NCERT Textbook: Separation of Substances | Science & Technology for UPSC CSE PDF Download

 Page 1


20 SCIENCE
3 Separation of Substances
Grain is separated from stalks, while
harvesting. Milk or curd is churned to
separate the butter (Fig. 3.2). We gin
cotton to separate its seeds from
the fibre.
Perhaps you might
have eaten salted
daliya or poha. If you
found that it had
chillies in it, you may
have carefully taken
them out before eating.
Suppose you are given a basket
containing mangoes and guavas and
asked to separate them. What would you
do? Pick out one kind and place them
in a separate container, right?
Seems easy, but what if the materials
we want to separate are much smaller
Fig. 3.1 Separating tea leaves with a strainer
Fig. 3.2 Butter is taken out by churning milk or
curd
But, why would we need to
separate substances like this
at all, is what Paheli wants
to know.
Activity 1
In Column 1 of Table 3.1, are given a
few processes of separation. The purpose
of separation and the way separated
components are used is mentioned in
Column 2 and 3 respectively. However,
the information given in Columns 2 and
3 is jumbled up. Can you match each
I
n our daily life, there are many
       instances when we notice a substance
   being separated from a mixture
of materials.
Tea leaves are separated from  the
liquid  with a strainer, while preparing
tea (Fig. 3.1).
than mango or guava? Imagine you are
given a glass of sand with salt mixed in
it. Impossible, even to think of
separating salt from this mixture by
picking out grains of sand by hand!
Rationalised 2023-24
Page 2


20 SCIENCE
3 Separation of Substances
Grain is separated from stalks, while
harvesting. Milk or curd is churned to
separate the butter (Fig. 3.2). We gin
cotton to separate its seeds from
the fibre.
Perhaps you might
have eaten salted
daliya or poha. If you
found that it had
chillies in it, you may
have carefully taken
them out before eating.
Suppose you are given a basket
containing mangoes and guavas and
asked to separate them. What would you
do? Pick out one kind and place them
in a separate container, right?
Seems easy, but what if the materials
we want to separate are much smaller
Fig. 3.1 Separating tea leaves with a strainer
Fig. 3.2 Butter is taken out by churning milk or
curd
But, why would we need to
separate substances like this
at all, is what Paheli wants
to know.
Activity 1
In Column 1 of Table 3.1, are given a
few processes of separation. The purpose
of separation and the way separated
components are used is mentioned in
Column 2 and 3 respectively. However,
the information given in Columns 2 and
3 is jumbled up. Can you match each
I
n our daily life, there are many
       instances when we notice a substance
   being separated from a mixture
of materials.
Tea leaves are separated from  the
liquid  with a strainer, while preparing
tea (Fig. 3.1).
than mango or guava? Imagine you are
given a glass of sand with salt mixed in
it. Impossible, even to think of
separating salt from this mixture by
picking out grains of sand by hand!
Rationalised 2023-24
21 SEPARATION OF SUBSTANCES
process with its purpose and the way
separated components are used?
We see that, before we use a
substance, we need to separate harmful
or non-useful substances that may be
mixed with it. Sometimes, we separate
even useful components if we need to
use them separately.
The substances to be separated may
be particles of different sizes or
materials. These may be in any three
states of matter i.e., solid, liquid or gas.
So, how do we separate substances
mixed together if they have so many
different properties?
3.1  METHODS OF SEPARATION
We will discuss some simple methods of
separating substances that are mixed
together. You may come across some of
these methods being used in day to day
activities.
Handpicking
Activity 2
Bring a packet of food grain purchased
from a shop to the classroom. Now,
spread the grains on a sheet of paper.
Do you find only one kind of grain on
the sheet of paper? Are there pieces of
stone, husks, broken grain and particles
of any other grain in it? Now, remove
with your hand the pieces of stone,
husks and other grains from it.
This method of handpicking can
be used for separating slightly larger
sized impurities like the pieces of dirt,
stone, and husk from wheat, rice or
pulses (Fig. 3.3). The quantity of such
impurities is usually not very large.
