پنجشنبه ۲۲ شهریور ۰۳ | ۱۹:۰۲ ۱۰ بازديد
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Major Primary pollutants include:
Carbon monoxide (CO) is produced by incomplete combustion of fuels like
petrol, natural gas, coal or wood. It is a colourless and odourless gas but very
poisonous in nature.
Carbon dioxide (CO2
) is produced by complete combustion of fuels in motor
vehicles and various industries. It is a colourless, odourless and non-toxic gas. (A
person dies in atmosphere of carbon dioxide due to lack of oxygen and not due
to its toxic nature). (Read details in lesson 30, section 30.8.2)
Sulphur oxides (SOx) (mainly sulphur dioxide, SO 2
) are produced by combustion
of coal and petroleum and also produced in volcanoes. It is also produced in
various industrial processes. Oxidation of sulphur dioxide (SO2
) to sulphur
trioxide (SO3
) results in formation of sulphuric acid (H2
SO 4
) which causes acid
rain. (See Lesson-30, Section 30.8.4)
Nitrogen oxides (NOx) especially nitrogen dioxide, NO2 is a reddish brown gas
with pungent smell. It catalyses the oxidation of SO 2 to SO3 and indirectly causes
acid rain.
Volatile organic compounds (VOCs) include methane, benzene, toluene and
xylene. While methane is a major green house gas, others are suspected to be
carcinogens (cancer inducing).
Particulate matter consists of tiny particles of solids or liquids suspended in air.
These are also called ‘suspended particulate matter (SPM)’. The major sources
for these include volcanoes, dust storms and burning of fuels. These can cause
heart and lung diseases and breathing disorders.
Chloro-fluorocarbons (CFCs) are used as refrigerants in air conditioners and
refrigerators and are harmful to the ozone layer which protect us from harmful
ultraviolet rays. You shall read about the ozone hole in Lesson30, Section 30.8.1)
Major secondary pollutants include :
Photochemical smog (smoke + fog) formed by the action of ultraviolet light from
the sun on particulate matter or formed due to burning of coal and petrol in an
atmosphere containing SO2
. It prevents dissipation of pollutants and causes
breathing disorders. Read in detail from Lesson-30, Section 30.8.3
Ground level ozone (O3
) is formed from NOx and VOCs. It is a constituent of
smog. Normally ozone occurs in stratosphere and prevents UV radiations from
reaching earth’s surface. At ground level, when inhaled, it is harmful for health
of humans and animals.
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Notes
INTEXT QUESTIONS 26.3
1. What happens to atmospheric pressure as we climb a mountain?
___________________________________________________________
2. At high altitude the people find their nose bleeding. Why?
___________________________________________________________
3. Which layer of atmosphere is the closest to the earth’s surface and which is
the farthest from earth’s surface?
___________________________________________________________
4. In which layer of atmosphere is ozone layer present?
___________________________________________________________
5. Name (i) a green house gas (ii) gas responsible for acid rain (ii) chemicals
causing ozone hole
___________________________________________________________
26.4 WATER - ITS SOURCES AND PROPERTIES
Next to air, water is the most important substance needed for survival of living
beings. Living beings cannot live long without water. Water is available in plenty
on earth. More than three-fourth of the earth’s surface is covered with water in the
form of seas, rivers and lakes. It is also found inside the earth’s crust Most of the
water that we get from the wells comes from this source.
26.4.1 Sources of water
The natural sources of water are rain, springs, wells, rivers and seas.
(a) Rain water: Rain water is considered to be the purest form of natural water
(distilled water) free from impurities. Water from sea and rivers get evaporated
into water vapour by the heat of sun. During this process of evaporation,
impurities are left behind. When the water vapours go high up in the air they
condense to form clouds. The water drops come down as rain.
(b) Spring water: Springs are formed by percolation of rain water into soil.
Springs supply water to wells and lakes.
(c) Well water: The rain water seeps through the soil and goes down and is
stored over rocks or hard earth crust. On digging the well this underground water
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becomes available to us. This is known as well water. This water may not be pure
and may contain impurities such as suspended particles, bacteria and other
microorganisms.
(d) River water: Rivers are formed by melting of snow on the mountain, and
also sometimes from the rain water. River water is also not pure and is not fit for
drinking.
(e) Sea water: Out of all the sources, sea water is the largest natural source of
water. However, it is also the source of common salt and other important
chemicals. It is the most impure form of water. All the impurities dissolved in river
water are carried into the sea. As such, sea water cannot be used for drinking
purpose because of high salinity and impurities.
26.4.2 Potable and Non-potable water
Potable water means water which is fit for drinking by humans and other animals.
It can be consumed with low risk of immediate or long term harm. Non-potable
water is that which is not safe for drinking. It may carry disease causing microbes,
and high levels of dissolved salts and minerals, heavy metals and suspended
solids. Drinking or using such water for cooking leads to illnesses and may even
cause death.
Contaminated or non-potable water can be treated to turn it into potable or
drinking water. Let us learn about simple methods of purifying water.
