Page 37 - NCERT Science Class 10 English Medium

What does 10H O signify? Does it make Na CO wet? We will address
2 2 3
this question in the next section.
Sodium carbonate and sodium hydrogencarbonate are useful
chemicals for many industrial processes as well.

Uses of washing soda
(i) Sodium carbonate (washing soda) is used in glass, soap and
paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic
purposes.
(iv) It is used for removing permanent hardness of water.

2.4.4 Are the Crystals of Salts really Dry?

Activity
Activity
Activity 2.15
Activity 2.15
2.15
Activity 2.152.15
n Heat a few crystals of copper sulphate
in a dry boiling tube.
n What is the colour of the copper
sulphate after heating?
n Do you notice water droplets in the
boiling tube? Where have these come
from?
n Add 2-3 drops of water on the sample
of copper sulphate obtained after
heating.
n What do you observe? Is the blue
colour of copper sulphate restored?

Figure 2.9
Figure 2.9
Figure 2.9 Copper sulphate crystals which seem to be dry contain water of
Figure 2.9
Figure 2.9
Removing water crystallisation. When we heat the crystals, this water is removed and the
of crystallisation
salt turns white.
If you moisten the crystals again with water, you will find that blue
colour of the crystals reappears.
Water of crystallisation is the fixed number of water molecules present
in one formula unit of a salt. Five water molecules are present in one
formula unit of copper sulphate. Chemical formula for hydrated copper
sulphate is Cu SO . 5H O. Now you would be able to answer the question
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whether the molecule of Na CO .10H O is wet.
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One other salt, which possesses water of crystallisation is gypsum.
It has two water molecules as water of cyrstallisation. It has the chemical
formula CaSO .2H O. Let us look into the use of this salt.
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Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes
1
calcium sulphate hemihydrate ( CaSO . H O ). This is called Plaster of
4
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