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Types of Hardness of Water
Identify types of hardness of water
The hard water in some areas can be softened simply by
boiling the water, but this is not true in all cases. This means that the
hardness in water can be divided into two types – temporary and permanent
hardness.
Temporary hardness
Temporary hardness in water is caused by dissolved calcium
or magnesium hydrogencarbonates. The most important characteristic of
temporarily hard water is that it can be softened by simply boiling. When the
water is boiled, the soluble sodium hydrogencarbonate is decomposed to form the
insoluble calcium carbonate.
The decomposition causes the “furring” of kettles, hot water
pipes and shower heads. This means that the inside of kettles, pipes and shower
heads become coated with a layer of calcium carbonate (limescale) caused by the
decomposition of the hydrogen carbonate according to the equation above.
In many supermarkets, it is possible to buy a limescale
remover. This is often a solution of methanoic acid (formic acid). This weak
acid is strong enough to react with limescale but not with the metal. The
insoluble limescale (carbonate) is probably dissolved to a soluble compound,
calcium methanoate that can be flushed away with water.
2COOH(aq) + CaCO3(s)(insoluble)→ Ca(HCOO)2(aq) + H2O(l) +
CO2(g)calcium methanoate (soluble)
Permanent hardness
Permanent hardness in water is caused by soluble sulphates
and chlorides of calcium and magnesium (CaSO4, MgSO4, CaCl2 and MgCl2). This
type of hardness cannot be removed by boiling the water. This is because
boiling does not decompose the chlorides of calcium or magnesium. Such water
may only be softened by chemical treatment or ion exchange methods
Activity 2:
Distinction between temporarily and permanently hard water
Materials:
Calcium carbonate
Dilute
hydrochloric acid
4 test tubes
Test tube rack
1 litre of
distilled water
Calcium chloride
Soap solution
Beaker
Heat source
Method:
Prepare carbon
dioxide gas in the laboratory by mixing calcium carbonate with hydrochloric
acid in a gas generator.
Bubble the gas
through a suspension of calcium carbonate in water. Shake well as you bubble
the gas until most of calcium carbonate has dissolved. Filter and divide the
filtrate into two test tubes M and N.
Prepare a 0.5M
solution of calcium chloride and divide it in two test tubes P and Q.
Prepare soap
solution in a large beaker.
Arrange the four
test tubes in a rack
Heat the solutions
in test tubes M and P to boiling. Allow them to cool.7. Add 10cm3 of the soap
solution to each of the four test tubes, M, N, P and Q. Shake well and allow to
rest. Observe lathering and scum formation.
Note:
When calcium carbonate is reacted with dilute hydrochloric
acid, carbon dioxide gas is produced.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
When carbon dioxide gas is bubbled through a suspension of
calcium carbonate in water for a long time, the insoluble calcium carbonate
dissolves to give the soluble bicarbonate, the presence of which makes the
water hard.
CO2(g) + CaCO3(s) + H2O(l) → Ca(HCO3)2(aq)
The purpose of heating solutions in test tubes M and P was
to try to remove water hardness. However, only the hardness in test tube M was
merely removed by boiling because it contains the temporarily hard water.
The hardness in test tube P could not be removed by just
boiling because it contained the hard water. Calcium chloride cannot be
decomposed by heat. So, no change is expected after heating.
Results:
- Scum was formed in test tubes N, P and Q but P and Q contained more scum than N.
- Lather was formed in test tube M only.
- Test tube M contained temporarily hard water and test tube P contained permanently hard water. The hardness in test tube M was removed by boiling while that in test tube P was not.
- Test tubes P and Q contained permanently hard water. The hardness in this water could not be removed by mere boiling.
