WATER
UNIT-8
Revision-importance
of water, general properties of water, water cycle, sources of water, causes of
water pollution, controlling water pollution, need for conservation of water,
chemical composition of water, specific properties, potable water, impurities in
water (soluble and insoluble), purification, distillation, uses of distilled
water.
8.1
Importance of water
Water is
an invaluable natural resource. It is impossible even to guess either the
water less earth or life on earth without water. You have already learnt about
usefulness of water. Do the following activity in order to recall what you have
already learnt.
Activity
8.1 : Each student in the class has to list the uses of water. Do not omit any
use thinking it to be unimportant or insignificant Prepare a consolidated
list. Classify the uses of water based on their nature and importance
(Important uses such as agricultural, cleaning etc.). Prepare a chart
containing the classified list and display it in the class.
This
important resource i.e., water, is available on the earth abundantly.
Astronauts have reported that the earth appears as a blue ball when viewed from
outer space. The reason for this phenomenon is the fact that the out of total
quantity of water available on earth. 97.2% is in oceans which cover 70% of the
earth’s surface. Where do we find the remaining water ? Examine the table 8.1
It is clear from the table that only a small portion of fresh water is
available for the use of organisms. Hence there is a need for conservation of
fresh water and to use it carefully.
Note :
Ocean water is saline and hence it is not potable.
Table 8.1
percentage
distribution of water on earth
Oceans 97.2
Glaciers
and ice caps 2.15
Ground
water 0.62
tanks 0.017
Atmosphere 0.001
Rivers 0.0001
Biomass 0.0001
More than
70% of the total body weight of many living beings constitutes water. We cannot
forget the fact that the food we eat also contains water.
If the
water content in the organisms becomes too low then they die. This shows the
importance of water as a constituent needed for the survival of living beings.
Remember :
The phenomenon of dehydration of the body means that there is decrease in
quantity of water in the body to a very low level. If this phenomenon is
ignored, death occurs. Therefore make sure that your body does not dehydrate for
any reason.
Activity
8.2 : List out the disease and other situation that lead to the dehydration of
human body. Collect information about its prevention. (Contact a doctor)
Table 8.2
Approximate
percentage of water in some food materials
Cereals
3-4
Egg 75
Potato 75
Milk 85
Tomato 90
Green
vegetables 90
8.2
Sources of water
Which are
the sources of water, which is so essential for the sustenance of life ? The
major sources of fresh water are tanks, ponds, open wells, bore wells, streams
and rivers. Ocean is the source of salt water. Which are the sources that
fulfill water requirements in your town or village ?
Note
We can
survive without food for a few days. But we cannot survive without water for
such a long period.
Drinking
of more water is necessary while fasting. If not, the body gets exhausted
rapidly.
8.3 Water
cycle
From where
do these water bodies get the water ? In case water is not continuously
supplied to these water bodies, they should have dried up long back. Because of
water cycle which happens in nature such drying up is prevented. The water on
the surface of the earth vaporizes and rises up in the atmosphere There it
condenses to form rain, snow, hailstorm and falls back to earth’s surface. this
cyclic process is called “water Cycle.”
Do you
know ?
Solid,
liquid and gas are the three states of matter. Water occurs in all these three
states in nature.
Only
0.005% of total quantity of water on the earth circulates through water cycle.
The major steps of this water cycle are as follows :
1.
Evaporation of
water from the surface of Ocean and other water bodies from the soil to the
atmosphere.
Do you
know the meanings of the following words ?
Evaporation
Condensation
Transpiration
2.
Leaves allow
water vapors to enter the atmosphere through the process of transpiration.
3.
Water vapor in
the atmosphere moves horizontally from one place to another.
4.
Water vapor in
the atmosphere, condenses and returns to the earth in the form of rain, snow,
hail storm etc. This condensation is due to the low temperature at high
altitudes.
5.
A Part of the
rain water flows down to the lower regions and finally reaches the oceans.
8.4
Characteristics of pure water
- You have already studied the physical properties of water. Recapitulate them using the table 8.3
Note
If this
water get cycled continuously. supply of energy has to be continuous. What are
the sources of energy which run the water cycles ?
