Flame and fuel : Science Ncert / Cbse Revision Notes



The substances which vapourise while burning, give flames. Kerosene oil and molten wax are substances that give a flame while burningWood and charcoal are substances that do not vaporise, but still burn, without any flame.

There are two types of flames. 

The luminous flame and the non-luminous flame.
A luminous flame is a bright yellow flame that gives off light. A luminous flame undergoes incomplete combustion as it does not get the oxygen that it requires.
A non-luminous flame is colourless and is much hotter. A non-luminous flame undergoes complete combustion as it draws much more oxygen and gets much hotter.
There are several "zones" within a non-luminous flame, and each zone has a different temperature. The outermost zone of the flame is blue in colour and it is the hottest part. This is due to complete combustion. The middle zone is moderately hot and is yellow in colour. This is because of partial combustion. The innermost zone is the least hot and black in colour. This is due to the presence of unburned wax vapours


Fuel is any material that is burned to obtain energy that can be used to heat or move another object. Fuel releases energy through a chemical reaction known as combustion.

When something burns, it produces heat energy, but that does not necessarily make it a fuel. Or at least, not a good fuel.

A good fuel must:
  • Be readily available.
  • Be cheap.
  • Burn easily at a moderate rate.
  • Produce a large amount of heat.
  • Not leave behind any undesirable substances.

Unfortunately, there is no fuel that has all these qualities, and hence no fuel can be considered as an ideal fuel.

There are different types of fuels – solid, liquid and gaseous. This classification is based simply on the state of the fuel. Wood was the first fuel that was used 2 million years ago by homo erectus, the predecessor of human beings.

Flame and fuel, Science Cbse / Ncert Revision Notes, types of fuel.

Calorific value is defined as the amount of heat energy produced on complete combustion of 1 kilogram of a fuel. It is expressed in a unit called kilojoule per kg. The higher the calorific value of a fuel, the more is its efficiency.

Each kilogram of LPG produces much more heat than one kilogram of wood or coal. The calorific value of LPG is the highest among wood, charcoal and LPG. In rural areas, cow dung and wood are still used as fuel because these are very cheap and easily available.
However, burning wood produces a lot of smoke, which is very harmful to humans, since it causes respiratory problems. Cutting down trees for fuel also leads to deforestation, which harms the environment and also deprives us of all the other benefits of trees.
Unburned carbon particles released when carbon fuels like wood, coal and petroleum burn, cause pollution and respiratory diseases such as asthma.
Incomplete combustion of carbon fuels causes the release of carbon monoxide – a very harmful gas. Combustion of fuels causes the release of carbon dioxide, which leads to global warming. Such rise in temperatures can cause melting of polar glaciers, a rise in sea level, and the flooding of low-lying areas of the world. Oxides of sulphur and nitrogen dissolve in rain water to form acid rain, which ruins soil, crops and buildings.

Must Read : Natural Gases

By choosing the right fuel, we can reduce the negative impact on the environment. A great example of this is cars, buses and auto rickshaws that run on Compressed Natural Gas, or CNG, instead of petrol. CNG is a much cleaner and cheaper fuel.

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