Page 75 - NCERT Science Class 10 English Medium

mixture is burnt to give a clean blue flame. If you observe the bottoms of
cooking vessels getting blackened, it means that the air holes are blocked
and fuel is getting wasted. Fuels such as coal and petroleum have some
amount of nitrogen and sulphur in them. Their combustion results in
the formation of oxides of sulphur and nitrogen which are major
pollutants in the environment.

Why do substances burn with or without a flame?
Have you ever observed either a coal or a wood fire? If not, the next time you get a
chance, take close note of what happens when the wood or coal starts to burn. You
ou Know? have seen above that a candle or the LPG in the gas stove burns with a flame. However,
you will observe the coal or charcoal in an ‘angithi’ sometimes just glows red and
gives out heat without a flame. This is because a flame is only produced when gaseous
substances burn. When wood or charcoal is ignited, the volatile substances present
vapourise and burn with a flame in the beginning.
Do Y A luminous flame is seen when the atoms of the gaseous substance are heated and
start to glow. The colour produced by each element is a characteristic property of
that element. Try and heat a copper wire in the flame of a gas stove and observe its
colour. You have seen that incomplete combustion gives soot which is carbon. On
this basis, what will you attribute the yellow colour of a candle flame to?

Formation of coal and petroleum
More to Know! other plants that lived millions of years ago. These were crushed into the earth,
Coal and petroleum have been formed from biomass which has been subjected to
various biological and geological processes. Coal is the remains of trees, ferns, and

perhaps by earthquakes or volcanic eruptions. They were pressed down by layers of
earth and rock. They slowly decayed into coal. Oil and gas are the remains of millions
of tiny plants and animals that lived in the sea. When they died, their bodies sank to
the sea bed and were covered by silt. Bacteria attacked the dead remains, turning
them into oil and gas under the high pressures they were being subjected to.
Meanwhile, the silt was slowly compressed into rock. The oil and gas seeped into the
porous parts of the rock, and got trapped like water in a sponge. Can you guess why
coal and petroleum are called fossil fuels?

4.3.2 Oxidation

Activity 4.5 You have learnt
4.5
Activity 4.54.5
Activity
Activity
Activity 4.5
about oxidation reactions in
n Take about 3 mL of ethanol in a test tube and warm it the first Chapter. Carbon
gently in a water bath. compounds can be easily
n Add a 5% solution of alkaline potassium permanganate oxidised on combustion. In
drop by drop to this solution. addition to this complete
n Does the colour of potassium permanganate persist when oxidation, we have reactions
it is added initially? in which alcohols are
n Why does the colour of potassium permanganate not converted to carboxylic
disappear when excess is added?
acids –

70 Science

2024-25