Class 8 Science - Combustion and Flame
1. Combustion
The process in which a substance combines chemically with oxygen with the simultaneous evolution of heat and light is called combustion.
Examples: Burning of magnesium ribbon, charcoal, coal, etc.
2. Conditions Necessary for Combustion
For combustion (or burning) to occur, three conditions must be satisfied:
- Presence of a combustible substance (like wood, coal, petrol, kerosene, etc.)
- Presence of a supporter of combustion (usually oxygen in the air)
- Heating up to ignition temperature (the minimum temperature at which a substance catches fire)
➡️ If any of these conditions is not present, combustion does not take place.
3. Terms Used in Combustion
- Combustible substances: Substances that can burn in air/oxygen.
Examples: Petrol, kerosene, LPG, coal, wood, paper, wax.
- Non-combustible substances: Substances that do not burn in air/oxygen.
Examples: Water, glass, sand, cement, stone.
- Supporter of Combustion: The substance that helps combustion.
Example: Air (oxygen) is the most common supporter of combustion.
- Ignition Temperature (Kindling Temperature):
The lowest temperature at which a combustible substance catches fire.
Petrol has a lower ignition temperature than kerosene and thus catches fire more easily.
- Inflammable Substances: Substances with a very low ignition temperature that catch fire quickly.
Examples: Petrol, LPG, kerosene, alcohol.
4. Flame
A flame is the shining zone in which a gaseous combustible substance undergoes combustion producing heat and light.
Nature of flame (colour and shape) depends on:
- The chemical nature of the fuel
- The apparatus used for burning
Examples:
- Matchstick → yellow flame
- LPG → blue flame
- Kerosene in lamp → yellow smoky flame
- Kerosene in stove → blue smokeless flame (because of better air supply)
General Rule:
- Gaseous fuels → always burn with a flame
- Liquid fuels → burn with a flame after vaporising (e.g., kerosene)
- Solid fuels → burn with a flame only when they vaporise on heating (e.g., wax in candles)
5. Types of Combustion
1. Rapid Combustion
A substance burns quickly with the release of a large amount of heat and light.
Example: Burning of LPG, kerosene, petrol, magnesium.
2. Slow Combustion
A substance reacts with oxygen slowly at a low temperature without flame.
Example: Respiration (digestion of food in our body).
3. Spontaneous Combustion
A substance catches fire suddenly without external heating.
Examples: White phosphorus, haystack, coal dust.
4. Incomplete Combustion
Occurs when there is insufficient supply of oxygen. Produces poisonous gases like carbon monoxide (CO) along with less heat.
Example: Carbon (C) + limited oxygen → Carbon monoxide (CO).
Dangerous because CO is a deadly poison.
5. Complete Combustion
Occurs when there is a sufficient supply of oxygen. Produces carbon dioxide, water, heat and light.
Example: C + O₂ → CO₂ + heat + light.
6. Control of Fire
Fire can be extinguished by removing one or more conditions needed for combustion:
- Removing combustible substance from the area
- Cutting off oxygen (air supply)
- Lowering the temperature below ignition temperature
Methods of Extinguishing Fire
- Spraying water: Lowers temperature + blocks air by steam
- Fire extinguishers (CO₂ / Foamite / dry chemical powder): Cut off oxygen supply and stop the fire
- Blanket method: If clothes catch fire, wrapping in a thick blanket cuts off air supply
⚠️ Not used for oil fires (oil floats on water and fire spreads)
⚠️ Not used for electrical fires (risk of electrocution)
7. Types of Flames
Non-luminous Flame
- Blue in colour
- Produces very little light
- Formed during complete combustion (sufficient air supply)
Examples: LPG stove, kerosene stove, Bunsen burner with open air-hole
Luminous Flame
- Yellow in colour
- Produces heat and significant light
- Formed during incomplete combustion (limited air supply)
Examples: Candle flame, kerosene lamp, matchstick
8. Structure of Candle Flame
Candle is made of paraffin wax, a petroleum product. Wax vaporises on heating, and the vapour burns to form a candle flame.
The candle flame consists of four zones (J.J. Berzelius, 1822):
Complete combustion. Hottest part of flame.
Incomplete combustion of wax vapour. Contains glowing carbon particles. Makes the flame bright. Moderately hot.
Near the wick. No (or very little) combustion due to lack of oxygen. Least hot region. Contains unburnt wax vapour and carbon particles.
Found at the base of the flame near wick. Blue colour due to burning of carbon monoxide formed in the dark zone.
👉 Thus, a candle flame is yellow and luminous mainly because of incomplete combustion of wax vapour.
9. Why Different Flames in Lamp and Stove?
Kerosene Lamp
- Yellow flame
- Kerosene vapour burns in limited air
- Incomplete combustion
Kerosene Stove
- Blue flame
- Air holes supply extra air
- Complete combustion
10. Fuels
A fuel is a combustible substance which on burning produces a large amount of heat and light.