In such situations, we find that
handpicking is a convenient method of
separating substances.
Table 3.1 Why do we separate substances?
n o i t a r a p e S
s s e c o r p
o d e w h c i h w r o f e s o p r u P
n o i t a r a p e s e h t
e h t h t i w o d e w o d t a h W
? s t n e n o p m o c d e t a r a p e s
e t a r a p e S ) 1
e c i r m o r f s e n o t s
, t n e r e f f i d o w t e t a r a p e s o T ) a
. s t n e n o p m o c l u f e s u t u b
d l i o s e h t y a w a w o r h t e W ) i
. t n e n o p m o c
k l i m g n i n r u h C ) 2
r e t t u b n i a t b o o t
l u f e s u - n o n e v o m e r o T ) b
. s t n e n o p m o c
e h t y a w a w o r h t e W ) i i
. s e i t i r u p m i
a e t e t a r a p e S ) 3
s e v a e l
r o s e i t i r u p m i e v o m e r o T ) c
. s t n e n o p m o c l u f m r a h
e h t h t o b e s u e W ) i i i
. s t n e n o p m o c
Fig. 3.3 Handpicking stones from grain
Threshing
You must have seen bundles of wheat
or paddy stalks lying in fields after
Rationalised 2023-24
Page 3


20 SCIENCE
3 Separation of Substances
Grain is separated from stalks, while
harvesting. Milk or curd is churned to
separate the butter (Fig. 3.2). We gin
cotton to separate its seeds from
the fibre.
Perhaps you might
have eaten salted
daliya or poha. If you
found that it had
chillies in it, you may
have carefully taken
them out before eating.
Suppose you are given a basket
containing mangoes and guavas and
asked to separate them. What would you
do? Pick out one kind and place them
in a separate container, right?
Seems easy, but what if the materials
we want to separate are much smaller
Fig. 3.1 Separating tea leaves with a strainer
Fig. 3.2 Butter is taken out by churning milk or
curd
But, why would we need to
separate substances like this
at all, is what Paheli wants
to know.
Activity 1
In Column 1 of Table 3.1, are given a
few processes of separation. The purpose
of separation and the way separated
components are used is mentioned in
Column 2 and 3 respectively. However,
the information given in Columns 2 and
3 is jumbled up. Can you match each
I
n our daily life, there are many
       instances when we notice a substance
   being separated from a mixture
of materials.
Tea leaves are separated from  the
liquid  with a strainer, while preparing
tea (Fig. 3.1).
than mango or guava? Imagine you are
given a glass of sand with salt mixed in
it. Impossible, even to think of
separating salt from this mixture by
picking out grains of sand by hand!
Rationalised 2023-24
21 SEPARATION OF SUBSTANCES
process with its purpose and the way
separated components are used?
We see that, before we use a
substance, we need to separate harmful
or non-useful substances that may be
mixed with it. Sometimes, we separate
even useful components if we need to
use them separately.
The substances to be separated may
be particles of different sizes or
materials. These may be in any three
states of matter i.e., solid, liquid or gas.
So, how do we separate substances
mixed together if they have so many
different properties?
3.1  METHODS OF SEPARATION
We will discuss some simple methods of
separating substances that are mixed
together. You may come across some of
these methods being used in day to day
activities.
Handpicking
Activity 2
Bring a packet of food grain purchased
from a shop to the classroom. Now,
spread the grains on a sheet of paper.
Do you find only one kind of grain on
the sheet of paper? Are there pieces of
stone, husks, broken grain and particles
of any other grain in it? Now, remove
with your hand the pieces of stone,
husks and other grains from it.
This method of handpicking can
be used for separating slightly larger
sized impurities like the pieces of dirt,
stone, and husk from wheat, rice or
pulses (Fig. 3.3). The quantity of such
impurities is usually not very large.
In such situations, we find that
handpicking is a convenient method of
separating substances.