Table 8.3
Physical
properties of pure water
·
Colourless
transparent liquid.
·
Odourless.
·
Tasteless
·
At normal
pressure, boils at 1000C and freezes at 0 0C.
·
Non-conductor of
heat.
·
Non-conductor of
electricity.
·
Density at 40C is
1 kg/litre.
Note
Rain water
harvesting is the need of the day. This has to be done at the house level and
community level. This is an easy and cost effective procedure.
Apart from
these general physical properties, water also has some unique properties.
In
general, the substances expend on heating and contract on cooling. Water
exhibits a different behaviour. It contracts up to 40C but it starts expanding
below 40C. This peculiar property of water is called anamolous expansion ?
Think it
over
1.
Why does ice
float on water ?
2.
Why do water
piper break in cold countries when the temperature of the surroundings falls to
00C or less than that ?
(Hint :
What should be the nature of change in the volume if mass should remain
constant whereas density decreases ?)
Water also
has another special property. Water can dissolve more substances when compared
to any other solvent. Hence it is called ‘universal solvent’. Remember that
water which appears to be pure may not be really pure.
Think it
over.
1.
Though fresh
water is a colouriess, transparent liquied, the bottom of ponds, wells, etc.,
are not visible ar certain times. Why ?
2.
Water from different
sources have different colours, tastes and odours. Why ?
3.
Why do tap water,
rain water, etc., conduct electricity ?
4.
Though the uppear
surface of a lake is covered with layers of ice in arctic region. the water
below it does not freeze. Aquatic animals live without facing any problems.
Which property of water is responsible for this ?
8.5 Water –
Pollution and its prevention
The
presence of foreign substances in water to such an extent that it is harmful to
the consumer is known as water pollutin. These foreign substances change the
physical and chemical properties of water. In many instances the change may be
so subtle that it is not easily noticeable.
The impact
of such polluted water on the consumer can only be noticed over a considerable
period of time. Sometimes all water bodies including ground water get polluted.
The fast growing population, industrialization, urbanization, unwise activities
of man, lack of concern for long term effects of these activities are major
causes of water pollution. All these contribute to the water pollution. Sewage
from cities, wastes, silt and hazardous minerals, infectants, dissolved toxic
substances, chemical compounds, radioactive substances, thermal wastes (super
heated water), organic matter etc., are some types of pollutants of surface
water. House hold wastes, industrial wastes, agricultural wastes, especially
effluents from cities and industries are major categories of pollutants of
ground water.
Activity
8.3
Identify
the sources of water which meet your water requirements. Find out wheather they
are being polluted. If so, identify the nature of pollutants.
Do you know
?
The
effluents from the industries on the banks of major rivers of India
Do you know
water was
believed tobe an element earlier, Hence its identification as a compound was
delayed.
8.6
Chemical composition of water
Water is a
compound, Each molecule of water, contains 2 atoms of hydrogen and an atom of
oxygen. Hence molecular formula of water is H2O. Henry Cavendish proved that
water is a compound of hydrogen and oxygen. In A. D. 1781. he prepared water in
the laboratory by combining two parts by volume of hydrogen and one part by
volume of oxygen.
Henry
Cavendish
1731 –
1810.
He was a
British scientist conducted innumerable experiments for ten years to prove that
water is not an element but a compound made of two colourless gases. Discovery
of hydrogen and nitrogen, composition of water and air. Innovative experiments
and analytical techniques are some of his contributions.
Caution :
water can be prepared in the labouratory. Using Eudiometer or by burning pure
hydrogen in oxygen. When a mixture of hydrogen and oxygen is burnt, it explodes
producting water. Explosion may causes extensive damage if precautionary
measures are not satisfactory.
2H2
+ O2 – 2H2O
Experiment :
To study the volumetric composition of water.
required
equipments : Hoffmann’s Volta meter, pure
water, dilute sulphuric acid, battery, conducting wire.
Know the
meanings of the following words
Compound,
Molecule, atom, molecular formula, combination, Synthesis electroysis.
