Combustion & Flame Summary Class 8 NCERT Summary

Introduction

• Fuels are used to get energy in the form of heat. Cow dung, wood, coal, charcoal, petrol, diesel, compressed natural gas (CNG), etc. are the examples of fuels.
• Candle burns with a flame whereas coal does not.

Combustion

• It is a chemical process in which a substance reacts with oxygen to give off heat and light.
• Oxygen (in air) is essential for combustion.
• Substances that burn in air are called combustible substances (also called fuels) and those that do not burn in air are non-combustible substances.

Ignition temperature

It is the lowest temperature at which a substance catches fire.

Substances like kerosene oil and petrol have low ignition temperatures and are called inflammable substances.

Supporter of combustion

• The gaseous environment that supports combustion of a combustible substance is called supporter of combustion.
• Smaller the size of combustible particles, faster is the rate of combustion.
• Nature of combustible substances: Inflammable substances burn faster as compared to substances such as wood.

Control of fire

• Water is commonly used to extinguish fire. It is not suitable for fires involving oil, petrol, and electrical equipments.
• For fires involving oil, petrol, and electrical equipments, carbon dioxide is the best extinguisher.

Types of combustion

(i) Rapid combustion

The combustion in which substances burn rapidly to produce heat and light. In rapid combustion, external heat must be supplied (e.g., gas stove).

(ii) Spontaneous combustion

The combustion in which substances suddenly burst into flames, without the application of any apparent cause (e.g., phosphorus, coal dust).

(iii) Explosion

The combustion in which sudden reactions take place on ignition of some substances to produce heat, light, and sound (e.g., fireworks).

Zones of candle flame

• Dark zone (middle zone), luminous zone (innermost zone), and non-luminous zone (outer zone).

• A good fuel is one which is cheap is readily available burns easily in air at a moderate rate produces large amount of heat does not leave behind any undesirable substances.

Fuel efficiency

• It is expressed in terms of calorific value. The unit is kilo joule per kg.

Calorific value

• It is the amount of heat energy produced by complete combustion of 1 kg of a fuel.
• It is expressed in Kilo joule per kg (kJ/kg).