Table 3.1 Why do we separate substances?
n o i t a r a p e S
s s e c o r p
o d e w h c i h w r o f e s o p r u P
n o i t a r a p e s e h t
e h t h t i w o d e w o d t a h W
? s t n e n o p m o c d e t a r a p e s
e t a r a p e S ) 1
e c i r m o r f s e n o t s
, t n e r e f f i d o w t e t a r a p e s o T ) a
. s t n e n o p m o c l u f e s u t u b
d l i o s e h t y a w a w o r h t e W ) i
. t n e n o p m o c
k l i m g n i n r u h C ) 2
r e t t u b n i a t b o o t
l u f e s u - n o n e v o m e r o T ) b
. s t n e n o p m o c
e h t y a w a w o r h t e W ) i i
. s e i t i r u p m i
a e t e t a r a p e S ) 3
s e v a e l
r o s e i t i r u p m i e v o m e r o T ) c
. s t n e n o p m o c l u f m r a h
e h t h t o b e s u e W ) i i i
. s t n e n o p m o c
Fig. 3.3 Handpicking stones from grain
Threshing
You must have seen bundles of wheat
or paddy stalks lying in fields after
Rationalised 2023-24
22 SCIENCE
harvesting the crop. Stalks are dried in
the sun before the grain is separated
from them. Each stalk has many grain
seeds attached to it. Imagine the
number of grain seeds in hundreds of
bundles of stalk lying in the field! How
does the farmer separate grain seeds
from those bundles of stalks?
One may pluck mangoes or guavas
from the trees. But, grain seeds are
much smaller than mangoes or guavas.
So, plucking them from their stalks
would be impossible. How does one
separate grain seeds from their stalks?
The process that is used to separate
grain from stalks etc. is threshing. In
this process, the stalks are beaten to free
the grain seeds (Fig. 3.4). Sometimes,
this mixture on a plate or a newspaper.
Look at this mixture carefully. Can the
two different components be made out
easily? Are the sizes of particles of the
two components similar? Would it be
possible to separate the components by
handpicking?
Now, take your mixture to an open
ground and stand on a raised platform.
Put the mixture in a plate or sheet of
paper. Hold the plate or the sheet of
paper containing the mixture, at your
shoulder height. Tilt it slightly, so that
the mixture slides out slowly.
What happens? Do both the
components — sand and sawdust (or
powdered leaves) fall at the same place?
Is there a component that blows away?
Did the wind manage to separate the
two components?
This method of separating
components of a mixture is called
winnowing. Winnowing is used to
separate heavier and lighter components
of a mixture by wind or by blowing air.
Fig. 3.4 Threshing
threshing is done with the help of
bullocks.  Machines are also used to
thresh large quantities of grain.
Winnowing
Activity 3
Make a mixture of dry sand with
sawdust or powdered dry leaves. Keep Fig. 3.5 Winnowing
soop
husk
grain
direction of the air
Rationalised 2023-24
Page 4


20 SCIENCE
3 Separation of Substances
Grain is separated from stalks, while
harvesting. Milk or curd is churned to
separate the butter (Fig. 3.2). We gin
cotton to separate its seeds from
the fibre.
Perhaps you might
have eaten salted
daliya or poha. If you
found that it had
chillies in it, you may
have carefully taken
them out before eating.
Suppose you are given a basket
containing mangoes and guavas and
asked to separate them. What would you
do? Pick out one kind and place them
in a separate container, right?
Seems easy, but what if the materials
we want to separate are much smaller
Fig. 3.1 Separating tea leaves with a strainer
Fig. 3.2 Butter is taken out by churning milk or
curd
But, why would we need to
separate substances like this
at all, is what Paheli wants
to know.
Activity 1
In Column 1 of Table 3.1, are given a
few processes of separation. The purpose
of separation and the way separated
components are used is mentioned in
Column 2 and 3 respectively. However,
the information given in Columns 2 and
3 is jumbled up. Can you match each
I
n our daily life, there are many
       instances when we notice a substance
   being separated from a mixture
of materials.
