NCERT Textbook - Physical and Chemical Changes Class 7 Notes | EduRev

General Science for UPSC (Civil Services) Prelims

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Class 7 : NCERT Textbook - Physical and Chemical Changes Class 7 Notes | EduRev

 Page 1


SCIENCE 58
Physical and Chemical
Changes
6
E
very day you come across many
changes in your surroundings.
These changes may involve one
or more substances.  For example, your
mother may ask you to dissolve sugar
in water to make a cold drink. Making a
sugar solution is a change. Similarly,
setting curd from milk is a change.
Sometimes milk becomes sour.  Souring
of milk is a change. Stretched rubber
band also represents a change.
Make a list of ten changes you have
noticed around you.
In this chapter we shall perform some
activities and study the nature of these
changes.  Broadly, these changes are of
two kinds, physical and chemical.
Fig. 6.1   Paper pieces
6.1  PHYSICAL CHANGES
Activity 6.1
Cut a piece of paper in four square
pieces. Cut each square piece further
into four square pieces.  Lay these pieces
on the floor or a table so that the pieces
acquire the shape of the original piece
of paper (Fig. 6.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 6.2
Collect the chalk dust lying on the floor
near the blackboard in your classroom.
Or, crush a small piece of chalk into
dust. Add a little water to the dust to
make a paste. Roll it into the shape of a
piece of chalk. Let it dry.
Did you recover chalk from the
dust?
Activity 6.3
Take some ice in a glass or plastic
tumbler. Melt a small portion of ice by
placing the tumbler in the sun. You have
now a mixture of ice and water. Now
place the tumbler in a freezing mixture
(ice plus common salt).
Does the water become solid ice once
again?
© NCERT
not to be republished
Page 2


SCIENCE 58
Physical and Chemical
Changes
6
E
very day you come across many
changes in your surroundings.
These changes may involve one
or more substances.  For example, your
mother may ask you to dissolve sugar
in water to make a cold drink. Making a
sugar solution is a change. Similarly,
setting curd from milk is a change.
Sometimes milk becomes sour.  Souring
of milk is a change. Stretched rubber
band also represents a change.
Make a list of ten changes you have
noticed around you.
In this chapter we shall perform some
activities and study the nature of these
changes.  Broadly, these changes are of
two kinds, physical and chemical.
Fig. 6.1   Paper pieces
6.1  PHYSICAL CHANGES
Activity 6.1
Cut a piece of paper in four square
pieces. Cut each square piece further
into four square pieces.  Lay these pieces
on the floor or a table so that the pieces
acquire the shape of the original piece
of paper (Fig. 6.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 6.2
Collect the chalk dust lying on the floor
near the blackboard in your classroom.
Or, crush a small piece of chalk into
dust. Add a little water to the dust to
make a paste. Roll it into the shape of a
piece of chalk. Let it dry.
Did you recover chalk from the
dust?
Activity 6.3
Take some ice in a glass or plastic
tumbler. Melt a small portion of ice by
placing the tumbler in the sun. You have
now a mixture of ice and water. Now
place the tumbler in a freezing mixture
(ice plus common salt).
Does the water become solid ice once
again?
© NCERT
not to be republished
PHYSICAL AND CHEMICAL CHANGES 59
6.3 and 6.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 6.5, the hack-saw blade
changed colour on heating.
Properties such as shape, size, colour
and state of a substance are called its
physical properties. A change in which
a substance undergoes a change in its
physical properties is called a physical
change. A physical change is generally
reversible. In such a change no new
substance is formed.
Let us now consider the other kind
of change.
6.2  CHEMICAL CHANGE
A change with which you are quite
familiar is the rusting of iron.  If you
leave a piece of iron in the open for some
time, it acquires a film of brownish
substance.  This substance is called rust
and the process is called rusting
(Fig. 6.2). Iron gates of parks or
farmlands, iron benches kept in lawns
and gardens, almost every article of iron,
kept in the open gets rusted. At home
you must have seen shovels and spades
getting rusted when exposed to the
Activity 6.4
Boil some water in a container. Do you
see the steam rising from the surface of
water? Hold an inverted pan by its
handle over the steam at some distance
from the boiling water.  Observe the
inner surface of the pan.
Do you see any droplet of water
there?
Activity 6.5
Fig. 6.2   Rusting iron
CAUTION
Be careful while handling a flame.
Hold a used hack-saw blade with a
pair of tongs. Keep the tip of the free
end on the flame of a gas stove. Wait for
a few minutes.
Does the colour of the tip of the blade
change?
Remove the blade from the flame.
Observe the tip once again after some
time.
Does it get back its original colour?
In Activities 6.1 and 6.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
© NCERT
not to be republished
Page 3