Experimental
procedure : Voltameter is a device that decomposes water with the help of
electricity. Fill the voltameter with pure water through its middle tube. Add a
few drops of dilute sulphuric acid. Connect the two platinum electrodes at the
bottom of voltameter to the two poles of a battery by conducting wires. One can
see the risting of gas bubbles from the electrodes. The volume of gas collected
in the tube over the negative electrode is double the volume of the gas
collected in the tube over the positive electrode. The two gases can be tested
by using two separate burning splinters In the gas with lesser volume, the
splinter burns brightly. Hence it is oxygen. In another the gas burns noisily
with blue flame. Therefore it is hydrogen. Notice that hydrogen collects
quickly in large quantities when compared to oxygen.
Conclusion :
In water, hydrogen and oxygen are combined in the ration of 2 : 1 by volume.
Observe :
Pure water is a bad conductor. Water is acidified to make it conducting.
Note : “The
exact chemical formula of the water molecule was established in 1860 by the
Italian scientist S. Cannizzarro”
Do you know
this
The
electrolytic cell used to measure the strength of current flowing through the
circuit is known as voltameter. It is also called as coulombmeter.
Do it
yourself
In case
there is no Hoffmann’s voltameter in your school laboratory, it is still
possible to conduct the electrolysis of water using the equipment assembled by
you. A small plastic container to fill water (cut the plastic container of the
mineral water). Two lead pieces (available in welding shops can be hammered to
obtain a rectangular lamina) two test tubes, test tube holders made of card
board, water, alum, battery, conducting wire, wax or M-seal, are the required
materials. Fix tightly the lead electrodes at the bottom of the container and
see that there is no leakage of water. This can be done with the help of wax or
M-seal. Fill the container with water in which a small quantity of alum is
dissolved. Fill the test tubes with the water and keep them inverted over
electrodes with the help of test tube holders, Complate the electric circuit as
shown in the figure. Observe the collection of gases in the test tubes. Examine
whether the facts explained earlier are true of false. (fig 8.3)
Note : it is
easy to do the experiment using lead, plastic container and alum solution. You
will succeed.
Points to
remember
1.
Water is produced
by buring hydrogen in the atmosphere of oxygen.
2.
water is formed
by the chemical combination of two hydrogen atoms and one oxygen atom.
2H2 + O2 –
2H2O
Do you
know ?
Only those
compounds which dissolve in water and from ions and those which can bind with
water, dissolve in water.
Remember
Hydride is
a binary compound of Hydrogen and another element.
8.7
Special properties of water
You have
studied certain physical properties of water in section 8.4. Out of these
properties anomalous expansion of water and of being a universal solvent are
the unusual properties of water. Water can be considered as a hydride of
oxygen. Theretically it should have been a gas at room temperature. But you
know that it is a liquied at ordinary temperature. The reason for this is its
distinctive molecular linkage. You will learn more about this is higher
classes.
Hydrogen
is a combustible gas. Oxygen is a supporter of combustion. The product formed
by the chemical combination of these two gases is a liquid that extinguishes
fire ! What are the distinctive chemical properties of this strange liquid ?
1.
Pure water is
neutral liquid. It is neither acidic nor basic. Hence it does not change the
colour of either blue litmus of red litmus.
2.
Water is
relatively a stable liquid. It cannot be chemically decomposed at ordinary
temperature except at high temperature (around 2273K) or by passing electric
current through it.
2H2O
– 2H2 + O2
Know these
words
Neutral,
acidic, basic, distillation, distil, distillate, crystal.
Do it
yourself
heat the
crystals of copper sulphate in a test tube white powder is obtained. Water
droplets are formed at the cooler parts of test tube. Add water. Once again the
white powder becomes blue.
3.
In case water
acts as medium it also facilitates the reaction to complete faster.
a)
Hydrogen normally
combains with chlorine with an explosion in sunlight. It does not do so in the
absence of traces of water.
b)
If sulphur is
distilled in compeletely dry oxygen, It doesnot burn.
c)
Sodium burning in
air is exitinguished in dry oxygen.
d)
Red hot carbon
doesnot burn in dry oxygen.
4.
Water is included
during the formation of crystals of salts in some cases. Water molecules do not
undergo any change and retain their characteristics even in the combined
states. these compounds are called hydrates.
Exampls :
Cacl2. 6H2O Calcium chloride hexahydrate.