Tea leaves are separated from  the
liquid  with a strainer, while preparing
tea (Fig. 3.1).
than mango or guava? Imagine you are
given a glass of sand with salt mixed in
it. Impossible, even to think of
separating salt from this mixture by
picking out grains of sand by hand!
Rationalised 2023-24
21 SEPARATION OF SUBSTANCES
process with its purpose and the way
separated components are used?
We see that, before we use a
substance, we need to separate harmful
or non-useful substances that may be
mixed with it. Sometimes, we separate
even useful components if we need to
use them separately.
The substances to be separated may
be particles of different sizes or
materials. These may be in any three
states of matter i.e., solid, liquid or gas.
So, how do we separate substances
mixed together if they have so many
different properties?
3.1  METHODS OF SEPARATION
We will discuss some simple methods of
separating substances that are mixed
together. You may come across some of
these methods being used in day to day
activities.
Handpicking
Activity 2
Bring a packet of food grain purchased
from a shop to the classroom. Now,
spread the grains on a sheet of paper.
Do you find only one kind of grain on
the sheet of paper? Are there pieces of
stone, husks, broken grain and particles
of any other grain in it? Now, remove
with your hand the pieces of stone,
husks and other grains from it.
This method of handpicking can
be used for separating slightly larger
sized impurities like the pieces of dirt,
stone, and husk from wheat, rice or
pulses (Fig. 3.3). The quantity of such
impurities is usually not very large.
In such situations, we find that
handpicking is a convenient method of
separating substances.
Table 3.1 Why do we separate substances?
n o i t a r a p e S
s s e c o r p
o d e w h c i h w r o f e s o p r u P
n o i t a r a p e s e h t
e h t h t i w o d e w o d t a h W
? s t n e n o p m o c d e t a r a p e s
e t a r a p e S ) 1
e c i r m o r f s e n o t s
, t n e r e f f i d o w t e t a r a p e s o T ) a
. s t n e n o p m o c l u f e s u t u b
d l i o s e h t y a w a w o r h t e W ) i
. t n e n o p m o c
k l i m g n i n r u h C ) 2
r e t t u b n i a t b o o t
l u f e s u - n o n e v o m e r o T ) b
. s t n e n o p m o c
e h t y a w a w o r h t e W ) i i
. s e i t i r u p m i
a e t e t a r a p e S ) 3
s e v a e l
r o s e i t i r u p m i e v o m e r o T ) c
. s t n e n o p m o c l u f m r a h
e h t h t o b e s u e W ) i i i
. s t n e n o p m o c
Fig. 3.3 Handpicking stones from grain
Threshing
You must have seen bundles of wheat
or paddy stalks lying in fields after
Rationalised 2023-24
22 SCIENCE
harvesting the crop. Stalks are dried in
the sun before the grain is separated
from them. Each stalk has many grain
seeds attached to it. Imagine the
number of grain seeds in hundreds of
bundles of stalk lying in the field! How
does the farmer separate grain seeds
from those bundles of stalks?
One may pluck mangoes or guavas
from the trees. But, grain seeds are
much smaller than mangoes or guavas.
So, plucking them from their stalks
would be impossible. How does one
separate grain seeds from their stalks?
The process that is used to separate
grain from stalks etc. is threshing. In
this process, the stalks are beaten to free
the grain seeds (Fig. 3.4). Sometimes,
this mixture on a plate or a newspaper.
Look at this mixture carefully. Can the
two different components be made out
easily? Are the sizes of particles of the
two components similar? Would it be
possible to separate the components by
handpicking?
Now, take your mixture to an open
ground and stand on a raised platform.
Put the mixture in a plate or sheet of
paper. Hold the plate or the sheet of
paper containing the mixture, at your
shoulder height. Tilt it slightly, so that
the mixture slides out slowly.
What happens? Do both the
components — sand and sawdust (or
powdered leaves) fall at the same place?
Is there a component that blows away?