26.4.3 Purification of water to make it suitable for drinking
• By decantation, insoluble impurities can be removed. Decantation is the-
process of separation of solid from the liquid by allowing the former to settle
down and pouring off the latter. Water is kept in a vessel for some time. The
suspended insoluble impurities settle down at the bottom. Clean water can
now be carefully poured into another clean vessel without disturbing the
settled impurities which are left behind. But, this water has to be made fit for
drinking through further treatment.
• By filtration also, the insoluble impurities can be removed. It is a more
effective method than decantation and can remove even very fine particles of
insoluble impurities. A piece of clean and very fine cloth can be used as a cheap
and easily available filter. When water is poured through it, the insoluble
impurities are stopped by the filter and clean water passes through it.
Commercially available water filters use ‘candles’ made of porous material
(figure 26.5). Pure water passes through it leaving the impurities on its outer
surface. These candles must be cleaned and washed periodically to maintain their
effectiveness.
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• By boiling, bacteria and other germs in the
water get killed. When boiled water is allowed
to cool, heavy impurities collect at the bottom
and dissolved salts form a thin layer on the
surface called scum. Now if we filter the
water, it becomes safe for drinking.
• By chlorine treatment small living organisms
and bacteria are killed. If required, treated
water may be filtered to remove insoluble
impurities.
26.4.4 Properties of water
Water, is a common ordinary substance of everyday use. However it is its unusual
and unique properties which make its use important and essential in our daily life.
26.4.4a Water acts as universal solvent
Water is certainly one of the best and most useful solvents that we have. It has a
unique property of dissolving a large number of substances starting from solids
such as common salt, sugar, to gases like oxygen, carbon dioxide etc. Indeed, as
so many substances dissolve in water, it is called a universal solvent.-This
property of water is useful for plants to take their food materials and minerals from
the soil. It helps us to absorb food that we eat. Many chemical reactions also take
place only in aqueous solution.
26.4.4b Hard water and Soft water
Water forms lather with soap which is used for cleaning purposes. It is called soft
water. Sometimes water from some sources like rivers or hand pumps does not
produce any lather with soap. It is called hard water.
Water, which we get from taps, contain lesser amounts of dissolved salts in it than
water that we get from hand pumps. The dissolved salts are usually bicarbonates,
sulphates and chlorides of calcium and magnesium. Their presence prevents
formation of soap lather. But why?
Soap is a sodium salt called sodium stearate. It is soluble in water. When soap is
added to hard water, which contains calcium and magnesium ions, a precipitate
of Ca or Mg stearate is formed. These calcium and magnesium steartes are
insoluble in water and appears as a greasy scum. The formation of scum in place
of lather makes it more difficult to clean things.
Sodium stearate + Calcium sulphate ⎯→ Calcium stearate + Sodium sulphate
(Soap) (Scum)
Fig. 26.5 Candles
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Accordingly, we can say that,
• Water which forms lather with soap is called soft water.
• Water which does not form lather is called hard water.
• The hardness of water is due to the presence of salts of magnesium and
calcium in water.
26.5.4c Conversion of hard water into soft water
Hard water does not form lather with soap. Can this hard water be converted into
soft water? Yes, hard water can be converted into soft water, by removal of Ca and
Mg ions which are responsible for hardness. This is called softening of water.
Hardness of water is of two types :
• Temporary hardness
• Permanent hardness
a) Temporary hardness
Temporary hardness of water is due to the presence of soluble bicarbonates of
calcium and magnesium. It is also called carbonate hardness. It can be removed
by boiling and by soda lime process.
(i) By boiling: Upon boiling hard water, calcium or magnesium bicarbonate present
in it are decomposed to give magnesium or calcium carbonate. These carbonate
salts are insoluble in water. They settle down easily and water can be decanted.
Ca (HCO 3
)2 Heat
⎯⎯⎯→ CaCO3 + H 2
O + CO 2
Calcium bicarbonate Calcium Carbonate
(Soluble) (Insoluble)
Mg (HCO 3
)2 Heat
⎯⎯⎯→ MgCO 3 + H2
O + CO 2
Magnesium bicarbonate Magnesium Carbonate
(Soluble) (Insoluble)
(ii) By soda lime process (Clark’s method): When a calculated amount of lime
is added to hard water, then the soluble bicarbonates are converted to insoluble
carbonates as follows:
Ca(HCO 3
)2 + Ca(OH) 2 Heat
⎯⎯⎯→ CaCO 3 + 2H 2
O
Lime (Insoluble)
Mg(HCO 3
)2 + Ca(OH) 2 Heat
⎯⎯⎯→ 2MgCO 3 + CaCO 3 + 2H 2
O
Lime (Insoluble)
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b) Permanent hardness
Permanent hardness of water is due to the presence of soluble chlorides and
sulphates of calcium and magnesium. It is also known as non-carbonate
hardness.
It can be removed by addition of washing soda or by the ion exchange method.
(i) By addition of washing soda: The hard water is treated with the ‘calculated’
quantity of washing soda (sodium carbonate). Washing soda reacts with chloride
and sulphate of calcium and magnesium to form precipitate of calcium and
magnesium carbonate.