Precaution
: Copper sulphate crystals are poisonous. Carry out the experiment under the
supervision of a teacher.
5.
Water reacts with
very reactive metals like sodium, potassium, calcium and barium, at room
temperature producing hydrogen and respective metallic hydroxides. Water reacts
with sodium and potassium vigorously and relatively slowly with other reactive
metals.
Eg : 2Na +
2H2O – 2NaOH + H2
Ca + 2H2O
– Ca(OH)2 + H2
Caution :
During the preparation of hydrogen by action of water on sodium or potassium,
the reaction is accompanied by evolution of heat, and hydrogen starts burning.
Sodium or potassium is mixed with mercury to reducr the vigorousness of
reaction and hydrogen can be collected ithout any danger.
Know these
words
Reactive
metal, reversible reaction, water gas, hydrolysis.
6.
When steam is
passed over certain metals like zinc, iron and non-metals like carbon, silicon
at high temperatures, their respective oxides and hydrogen can be obtained.
Eg : 3Fe +
4H2O Fe3O4 4H2
(1273K)
(Steam)
Note :
When the iron oxide is heated in a current of hydrogen, iron and water are
obtained. Such reactions are called reversible under suitable conditions.
Know these
words
Feactive
metal, reversible reaction, water gas, hydrolysis
Note :
Esters are obtained by the elimination of water between acid and alcohol, Many
essences available in the marker (substance which give desired artificial smell
to food) are esters.
Eg : C + H2O
– CO + H2
(873K)
(steam)
Note : The
mixture of CO and H2 in the ratio 1 : 2 is called water gas. it is
used in the manufacture of hydrogen, synthetic petrol. It is used as a fuel in
industries.
Note :
Acetylene
is obtained by adding water to calcium carbide. This gas is used as a fuel. It
is a wonder that a fuel is got from water while water itself exitinguishes
fire.
Do you
know ?
Boiled
water has no taste. The dissolved oxygen is removed while boiling.
7.
Oxides of the
elements which are highy acidic and basic react with water.
Eg : SO3
+ H2O – H2SO4
CaO + H2O
– Ca(OH)2
8.8
Drinking water
Water
which is free from the hazardous substances and is free from disense causing
mircro organisms is fit for drinking and cooking purposes. Naturally occurring
water is usually impure due to the contamination during the course of its flow.
Standards of assessing the quantity of drinking water have been formulated.
(W.H.O. standards, I.S.I. standards). The water which contains foreign
substances within these limits set by the standards is considered to be fit for
drinking.
Note
If clean,
safe water was made available for the world’s population. 80% of sickness and
disense would be prevented.
Do you
know ?
‘Cusec’ is
the unit that denotes the quantity of water flow. No. of cubic feet of water
flowing per second is expressed as cusecs. Cusec is the simplified version of
cubic feet per second.
Do you
know ?
T.M. C is
a measure of volume of water. 109 cubic feet of thousand million cubic feet
constitutes T.M.C. (T.M.C. is abbreviated form of thousand million cubic feet)
Eg : *
Chlorides : 250 ppm
·
soluble solids :
Less than 500 ppm
·
Coliform bacteria
: 100 ppm
·
Disinfectants :
0.005 ppm
(Note : ppm
– Parts per million parts.)
Pure water
is not available. Moreover it will not be tasty. Water containing dissolve
gases and mineral salts in it within the standard limits is both tasty to drink
and also good for health. But, where do we get such water ?
8.9 Sources
of potable water
Which are
the sources of drinking water ?
Major
sources of drinking water are : rivers, rain water, ground water dependent
tanks and takes, wells, bore wells, back water of dams constructed across
rivers and canals. Relatively pure water available in nature is rain water,
which is the product of natural process of distillation. But many impurities in
the atmosphere are carried in rain water before reaching the earth rain water
many be harmful in the regions where the level of air pollution is high.
Organic compounds and inorgan salts are dissolved in river water, well water,
tank water and bore well water depending upon the source. Water in different
places have distinctive tastes. Industrial activities and modern arricultural
practices and harmful pollutants to these water bodies. Hence there is a need
for the treatment of water to make it fir for human consumption.
