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.

Inflammable substances

• They have very low ignition temperature and can easily catch fire with flame.

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.

(ii) Spontaneous combustion

• The combustion in which substances suddenly burst into flames, without the application of any apparent cause.

(iii) Explosion

• The combustion in which sudden reactions take place on ignition of some substances to produce heat, light, and sound.

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).