These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 7 Notes – Combustion act as excellent revision resources, particularly in preparation for board exams.
Combustion Class 8 DAV Notes
→ Combustion: When a material burns in the presence of oxygen and produces heat and light, the process is called combustion.
→ Combustible Material: A material which easily catches fire and produces heat and light; on heating in the presence of oxygen is called combustible material.
→ Fuel: A substance which undergoes combustion is called a combustible substance or fuel.
→ Conditions Required for Combustion:
- A combustible substance.
- Continuous supply of oxygen or air.
- The temperature of combustible material should be above its ignition temperature.
→ Ignition Temperature: The lowest temperature at which a substance starts burning is called its ignition temperature.
→ Types of Combustion
(a) Slow Combustion: When a combustive material burns at slow or moderate rate, it is called slow combustion. Examples: burning of cowdung cakes, wood, etc.
(b) Rapid Combustion: When a combustible material burns at fast rate, it is called rapid combustion. Examples: burning of petrol and LPG.
(c) Spontaneous Combustion: When a combustible material starts burning at room temperature, it is called spontaneous combustion. Example: Sodium and phosphorus catch fire at room temperature instantaneously.
(d) Explosive Combustion: When a combustible substance completely burns in a very short period of time and releases lot of heat, light and gases, it is called explosive combustion. Examples: burning of firecrackers, gunpowder, etc.
→ Fire Control:
Fire can take place under following circumstances:
- There is a combustible material.
- There is continuous supply of air.
- Temperature of combustible material is higher than its ignition temperature.
→ Goals of fire control:
- Removing the combustible material.
- Cutting off air supply.
- Bringing down the temperature below ignition temperature.
In case a building catches fire, it is not possible to remove the combustible material, i.e. the building. So, cutting off air supply and/or bringing down the temperature below ignition temperature remain the primary goals of fire control.
→ Water as fire extinguisher: Water is the most commonly used as fire extinguisher. It helps to bring down the temperature below ignition temperature. Moreover, water vapours form a blanket around the fire and cut off air supply. Thus, water helps in controlling fire. But water cannot be used in case of fire because of short circuit or fire because of oil. Normal water is a good conductor of electricity and hence in case of fire because of short circuit, use of water may lead to electric shock to the firefighters. Water is heavier than oil, so in case of fire because of oil, oil would come over water and it would be impossible to control fire.
→ Soda Acid Fire Extinguisher: Soda-acid fire extinguisher contains sodium hydrogen carbonate in a container and sulphuric acid in a bottle. The bottle is attached with a plunger which can break the bottle. When plunger is pressed, the bottle breaks open and acid starts reacting with sodium hydrogen carbonate. This results in evolution of carbon dioxide gas. We know that carbon dioxide gas does not support combustion and thus it is able to control fire.
Blankets and sand are also used to control fire, albeit on small scale.
→ Incomplete Combustion: When there is insufficient supply of oxygen, combustion remains incomplete. Following are the results of incomplete combustion:
- A part of unbumt carbon passes into the atmosphere in the form of soot. This causes atmospheric pollution, apart from wastage of fuel.
- Carbon monoxide is formed. This is a highly poisonous gas and can even prove fatal.
→ Flame: A flame is the region of burning gases. A flame has following zones:
- Innermost Zone: This is coldest part of the flame. This zone consist of unburnt vapour.
- Middle Zone or Luminous Zone: In this zone, partial combustion takes place and lot of energy is liberated. This zone is hotter than the innermost zone and is yellow in colour.
- Outermost Zone or non-luminous Zone: In this zone, complete combustion takes place. This is the hottest part of the flame and is colourless.
→ Fuel and Calorific Value: The amount of heat energy produced on complete combustion of one kilogram of a fuel is called the calorific value of a fuel.
The unit of calorific value is kilojoules per kilogram (KJ/kg)
→ Characteristics of good fuel:
- It should be cheap and readily available.
- It should be easy to store.
- It should have high calorific value.
- It should not produce harmful gases as pollutants.
- It should have low ignition temperature but that should not be below room temperature.
- It should undergo complete combustion and should be controllable.
→ Benefits of Gaseous Fuels:
- Gaseous fuels can be transported in cylinders or through pipelines. This saves on the cost of transportation.
- Gaseous fuels produce large amount of energy and do not leave behind any residue.
- Compared to solid or liquid fuels, gaseous fuels produce least amount of harmful gases.
- Gaseous fuels can be easily ignited with a small spark and the rate of combustion can be easily controlled with the help of a control valve.
→ Harmful Effects of Fuel:
- Carbon fuels release unburnt particles in the air. Excess amount of these particles in the atmosphere causes respiratory problems. Moreover, air pollution is harmful for plants and animals also.
- Incomplete combustion of carbon fuels produces carbon monoxide gas. It is a highly poisonous gas and even proves fatal.
- Combustion of carbon fuels also produces carbon dioxide gas. Excess amount of carbon dioxide in the atmosphere leads to global warming.
- Combustion of carbon fuels also produces oxides of sulphur and nitrogen. These oxides mix with rainwater and form acid rain. Acid rain damages plants and buildings.
→ Fuel : A substance which gives heat and light on burning in air is called fuel.
→ Fossil fuel : Fuels which were formed from remains of plants and animals over millions of years.
→ Combustion : When a substance bums in the presence of oxygen to produce heat and light, the process is called combustion.
→ Inflammable Substance : A substance which easily catches fire is called inflammable substance.
→ Air pollution : Addition of impurities in air which makes it harmful for living beings.
→ LPG : Liquefied Petroleum Gas
→ CNG : Compressed Natural Gas