WATER
Hard
and soft water – experiments with hard water – removal of hardness –
conservation of water.
REVIEW
You
have studied many aspects of water in your previous classes. Water is a
wonderful substance. It is one of our invaluable resources. Without it life can
neither exist on earth nor can the bio-chemical reactions take place. Let us
once again recall some of the important points that you have already studied
about water.
Water
is a compound made of hydrogen and oxygen.
Most
of the land on earth is Covered with water.
Water
exists on earth in all the three states – solid, liquid, and vapor.
Water
can dissolve the highest number of substances in it.
Most
of the substances that dissolve in water are inorganic ionic compounds.
The
ionic compounds dissociate into positive and negative ions when dissolved in
water.
Seawater
is saline and hence not potable. Distillation is a method of obtaining pure
water from seawater.
Even
distilled water is not potable.
Water
gets cycled continually through the biosphere and this movement is called water
cycle.
Water
sources are being polluted due to human activities.
Indian
philosophers regarded water as one of the five elements that constitute the
universe. Even the westerners had similar views. A British chemist, Sir Henry
Cavendish showed for the first time in 1781, that water is produced when
hydrogen burns is air. Lavosier, a Frech chemist proposed that water is not an
element but a compound made of hydrogen and oxygen.
INTRODUCTION
The
water obtained from natural sources have several salts dissolved in it. Salts
such as nitrates, sulphates, chlorides, and bicarbonates might have been
dissolved in water. Pure water has neither color and taste nor smell.
Water
gets colour, teste, and smell due to impurities. Water acquires certain
properties other than colour, taste and smell, when certain salts are dissolved
in it. The change in the way it interacts with soap is one such property. Based
on the way in which it acts with soap, water is classified as soft water and
hard water.
Note
Rainwater
dissolves small amount of atmospheric carbon dioxide in it to form a weak acid
called carbonic acid. The other chemicals that are let into the atmosphere may
also e dissolved in it. Hence even rainwater is not pure.
SOFT
WATER AND HARD WATER
Water
that gives lather with soap easily and readily is called soft water. Pure water
is always soft. Water may be soft even when some salts are dissolved in it.
Experiment
1 :
Collect samples of water from different sources – tap, bore well, pond or
lake etc. Take equal quantity of the samples collected in different test tubes.
Add about 1 gram of soap powder to each of them. Shake well. Which of them gave
lather easily and spontaneously ? Which sample does not give lather readily ?
 |
| Experiment Soap formation in different water |
 |
| Experiment of soap formation in different water |
Experiment
2 :
Collect samples of water from various sources as mentioned in the
experiment cited above. Add 1 gram of soap powder to each of them and stir
until the soap dissolves in water. Now blow the contents of each of the test
tubes using a straw few times. Which of them gives lather profusely ?
|
| Experiment of soap by air in different water sample |
 |
| Experiment of soap by air in different water sample |
Experiment
3 :
Does the water you use at home give lather readily with soap ? Test and
find out.
Experiment
4 :
Take equal quantities of water in two beakers. Add about 1 gram of calcium
chloride to one of them and stir well. Now add about 1 gram of soap powder to
each beaker. Blow the contents of the beakers using a straw. Which of them give
lather well ? Repeat the experiment by using sodium chloride, sodium
bicarbonate, magnesium chloride, calcium sulphate, and magnesium sulphate
instead of calcium chloride. Which of them give lather ? Which of them does not
give lather ? Examine.
 |
| Test of water with different substance like Calcium Chloride |
 |
| Test of water with different substance like Calcium Chloride |
 |
| Test of water with different substance like Calcium Chloride |
It
is not practically possible to obtain calcium bicarbonate and magnesium
bicarbonate in solid form. This is because these salts chemically dissociate
much before the evaporation is complete and form their respective carbonates.
Therefore, it is only possible to have solutions of calcium bicarbonate and
magnesium bicarbonate.
Certain
salts dissolved in water from a scum with soap. Water that forms an insoluble
scum with soap before giving lather is called hard water. Hard water does not
readily give lather with soap. The dissolved salts such as calcium bicarbonate,
calcium sulphate, calcium chloride, magnesium bicarbonate, magnesium sulphate
and magnesium chloride are the chief salts that cause hardness of water. Let us
consider how these salts render water hard.
Note
that the salt that cause hardness are ionic compounds. These salts ionize to
give positive ions and negative ions when dissolved in water. For example,
magnesium sulphate when dissolved in water, dissociates into Mg++ ions and SO4-
ions. Similarly calcium sulphate (CaSO4) dissociates into Ca++ and SO4-. The
equations showing the process of ionization of salts causing hardness of water,
are given below.
The
salts that cause hardness in water include calcium fluoride and salts of iron. Ions
of iron and manganese will also cause hardness of water.
MgSo4 - Mg++
+ SO4
--
CaSO4 - Ca++ + So4--
CaCL2 - ca++ + So4--
MgCL2 - Mg++ + 2CL-
Ca(HCO3)2 - Ca++ + 2(HCO3)-
Mg(HCO3)2 - Mg++ + 2(HCO3)-
Of
the ions mentioned above, only Mg++ and Ca++ ions cause hardness. These ions
form an insoluble scum with soap. How does hard water form scum with soap ?
Soap (you will know more about it later) is a sodium salt or potassium salt
(Ex: sodium stearate or potassium stearate repectively). These salts react with
Mg++ and Ca++ ions present in water to form magnesium stearte and calcium stearate
respectively. These insoluble precipitates, from a scum on the surface of
contact. Soap begins to give lather only after all the Mg++ and Ca++ ions
present in this water samples are precipitated.
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