Did the wind manage to separate the
two components?
This method of separating
components of a mixture is called
winnowing. Winnowing is used to
separate heavier and lighter components
of a mixture by wind or by blowing air.
Fig. 3.4 Threshing
threshing is done with the help of
bullocks.  Machines are also used to
thresh large quantities of grain.
Winnowing
Activity 3
Make a mixture of dry sand with
sawdust or powdered dry leaves. Keep Fig. 3.5 Winnowing
soop
husk
grain
direction of the air
Rationalised 2023-24
23 SEPARATION OF SUBSTANCES
This method is commonly used by
farmers to separate lighter husk particles
from heavier seeds of grain (Fig. 3.5).
The husk particles are carried away
by the wind. The seeds of grain get
separated and form a heap near the
platform for winnowing. The separated
husk is used for many purposes such
as fodder for cattles.
Sieving
Sometimes, we may wish to prepare a
dish with flour. We need to remove
impurities and bran that may be present
in it. What do we do? We use a sieve
and pour the flour into it (Fig. 3.6).
Sieving allows the fine flour particles
to pass through the holes of the sieve
while the bigger impurities  remain on
the sieve.
In a flour mill, impurities like husk
and stones are removed from wheat
before grinding it. Usually, a bagful of
wheat is poured on a slanting sieve. The
sieving removes pieces of stones, stalk
and husk that may still remain with
wheat after threshing and winnowing.
to separate pebbles and stones from
sand (Fig. 3.7).
Activity 4
Bring a sieve and a small quantity of
flour from home, to the class. Sieve the
flour to separate any impurities in it.
Now, make a fine powder of chalk pieces
and mix it with the flour. Can we
separate the flour and the powdered
chalk by sieving?
Sieving is used when components of
a mixture have different sizes.
Sedimentation, Decantation
and Filtration
Sometimes, it may not be possible to
separate components of a mixture by
winnowing and handpicking. For
example, there may be lighter impurities
like dust or soil particles in rice or
pulses. How are such impurities
separated from rice or pulses before
cooking?
Rice or pulses are usually washed
before cooking. When you add water to
these, the impurities like dust particles
Fig. 3.6 Sieving
You may have also noticed similar
sieves being used at construction sites
Fig. 3.7 Pebbles and stones are removed from
sand by sieving
Rationalised 2023-24
Page 5


20 SCIENCE
3 Separation of Substances
Grain is separated from stalks, while
harvesting. Milk or curd is churned to
separate the butter (Fig. 3.2). We gin
cotton to separate its seeds from
the fibre.
Perhaps you might
have eaten salted
daliya or poha. If you
found that it had
chillies in it, you may
have carefully taken
them out before eating.
Suppose you are given a basket
containing mangoes and guavas and
asked to separate them. What would you
do? Pick out one kind and place them
in a separate container, right?
Seems easy, but what if the materials
we want to separate are much smaller
Fig. 3.1 Separating tea leaves with a strainer
Fig. 3.2 Butter is taken out by churning milk or
curd
But, why would we need to
separate substances like this
at all, is what Paheli wants
to know.
Activity 1
In Column 1 of Table 3.1, are given a
few processes of separation. The purpose
of separation and the way separated
components are used is mentioned in
Column 2 and 3 respectively. However,
the information given in Columns 2 and
3 is jumbled up. Can you match each
I
n our daily life, there are many
       instances when we notice a substance
   being separated from a mixture
of materials.
Tea leaves are separated from  the
liquid  with a strainer, while preparing
tea (Fig. 3.1).
than mango or guava? Imagine you are
given a glass of sand with salt mixed in
it. Impossible, even to think of
separating salt from this mixture by
picking out grains of sand by hand!
Rationalised 2023-24
21 SEPARATION OF SUBSTANCES
process with its purpose and the way
separated components are used?
We see that, before we use a
substance, we need to separate harmful
or non-useful substances that may be
mixed with it. Sometimes, we separate
even useful components if we need to
use them separately.
