Chemistry Reference
In-Depth Information
Ca
2
Ca
2
Ca
2
Ca
2
Ca
2
ions
in hard water
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Ca
2
Ca
2
Ca
2
Ca
2
ions
stay on
column
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
ions
on resin
b
Ion exchange resin.
When all the sodium ions have been removed
from the resin, it can be regenerated by pouring a
solution of a suitable sodium salt through it.
•
Distillation.
The water is distilled away from the
dissolved substances. This method, however, is far
too expensive to be used on a large scale.
Na
ions
in the
softened
water
column before use
Na
Na
Na
Na
Na
Na
column in use
Advantages and disadvantages of
hard water
Problems and advantages associated with hard water
are detailed in Table 11.8.
a
The ion exchange process.
Figure 11.36
The substances in permanently hard water are not
decomposed when heated and therefore cannot be
removed by boiling. Both types of hardness can
be removed by any of the following methods.
•
Addition of washing soda (Na
2
CO
3
.10H
2
O)
crystals.
In each type, the calcium or magnesium
ion, which actually causes the hardness, is removed
as a precipitate and can, therefore, no longer cause
hardness.
calcium ions + carbonate ions → calcium
(from hard water)
(from washing soda)
carbonate
Ca
2+
(
aq
) + CO
3
2−
(
aq
) → CaCO
3
(
s
)
•
Ion exchange.
The water is passed through a
container fi lled with a suitable resin containing
sodium ions. The calcium or magnesium ions
causing the hardness are exchanged for the sodium
ions in the resin (Figure 11.36).
calcium
ion
Ca
2+
(
aq
)
Table 11.8
Disadvantages and advantages of hard water.
Disadvantages
Advantages
Wastes soap
Has a nice taste
Causes kettles to fur
Calcium ions in hard water are required by
the body for bones and teeth
Can cause hot water
pipes to block
Coats lead pipes with a thin layer of lead(
ii
)
sulfate or lead(ii)
ii
) carbonate and cuts down
the possibility of lead poisoning
Can spoil the fi nish of
some fabrics
Some industries favour hard water. For
example, the leather industry prefers to
cure leather in hard water
Questions
1
Make a list of the main methods of softening hard water.
In each case write a suitable equation(s) to summarise the
chemical reactions involved.
2
One of the substances found in some temporary hard
waters is magnesium hydrogencarbonate. Write word and
balanced chemical equations to show the effect of heat on
this substance in aqueous solution.
+
sodium-resin
→
calcium-resin
+
sodium
ion
2Na
+
(
aq
)
+
Na
2
-R(
s
)
→
Ca-R(
s
)
+