Do you know
?
Carbon
dioxide present in atmosphere dissolves in rain water to import a slight
acidity ot water. But the rain water at some places will be more acidic than
citric acid. The excessive burning of fossil fuels liberate carbon dioxide,
sulphurdioxide, nitrogen oxides to the atmosphere. These gases dissolve in rain
water making it more acidic. This phenomena is called acid rain. It is
dangerous to living beings and also to the organic materials.
Do you Know
?
Calcium and
Magnesium bicarbonate cause temporary hardness and sulphates of calcium and
magnesium causes permanent hardness in water, If hardness is beyond the
specific limit it becomes unfit for drinking. it doesn’t produce lather with
soap solution. When temporary hard water is boiled. A layer of salt is formed
at the bottom of vessel. Temporary hardness can be eliminated by boiling. Permmanent
by treating with washing soda. Both types of hrdness can be removed by ion
exchange method.
8.10
Impurities likely to be present in water
You know
that the water obtained from many of the sources is not pure. Which are the
impurities that are likely to be present in such water samples ?
a)
Floating and
suspended impurties – these are the insoluble impurities. : Pieces of leaves,
dust, twigs, decayed or decaying organic matter, fine soil etc.
b)
Dissolved
impurtites ? : gases, salts etc.
c)
germs and other
micro-organisms.
Note : All
the dissolved gases and salts in water are not harmful. SOme of these are
useful to living beings. Example : All aquatic plants and aquatic animals live
using dissolved carbon dioxide and oxygen. Water containing dissolved chlorine
is a disinfected water.
Drinking
water should not contain impurities mentioned in (a) or (c) Impurities
classified in (b) may be present within the limits of national and
international standards, (see section 8.8).
Example :
Salts like magnesium or calcium bicarbonates or sulphates that make water hard
should be less than 500 ppm (W.H.O. standards). The water which does not
produce lather with soap is called Hard water.
Experiment
: to detect whether the given water contains dissolved solid substances or not
:
Required materials :
water to be tested tap water well water pond water watch glass, tripodstand
spirit lamp.
Experimental
procedure : Place the beaker half filled with water on the tripad stand, Close
the beaker with a watch glass. Pour a small quantity of water to be tested on
the watch glass. Hent the water in the beaker. the water evaporates, thecompletely
examine the watch glass. If dissolved solid substance are present they will be
left behind in the form of concentric circles on the watch glass. (fig 8.4)
8.11
Supply of drinking water
You now
know that it is dangerous to drink the untreated water obtained from different
sources of water. You can drink tap water as it has been treated properly to
make it fir for human consumption.
If the tap
water that is being supplied to houses either in rural or in urban areas is
coming from large overhead tanks to which it is being pumped directly, consider
it as untreated water. How can one make it potable ?
The
procedure of treating of water in order to reduce impurities to the permissive
limits is as follows :
Do you
know ?
In the
mixture of gases dissolved in water percentage of oxygen will be more and that
of nitrogen will be less unlike air which contains more nitrogen. This is so
because of higher solubility of oxygen in water than that of nitrogen.
Think it
over
When water
is boiled you can observe small bubbles rising up from the bottom of the
container. What is the reason for this ? the bubble formation stops after some
time, why ?
Note
Hardness
of water is due to the presence of dissolved calcium and mangnesium salts.
The main aim
of this process is to remove floating and suspended impurities, bacteria and
other microorganisms as well as unpleasant odour of water.
At the the
first stage, water is stored in settling tank and alum is added in limited
quantities. Due to this all suspended fine particles coagulate and settle down
at the bottom of the tank, Later clear water is transferred into aerating tank
where water is allowed to come in contact with air with the help of sprinklers.
This process is known as aeration. this eliminates the unpleasant odour of
water. Though it is better to deocourise water by passing it through activated
charcoal, the process works out costly. At the third stage, water is made to
pass through sand filter bed to remove remaining floating and suspended
impurities and micro-organisms to some extent. A thin layer of bacterial jelly
on this sand bed is formed and serve as good filter. In the final stage
bleaching powder or small dosage of stage bleaching powder or small dosage of
chlorine is added to the water to destroy disease causing germs. The whole
array of tanks are constructed in such a way as to allow sun rays to fall to
the maximum extent on water. Ultraviolet radiation of the solar radiation kills
micro-organisms.