Common fuels: Wood, Coal, LPG, Petrol, Diesel, Kerosene
Types of Fuels (based on physical state)
Solid at room temperature
Coal, Coke, Wood, Charcoal, Animal dung cakes, Agricultural waste
Volatile liquids which produce combustible vapour
Petrol, Diesel, Kerosene
Gases or mixtures of gases that burn
Natural Gas, CNG, LPG, Coal Gas, Water Gas, Producer Gas, Biogas
11. Fossil Fuels
Formed millions of years ago by decomposition of plants and animals under high pressure and temperature inside the earth.
Examples: Coal, Petroleum, Natural Gas
12. Characteristics of an Ideal Fuel
An ideal fuel should:
- Be cheap and easily available
- Burn at a moderate rate
- Not produce poisonous or irritating fumes
- Leave no residue or ash after burning
- Have a high calorific value (give large amount of heat per gram)
- Be safe and convenient to store and transport
- Have ignition temperature above room temperature (to be safe for use)
13. Fuel Efficiency - Calorific Value
Calorific Value: The amount of heat produced when 1 kg of fuel is completely burnt in air or oxygen.
SI unit: kJ/kg (kilojoules per kilogram)
Sometimes expressed in kJ/g or cal/g
| Fuel | Calorific Value (approx.) |
|---|---|
| Wood | 17,000–22,000 kJ/kg |
| Coal | 30,000–40,000 kJ/kg |
| Charcoal | 35,000 kJ/kg |
| Coke | 33,000 kJ/kg |
| Kerosene / Petrol / Diesel | 45,000 kJ/kg |
| Methane | 50,000 kJ/kg |
| LPG (Butane) | 50,000 kJ/kg |
| Biogas | 35,000–40,000 kJ/kg |
| Hydrogen | 1,50,000 kJ/kg (highest calorific value) |
14. Pollution and Problems
Burning of fuels releases harmful gases like carbon dioxide, carbon monoxide, nitrogen oxides, and sulfur oxides.
These gases cause air pollution, acid rain, respiratory diseases, and contribute to the greenhouse effect.
15. Greenhouse Effect
The heating up of Earth's atmosphere due to trapping of infrared radiation by greenhouse gases (mainly carbon dioxide, methane, and water vapour).
Excess CO₂ from burning fuels increases the greenhouse effect causing global warming.
16. Pollution and Problems Caused by Burning Fuels
When fuels (wood, coal, petrol, diesel, kerosene, LPG) are burnt, they produce:
- Carbon monoxide (CO)
- Carbon dioxide (CO₂)
- Oxides of sulphur (SO₂, SO₃)
- Oxides of nitrogen (NO, NO₂)
- Smoke (unburnt carbon particles)
- Ash (in the case of solid fuels)
Harms caused by these pollutants:
- Dust: Allergic reactions & respiratory problems
- Smoke: Breathing problems, may even cause death
- Carbon monoxide (CO): Poisonous; combines with haemoglobin to form carboxy-haemoglobin, reducing oxygen supply, causing suffocation and even death
- Carbon dioxide (excess): Greenhouse effect, rise in earth's temperature
- Oxides of sulphur (SO₂, SO₃): Cause acid rain, corrode structures, affect lungs
- Oxides of nitrogen (NO, NO₂): Cause smog, acid rain, lung diseases
17. Greenhouse Effect & Global Warming
Greenhouse effect: Heating of earth's atmosphere due to trapping of infrared radiation by greenhouse gases (mainly carbon dioxide, methane, water vapour).
Without greenhouse gases, the earth's temperature would be ~30°C lower.
Global warming: Gradual increase in earth's average temperature due to extra CO₂ and other greenhouse gases from fuel burning.
Effects of Global Warming:
- Changes in crop cultivation pattern
- Monsoon disturbance
- Melting of polar ice → rise in sea level → flooding & submerging of coastal areas
18. Acid Rain
Rainwater containing dissolved oxides of sulphur and nitrogen is called acid rain.
Sources: Burning of coal, petrol, diesel in industries, factories, and vehicles
Harmful Effects of Acid Rain:
- Highly acidic and corrosive; corrodes metallic structures
- Damages historical monuments (e.g. Taj Mahal's marble loses shine)
- Harms plants, fishes, and aquatic life
- Reduces soil fertility
19. Judicious Use of Energy Sources
Since fossil fuels (coal, petroleum, natural gas) are limited and non-renewable, we must use them wisely.
Methods to save energy:
- Reduce (slow down) use of petrol, coal, diesel, natural gas
- Encourage use of renewable energy (wind, solar, hydro, geothermal, bioenergy)
- Use fuel-efficient appliances (improved chulha, pressure cooker, smokeless stoves)
- Promote energy-efficient machines in industries and transport
Energy Crisis: Fast depletion of non-renewable energy resources (coal, petroleum, natural gas, uranium)
At present rate, coal may last ~100 years and petroleum ~50 years.
Comments
Post a Comment