Page 17 - NCERT Science Class 10 English Medium

What causes this? The white precipitate of BaSO is formed by the
4
2–
2+
reaction of SO and Ba . The other product formed is sodium chloride
4
which remains in the solution. Such reactions in which there is an
exchange of ions between the reactants are called double displacement
reactions.
t
i
v
Ac
Activity yc ivity y
c
1
1
.
i
1.2
1.
Recall lle all ll
e e
R R
2
R Recac ca l l Activitt tivit t y 1.2 2.2, where you have mixed the solutions of lead(II) nitrate
a
A
c
A
and potassium iodide.
(i) What was the colour of the precipitate formed? Can you name the compound
precipitated?
(ii) Write the balanced chemical equation for this reaction.
(iii) Is this also a double displacement reaction?
1.2.5 Oxidation and Reduction
Activity 1.11
Activity
Activity 1.11
Activity 1.111.11
Activity 1.11
n Heat a china dish containing about 1 g
copper powder (Fig. 1.10).
n What do you observe?
The surface of copper powder becomes coated with
black copper(II) oxide. Why has this black
substance formed?
This is because oxygen is added to copper and
copper oxide is formed.
Figure 1.10
Figure 1.10
Figure 1.10
Figure 1.10
Figure 1.10
Oxidation of copper to 2Cu + O Heat → 2CuO (1.28)
2 
copper oxide
If hydrogen gas is passed over this heated material (CuO), the black
coating on the surface turns brown as the reverse reaction takes place
and copper is obtained.
Heat
CuO +H  Cu+H O (1.29)
→
2
2
If a substance gains oxygen during a reaction, it is said to be oxidised.
If a substance loses oxygen during a reaction, it is said to be reduced.
During this reaction (1.29), the copper(II) oxide is losing oxygen and
is being reduced. The hydrogen is gaining oxygen and is being oxidised.
In other words, one reactant gets oxidised while the other gets reduced
during a reaction. Such reactions are called oxidation-reduction reactions
or redox reactions.
(1.30)
Some other examples of redox reactions are:
ZnO + C → Zn CO (1.31)
+
MnO + 4 HCl → MnCl + 2 H O + Cl (1.32)
2 2 2 2

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