Activity
8.5
Visit a
water treatment plant. Study the process of water treatment. prepare a report
on its effectiveness and present it in the class.
8.12
Precautionary measures to be taken about the drinking water.
Which ever
be the source of drinking water. follow the precautionary measures given below.
this is essential incase of untreated water.
a)
Fold a cotton
cloth about 3 – 4 folds and filter the water sample.
b)
Boil the filtered
water.
c)
Wash the water
container of daily, by using hot water.
d)
Use only a small
vessel or tumbler with a long handle to take out water from the container. This
also has to be washed with hot water. Don’t touch the drinking water or use
other vessels.
Do you
know ?
Heavy
water : Deuterium is an isotope of hydrogen its mass number is 2 and symbol is
D. There is one proton and one neutron in its nucleus. Deuterium oxide (D2O)
is called heavy water. It is used as moderator in nuclear reactors. It is
poisonous.
e)
If you are living
in rural area and drawing water from a tank or well. Do not pollute the source.
Motivate others also not to carry out such activities which pollute water.
8.13
Methods of obtaining pure water
a) By
distillation.
b) Partly
freezing and defreezing pure ice.
C) Burning
hydrogen in oxygen or reducting strongly heated oxides with hydrogen and
collecting the product. First method is easy and simple compared to others.
Hence it is described here.
Experiment
: To obtain pure or distilled water by distillation and condensation.
Required
materials : Round bottom flask, test tube stand, spirit lamp, Leibig condenser,
receiver, rubber tube, thermometer, water to be distilled.
Think it
over
If water
contains a liqid whose boiling point is lower than that of water. which one of
these is collected first in the receiver ?
Do you
know this ?
Detergents
produce lather even with hard water, They cleanse : But the also cause pollution.
Experimental
procedure : Set up the apparatus as shown in the (figure 8.6). take the water
to be distilled in the round bottom flask. Arrange cold water to flow in leibig
condenser in the direction of arrow mark. Boil water in the flask. The vapours
produced condenses in the condenser and collects in the receiver. Distillate is
relatively pure water. Record the temperature during distillation. Test for
colour. taste and odour before and after distillation.
Uses of
distilled water
Distilled
water is a relatively pure form of water. Hence it is used in
1)
manufacture of
medicine.
2)
medical
activities.
3)
manufacture of
chemicals.
4)
various
industries.
Sometimes
water is distilled several times to get ultra pure water.
8.15
Having studied this chapter you will be able to….
·
explain the
importance of water with the illustrations.
·
list the sourfces
of water and explain how and why they get polluted. You will be able to list
out the preventive measures.
·
describe the
chemical composition of water.
·
determine the
composition of water by volume experimentally.
·
list out general
as well as special physical and chemical properties of water.
·
explain the
qualities of drinking water and methods of obtaining water to make it fir for
human consumption.
·
explain the
process of obtaining distilled water and uses of distilled water.
8.16
EEXRCISE.
1) Explain
the importance of water with any five examples.
2) Even
though 70% of the earth’s surface is covered with water, why should it be
conserved ?
3) Which
are the major steps in water cycle ?
4) What
are the physical properties of pure water ?
5) List
out the chemical properties of pure water.
6) What is
the percapita consumption of drinking water ?
7) How is
water treated to make it fit for drinking ?
8) What is
water pollution ? How can it be prevented ?
9) Explain
with diagram the experiment to determine the composition of water by volume.
10)
Explain with a diagram the process of obtaining distilled water.
11) What
is dehydration of they body ? How is it prevented ?
12) What
is water cycle ? What is its importance ?
13) What
do you mean by anamolous expansion of water ?
14) Why is
water called a universal solvent ?
15) Give
the chemical composition of water.
16) State
one example of a chemical reaction in which water acts as a medium.
17) What
is water gas ? How is it obtained ?
18) State
an illustration to establish the reaction of water with the alkali metals.
19) State
an example for each of the reactions of water reacting with acidic and basic
oxides.
20)
Mention the uses of distilled water.
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