Combustion and Flame Class 8 Notes Science Chapter 4

Facts that Matter

• Fuels: Substances which provide energy on burning in air are called fuels. They may be of various types:
  • Solid fuels such as wood, coal, cowdung cakes, charcoal (prepared from wood)
  • Liquid fuels like kerosene (also known as  Mitti ka tel), petrol and diesel.
  • Gaseous fuels such as CNG (Compressed Natural Gas), LPG (Liquefied Petroleum Gas) and coal gas.
• Combustion: It is a chemical process in which a substance reacts with oxygen to give off heat. Sometimes light, either as a flame or as a glow is also given off during combustion.
• In the sun, heat and light are produced by nuclear reactions.
• Ignition Temperature: The lowest temperature at which a substance catches fire is known as its ignition temperature. A substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.
• Contents of a Modern Matchstick
  • Wooden matchstick, that has antimony trisulphide and potassium chlorate at its head
    
     
  • The rubbing surface, present on the matchbox, has powdered glass and a little red phosphorus.

When the head of a matchstick is rubbed against the rubbing surface—

Red phosphorus (of surface) → converts into white phosphorus White phosphorus Antimony trisulphide (in head) → starts combustion

• Inflammable substances: The substances that have very low ignition temperature and can easily catch fire with a flame are known as inflammable substances. For example: Alcohol, Petrol etc.
• Fire Extinguishers: A substance which disrupts the contact between air and the fire is called extinguisher.
  (a) Water is very good substance in firefighting as it cools down the fuel such as wood, below its ignition temperature.
  (b) Sand and soil may act as a good fire extinguisher particularly for burning oils.
  (c) If the clothes of a person catch fire it may be put off with the help of blanket.
  (d) Water should not be used to extinguish fire involving electrical equipment or oils.

In case of electrical fires, water may cause a shock to people fighting with fire.

(e) For fires involving electrical equipment and inflammable materials such as petrol, carbon dioxide (CO₂) is the best extinguisher. Carbon dioxide (CO₂), being heavier than oxygen, covers the fire like a blanket. As the contact between the fuel and oxygen is cut off, the fire is controlled. Compressed CO₂ stored in a cylinder when released on a fire causes
(i) formation of a blanket around the fire  
(ii) cools down the temperature of the fuel.
CO₂ does not harm electrical equipment.

• Types of Combustions:
  (a) Rapid combustion: When combustion occurs rapidly/immediately by applying a flame such as burning of LPG gas in kitchen stove, is known as rapid combustion.
  (b) Spontaneous combustion: The combustion in which a material suddenly bursts into flames, without the application of any apparent cause is known as spontaneous combustion. For example, burning of phosphorus (white) at room temperature, without the effort of burning it.
  (c) Explosion: During ignition of a substance such as cracker, a sudden reaction takes place with the release of heat, light and sound and evolution of a large amount of gas, such a combustion is called explosion.
• Structure of a Flame: Substances which form vapour during heating, burn with a flame. A flame is a region where combustion of gaseous substances or vapour takes place.

The flame has three distinct zones:

(i) Black zone: It is the innermost zone of unburnt vapours.

(ii) Bright and Luminous (yellow) zone: It is the middle zone. Brightness of this zone is due to the glow of unburnt carbon particles.

(iii) Blue zone: It is the outer zone of complete combustion. It is the hottest and non‑luminous zone.

•  Factors which help combustion:
  (a) Combustible substance
  (b) Oxygen
  (c) Surface area of combustible substance: Larger the surface area, more is the rate of combustion.

That is why a block of wood or a log does not burn easily. On splitting the block of wood, its pieces gives larger surface, so combustion becomes easier.

• Ideal (a good) Fuel: An ideal or a good fuel is one which is:
  (a) readily available
  (b) cheap
  (c) it burns easily in air at a moderate rate.
  (d) it produces a large amount of heat.
  (e) it does not leave behind any undesirable substances.

There is probably any fuel that could be considered as an ideal fuel.

• Calorific Value: The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit known as kilojoule per kg (kJ/kg).
• Burning of Fuels Leads to Harmful Products
  (i) Unburnt carbon particles produced by fossil fuels such as wood, coal and petroleum cause many respiratory diseases, such as asthma.
  (ii) Incomplete combustion of carbon (fossil) fuel produces carbon monoxide (CO) which is a very poisonous gas. It can kill persons sleeping in the room containing carbon monoxide.
  (iii) Carbon dioxide produced by burning of fuels causes global warming, i.e. the rise in temperature of the atmosphere of the earth. It leads to melting of polar ice.
  (iv) Burning of fossil fuels like coal, diesel, and petrol release sulphur dioxide and oxides of nitrogen. These gases are extremely suffocating and corrosive.

Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. Acid rain is very harmful for crops, buildings and soil.