Study Material and Notes of Ch 6 Combustion and Flame Class 8th Science
Topics in the Chapter
• Combustion
• Ignition temperature
→ Inflammable substances
• Supporter of combustion
• Control of fire
• Types of combustion
→ Rapid combustion
→ Spontaneous combustion
→ Explosion
• Zones of candle flame
• Fuel efficiency
• Calorific value
Introduction
→ Fuels are used to get energy in the form of heat. Cowdung, 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).