NCERT 8th Class (CBSE) Science: Combustion and Flame

Question: Discuss the impact of burning fuels on air.

Answer: The main impacts due to burning of fuels are as follows:

1. Burning of carbon-containing fuels releases ash and fine particles of unburnt carbon in the air. These fine particles, called SPM (suspended particulate Matter) are dangerous pollutants.
2. Combustion of fuels adds carbon dioxide to the environment. A percentage increase in carbon dioxide in the air leads to the greenhouse effect, which can cause global warming. This can result in melting of the Polar ice caps and rise in sea levels, leading to flooding of large portions of land on the sea coasts.
3. Carbon monoxide produced when fuels containing carbon burn in insufficient supply of air, is a very dangerous pollutant and can cause death if breathed in.
4. Coal contains sulphur, which produces sulphur dioxide on burning. Beside being a poisonous gas, it dissolves in rain to form Sulphuric acid. This gives rise to acid rain, which is very harmful for soil, crops, buildings, etc.
5. The oxides of nitrogen given off from exhausts of engines of vehicles are poisonous gases. Many vehicles now use catalytic converters to convert these to harmless gases.
6. Lead compounds are released in exhausts of vehicles. They are poisonous. Increasing use of unleaded petrol is expected to reduce lead pollution.

Question: Only gases burn with a flame. But when you burn wood, it initially burns with a flame. Later, it only glows without a flame. What do You think is the reason for this?

Answer: When wood is burnt, initially the gases trapped in it are released. These gases burn with a flame. Once the gases are exhausted, the wood only glows and does not burn with a flame.

Question: Would you consider coal as a good fuel? Give reasons.

Answer: While coal has many necessary attributes that a good fuel should have, its major drawbacks are that it causes air pollution and leaves behind residue (ash). Therefore in the existing situation, where pollution is a major worry, it will be better not to use it in its natural form but to convert it into coke.

Question: When a fuel is burnt, carbon dioxide (or carbon monoxide) and water vapour are given out. Can you name one fuel which burns without giving off water vapour? Why does it not give out water?

Answer: Hydrocarbon fuels give off water vapour when burnt due to the presence of hydrogen in them. Coal and coke, which are carbon and not hydrocarbons, will not give off water vapour when burnt.

Question: Why does a matchstick burn on rubbing it on the side of the matchbox?

Answer: When a matchstick is rubbed on the side of a matchbox, heat is given off due to friction with the rough surface. The ignition temperature of the material used in matchsticks is low enough to catch fire because of this heat.

Question: Your LPG gas stove at home is giving a yellow flame. What can this mean?

Answer: The LPG gas stove gives a yellow flame due to incomplete combustion of the gas. This can be due to blockage of burner holes, or improper adjustment of the air-gas mixture.