The substances to be separated may
be particles of different sizes or
materials. These may be in any three
states of matter i.e., solid, liquid or gas.
So, how do we separate substances
mixed together if they have so many
different properties?
3.1  METHODS OF SEPARATION
We will discuss some simple methods of
separating substances that are mixed
together. You may come across some of
these methods being used in day to day
activities.
Handpicking
Activity 2
Bring a packet of food grain purchased
from a shop to the classroom. Now,
spread the grains on a sheet of paper.
Do you find only one kind of grain on
the sheet of paper? Are there pieces of
stone, husks, broken grain and particles
of any other grain in it? Now, remove
with your hand the pieces of stone,
husks and other grains from it.
This method of handpicking can
be used for separating slightly larger
sized impurities like the pieces of dirt,
stone, and husk from wheat, rice or
pulses (Fig. 3.3). The quantity of such
impurities is usually not very large.
In such situations, we find that
handpicking is a convenient method of
separating substances.
Table 3.1 Why do we separate substances?
n o i t a r a p e S
s s e c o r p
o d e w h c i h w r o f e s o p r u P
n o i t a r a p e s e h t
e h t h t i w o d e w o d t a h W
? s t n e n o p m o c d e t a r a p e s
e t a r a p e S ) 1
e c i r m o r f s e n o t s
, t n e r e f f i d o w t e t a r a p e s o T ) a
. s t n e n o p m o c l u f e s u t u b
d l i o s e h t y a w a w o r h t e W ) i
. t n e n o p m o c
k l i m g n i n r u h C ) 2
r e t t u b n i a t b o o t
l u f e s u - n o n e v o m e r o T ) b
. s t n e n o p m o c
e h t y a w a w o r h t e W ) i i
. s e i t i r u p m i
a e t e t a r a p e S ) 3
s e v a e l
r o s e i t i r u p m i e v o m e r o T ) c
. s t n e n o p m o c l u f m r a h
e h t h t o b e s u e W ) i i i
. s t n e n o p m o c
Fig. 3.3 Handpicking stones from grain
Threshing
You must have seen bundles of wheat
or paddy stalks lying in fields after
Rationalised 2023-24
22 SCIENCE
harvesting the crop. Stalks are dried in
the sun before the grain is separated
from them. Each stalk has many grain
seeds attached to it. Imagine the
number of grain seeds in hundreds of
bundles of stalk lying in the field! How
does the farmer separate grain seeds
from those bundles of stalks?
One may pluck mangoes or guavas
from the trees. But, grain seeds are
much smaller than mangoes or guavas.
So, plucking them from their stalks
would be impossible. How does one
separate grain seeds from their stalks?
The process that is used to separate
grain from stalks etc. is threshing. In
this process, the stalks are beaten to free
the grain seeds (Fig. 3.4). Sometimes,
this mixture on a plate or a newspaper.
Look at this mixture carefully. Can the
two different components be made out
easily? Are the sizes of particles of the
two components similar? Would it be
possible to separate the components by
handpicking?
Now, take your mixture to an open
ground and stand on a raised platform.
Put the mixture in a plate or sheet of
paper. Hold the plate or the sheet of
paper containing the mixture, at your
shoulder height. Tilt it slightly, so that
the mixture slides out slowly.
What happens? Do both the
components — sand and sawdust (or
powdered leaves) fall at the same place?
Is there a component that blows away?
Did the wind manage to separate the
two components?
This method of separating
components of a mixture is called
winnowing. Winnowing is used to
separate heavier and lighter components
of a mixture by wind or by blowing air.
Fig. 3.4 Threshing
threshing is done with the help of
bullocks.  Machines are also used to
thresh large quantities of grain.
Winnowing
Activity 3
Make a mixture of dry sand with
sawdust or powdered dry leaves. Keep Fig. 3.5 Winnowing
soop
husk
grain
direction of the air
Rationalised 2023-24
23 SEPARATION OF SUBSTANCES
This method is commonly used by
farmers to separate lighter husk particles
from heavier seeds of grain (Fig. 3.5).