The reactions are as follows.
CaCl2 + Na2
CO 3 Heat
⎯⎯⎯→ CaCO 3 + 2NaCl
Calcium chloride Sodium carbonate Calcium carbonate Sodium chloride
(Insoluble)
MgSO 4 + Na2
CO 3 Heat
⎯⎯⎯→ MgCO3 + Na2
SO 4
Magnesium sulphate Sodium carbonate Magnesium carbonate Sodium sulphate
(Insoluble)
The precipitate settles down and can be removed by decantation.
(ii) By ion. exchange method: Two types of ion exchangers can be used, namely,
inorganic ion exchanger and organic ion exchanger. In inorganic ion exchange
process, complex compounds known as Zeolite are used to soften the hard water.
The salts causing hardness of water are precipitated as insoluble zeolite of
calcium and magnesium and are replaced by soluble sodium salts. On the large
scale, this process is carried
out in tanks as shown in
figure 26.6. After using it
for sometime the zeolite is
regenerated by soaking it in
10% solution of NaCl (brine)
and then washing away
chlorides. The washings are
removed and are replaced
by soluble sodium salts.
By using organic ion
exchanger, water obtained is
free from cations and anions
and is known as deionized
water or demineralized
water.
Fig. 26.6 Obtaining soft water on a large
scale using tanks
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26.5.4d Polar nature of water
Water is a very effective solvent for ionic compounds. Although water is an
electrically neutral molecule, it has a small positive charge (on the H atoms) and
a negative charge (on the O atom), Therefore, it is polar in nature and can dissolve
ionic compounds.
Fig 26.7 Structure of water
Let us perform an activity, which proves the polar nature of water
ACTIVITY 26.3
Aim: To study the polar nature of water
What is required? Burette, water, ebonite rod (negatively charged), glass rod
(positively charge) and burette stand.
What to do?
• Take a burette or a bottle with a fine opening and fill it with water.
• Fix the burette vertically in a burette stand/hold the put a clip a little above
the fine opening to regulate the water flow bottle in a suitable stand.
• Open the stopcock of the burette/clip of the bottle and allow the water to flow.
• Take an ebonite rod/ordinary straw (negatively charged by rubbing one end
with fur) near the water
What to observe?
You will see that the stream of water is attracted towards negatively charged rod
(figure 26.8a). Why? Because one end of water molecule has positive charge.
Similarly, now we take a glass rod/glass tumbler rubbed with fur near water,
which is positively charged. You will see the rod again attracts the stream of water.
This indicates that one end of water molecule also has negative charge (figure
26.8b). This proves the polar nature of water.
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26.4.4e Surface tension
Surface tension is the property of all the
liquids. Due to this tension water drops
try to occupy a minimum surface area.
Hence, water droplets always tend to take
the shape of a sphere.
The tension exerted by molecules of water
present on the surface layer is called
surface tension.
To understand this let us perform an
activity.
ACTIVITY 26.4
Aim: To study surface tension
What is required? Glass and razor blade.
What to do?
Take a glass full of water. Put a safety razor blade (having a coating of very thin
layer of wax), gently on the surface of water
What to observe?
You will find that the blade remains on the surface of water though it is heavier
than water.
Why is it so?
The upper layer of water acts like a tight sheet and holds the blade. Why is the
sheet tight? Due to intermolecular forces i.e. attractive forces between the
molecules on water surface and there is a tension or force acting on the surface
of the thin film of the liquid which behaves like a tight sheet.
26.4.4f Capillarity - Rise of water
When a capillary tube with a fine bore is dipped in water, water rises in the
capillary. The extent to which water rises depends on the diameter of the capillary.
The smaller the diameter of the capillary, the higher will be the rise of water in
the capillary tube.
This property of rise of water inside a capillary is called capillarity or capillary
action.
Fig. 26.8 a and b To show that water
is polar in nature
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This is the property, by which water from the soil enters the leaves and branches
of the plants through the stems.
When a piece of cloth or blotting paper is placed in water, it soaks the water by
this process of capillary action. The thread strands in the cloth and cellulose of the
blotting paper serves like very fine capillaries for the water to rise.
26.4.4g Density of water
Water behaves in an unusual way when it is heated from 0°C. As the temperature
rises from 0°C to 4°C it actually contracts. However, from 4°C upwards it
expands like any other liquid. This means that water takes up the least space at
4°C. It has the highest density at this temperature and will sink through warmer
or colder water around it. The density of water at 4°c is 1g/m3
Because of this property of water, we can explain why it takes months for a lake
to freeze while a small bucket of water can freeze overnight on a bitterly cold day.
The surface water cools down to 4°C and sinks to the bottom of the lake due to
its high density and hotter water comes up to the surface. Gradually the whole
water cools down to 4°C. Further cooling decreases the temperature of surface
water which finally freezes. Ice being lighter than water keeps floating on the
surface. It acts as an insulator and slows down the cooling and freezing of the
lower layers of water. This explains why aquatic animals living in water bodies
of very cold regions do not die in severe wint
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