be met, often referred to as the 'Fire Triangle'. These are:

1. Fuel: The combustible substance itself. Without a fuel source, there's nothing to burn. Fuels can be solids (wood, coal), liquids (petrol, kerosene), or gases (LPG, CNG).
2. Oxygen (or Air): A supporter of combustion. Most combustion reactions require oxygen. If the supply of oxygen is cut off, the fire will extinguish.
3. Ignition Temperature: This is perhaps one of the most critical conditions. The ignition temperature is the lowest temperature at which a combustible substance catches fire and starts burning. Different substances have different ignition temperatures. For example, a piece of paper catches fire much more easily (at a lower temperature) than a log of wood. Substances with very low ignition temperatures, like LPG, petrol, and alcohol, are termed inflammable substances.

When these three conditions are present, combustion can begin. The rate and characteristics of combustion can vary, leading to different types, such as rapid combustion where substances burn quickly, or spontaneous combustion where materials ignite on their own without external heat. Understanding these basics is key to both harnessing and controlling fire.

Types of Combustion

Fire Control: Methods of Extinguishing Fire

1. — The most direct way to extinguish a fire is to remove the combustible material. If there's no fuel, there's nothing left to burn. This is often impractical in large fires like buildings or forests but can be effective in smaller scenarios.
2. — Oxygen is a supporter of combustion. By cutting off its supply, the fire can be put out. This is why covering a small fire with a blanket or sand works. Carbon dioxide (CO2) extinguishers work by forming a blanket around the fire, displacing oxygen.
3. — Bringing down the temperature of the burning substance below its ignition temperature will extinguish the fire. Water is commonly used for this purpose as it cools the combustible material and also helps cut off the air supply by forming water vapour.

Exam Tip: Differentiating Fuels and Flames

When answering questions about fuels and flames, pay close attention to definitions and specific examples. Always clarify if a substance 'burns with a flame' or 'without a flame'. For instance, wood burns with a flame, but coal primarily glows. Also, remember the specific uses of fire extinguishers: Water is NOT suitable for electrical fires or oil fires. For electrical fires, water conducts electricity, posing a shock hazard. For oil fires, water is heavier than oil, so the oil floats on top and continues to burn. In these cases, carbon dioxide (CO2) extinguishers are preferred as CO2 is non-conductive and heavier than oxygen, blanketing the fire. Clearly stating these exceptions earns you full marks!

Practice Questions with Solutions

• Q: What are the three essential conditions required for combustion? A: The three essential conditions for combustion are Fuel, Oxygen (air), and Ignition Temperature.
• Q: Why is carbon dioxide (CO2) preferred over water for extinguishing electrical fires? A: Carbon dioxide is preferred because it is non-conductive, preventing electrical shocks, and it is heavier than oxygen, blanketing the fire and cutting off the air supply. Water is a conductor of electricity.
• Q: Give two examples of inflammable substances. A: Two examples of inflammable substances are petrol and Liquefied Petroleum Gas (LPG).
• Q: What is the difference between the luminous and non-luminous zones of a flame? A: The luminous zone is the middle, yellow part where incomplete combustion occurs due to insufficient oxygen, making it bright. The non-luminous zone is the outermost, blue, and hottest part where complete combustion takes place due to ample oxygen, making it less bright.

Frequently Asked Questions