Page 13 - NCERT Science Class 10 English Medium

1.2.2 Decomposition Reaction

Activity 1.5
Activity 1.5
Activity
1.5
Activity 1.51.5
Activity
n Take about 2 g ferrous sulphate crystals
in a dry boiling tube.
n Note the colour of the ferrous sulphate
crystals.
n Heat the boiling tube over the flame of
a burner or spirit lamp as shown in
Fig. 1.4.
n Observe the colour of the crystals after
heating.

Figure 1.4 Have you noticed that the green colour of the ferrous sulphate crystals
Correct way of heating
the boiling tube has changed? You can also smell the characteristic odour of burning
containing crystals sulphur.
of ferrous sulphate
Heat
and of smelling the 2FeSO (s)  Fe O (s) + SO (g) + SO (g) (1.19)
→
4 2 3 2 3
odour
(Ferrous sulphate) (Ferric oxide)
In this reaction you can observe that a single reactant breaks down
to give simpler products. This is a decomposition reaction. Ferrous
sulphate crystals (FeSO . 7H O) lose water when heated and the colour
4 2
of the crystals changes. It then decomposes to ferric oxide (Fe O ),
2 3
sulphur dioxide (SO ) and sulphur trioxide (SO ). Ferric oxide is a solid,
2 3
while SO and SO are gases.
2 3
Decomposition of calcium carbonate to calcium oxide and carbon
dioxide on heating is an important decomposition reaction used in
various industries. Calcium oxide is called lime or quick lime. It has
many uses – one is in the manufacture of cement. When a decomposition
reaction is carried out by heating, it is called thermal decomposition.
Heat
CaCO (s)  CaO(s) + CO (g) (1.20)
→
3 2
(Limestone) (Quick lime)
Another example of a thermal decomposition reaction is given
in Activity 1.6.

Activity
Activity 1.6
Activity 1.6
Activity 1.61.6
Activity 1.6
n Take about 2 g lead nitrate powder in a boiling
tube.
n Hold the boiling tube with a pair of tongs and
heat it over a flame, as shown in Fig. 1.5.
n What do you observe? Note down the change,
if any.

You will observe the emission of brown fumes.
Figure 1.5
Heating of lead nitrate and These fumes are of nitrogen dioxide (NO ). The
2
emission of nitrogen dioxide reaction that takes place is –

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