The husk particles are carried away
by the wind. The seeds of grain get
separated and form a heap near the
platform for winnowing. The separated
husk is used for many purposes such
as fodder for cattles.
Sieving
Sometimes, we may wish to prepare a
dish with flour. We need to remove
impurities and bran that may be present
in it. What do we do? We use a sieve
and pour the flour into it (Fig. 3.6).
Sieving allows the fine flour particles
to pass through the holes of the sieve
while the bigger impurities  remain on
the sieve.
In a flour mill, impurities like husk
and stones are removed from wheat
before grinding it. Usually, a bagful of
wheat is poured on a slanting sieve. The
sieving removes pieces of stones, stalk
and husk that may still remain with
wheat after threshing and winnowing.
to separate pebbles and stones from
sand (Fig. 3.7).
Activity 4
Bring a sieve and a small quantity of
flour from home, to the class. Sieve the
flour to separate any impurities in it.
Now, make a fine powder of chalk pieces
and mix it with the flour. Can we
separate the flour and the powdered
chalk by sieving?
Sieving is used when components of
a mixture have different sizes.
Sedimentation, Decantation
and Filtration
Sometimes, it may not be possible to
separate components of a mixture by
winnowing and handpicking. For
example, there may be lighter impurities
like dust or soil particles in rice or
pulses. How are such impurities
separated from rice or pulses before
cooking?
Rice or pulses are usually washed
before cooking. When you add water to
these, the impurities like dust particles
Fig. 3.6 Sieving
You may have also noticed similar
sieves being used at construction sites
Fig. 3.7 Pebbles and stones are removed from
sand by sieving
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24 SCIENCE
get separated. These impurities go into
water. Now, what will sink to the bottom
of the vessel — rice or dust? Why? Have
you seen that the vessel is tilted to pour
out the dirty water?
When the heavier component in a
mixture settles after water is added to
it, the process is called sedimentation.
When the water (along with the dust) is
removed, the process is called
decantation (Fig. 3.8).  Let us find a
few other mixtures that can be separated
through sedimentation and
decantation.
The same principle is used for
separating a mixture of two liquids that
do not mix with each other. For example,
oil and water from their mixture can be
separated by this process. If a mixture of
such liquids is allowed to stand for some
time, they form two separate layers. The
component that forms the top layer can
then be separated by decantation.
Let us again consider a mixure of a
solid and liquid. After preparing tea,
what do you do to remove the tea leaves?
Usually, we use stainer to remove tea
leaves. Try decantation. It helps a little.
But, do you still get a few leaves in your
tea? Now, pour the tea through a
strainer. Did all the tea leaves remain
in the strainer? This process is called
filtration (Fig. 3.1). Which method of
separating tea leaves from prepared tea
is better, decantation or filtration?
Let us now consider the example
of water that we use. Do all of us, at
all times, get safe water to drink?
Sometimes, water supplied through taps
may be muddy. The water collected from
ponds or rivers may also be muddy,
especially after rains. Let us see if we
can use some method of separation to
remove insoluble impurities like soil
from the water.
Activity 5
Collect some muddy water from a pond
or a river. If it is not available, mix some
soil to water in a glass. Let it stand for
half an hour. Observe the water
carefully and note your observations.
Does some soil settle at the bottom
of water? Why? What will you call this
process?
Now, slightly tilt the glass without
disturbing the water. Let the water from
the top flow into another glass (Fig. 3.8).
What will you call this process?
Is the water in the second glass still
muddy or brown in colour? Now filter
it. Did the tea strainer work? Let us try
filtering the water through a piece of
cloth. In a piece of cloth, small holes or
pores remain in between the woven
threads. These pores in a cloth can be
used as a filter.
If the water is still muddy, impurities
can be separated by a filter that has even
Fig. 3.8 Separating two components of a mixture
by sedimentation and decantation
undissolved
material
(soil)
mixture
(soil + water)
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