SCIENCE 58
Physical and Chemical
Changes
6
E
very day you come across many
changes in your surroundings.
These changes may involve one
or more substances.  For example, your
mother may ask you to dissolve sugar
in water to make a cold drink. Making a
sugar solution is a change. Similarly,
setting curd from milk is a change.
Sometimes milk becomes sour.  Souring
of milk is a change. Stretched rubber
band also represents a change.
Make a list of ten changes you have
noticed around you.
In this chapter we shall perform some
activities and study the nature of these
changes.  Broadly, these changes are of
two kinds, physical and chemical.
Fig. 6.1   Paper pieces
6.1  PHYSICAL CHANGES
Activity 6.1
Cut a piece of paper in four square
pieces. Cut each square piece further
into four square pieces.  Lay these pieces
on the floor or a table so that the pieces
acquire the shape of the original piece
of paper (Fig. 6.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 6.2
Collect the chalk dust lying on the floor
near the blackboard in your classroom.
Or, crush a small piece of chalk into
dust. Add a little water to the dust to
make a paste. Roll it into the shape of a
piece of chalk. Let it dry.
Did you recover chalk from the
dust?
Activity 6.3
Take some ice in a glass or plastic
tumbler. Melt a small portion of ice by
placing the tumbler in the sun. You have
now a mixture of ice and water. Now
place the tumbler in a freezing mixture
(ice plus common salt).
Does the water become solid ice once
again?
© NCERT
not to be republished
PHYSICAL AND CHEMICAL CHANGES 59
6.3 and 6.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 6.5, the hack-saw blade
changed colour on heating.
Properties such as shape, size, colour
and state of a substance are called its
physical properties. A change in which
a substance undergoes a change in its
physical properties is called a physical
change. A physical change is generally
reversible. In such a change no new
substance is formed.
Let us now consider the other kind
of change.
6.2  CHEMICAL CHANGE
A change with which you are quite
familiar is the rusting of iron.  If you
leave a piece of iron in the open for some
time, it acquires a film of brownish
substance.  This substance is called rust
and the process is called rusting
(Fig. 6.2). Iron gates of parks or
farmlands, iron benches kept in lawns
and gardens, almost every article of iron,
kept in the open gets rusted. At home
you must have seen shovels and spades
getting rusted when exposed to the
Activity 6.4
Boil some water in a container. Do you
see the steam rising from the surface of
water? Hold an inverted pan by its
handle over the steam at some distance
from the boiling water.  Observe the
inner surface of the pan.
Do you see any droplet of water
there?
Activity 6.5
Fig. 6.2   Rusting iron
CAUTION
Be careful while handling a flame.
Hold a used hack-saw blade with a
pair of tongs. Keep the tip of the free
end on the flame of a gas stove. Wait for
a few minutes.
Does the colour of the tip of the blade
change?
Remove the blade from the flame.
Observe the tip once again after some
time.
Does it get back its original colour?
In Activities 6.1 and 6.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
© NCERT
not to be republished
SCIENCE 60
light (Fig. 6.3). When it is completely
burnt it leaves behind a powdery ash.
Does the ash look like the
magnesium ribbon?
The change can be represented by
the following equation:
Magnesium (Mg) + Oxygen (O
2
) ?
Magnesium oxide (MgO)
Fig. 6.3  Magnesium ribbon burning
atmosphere for some time. In the
kitchen, a wet iron pan (tawa) often gets
rusted if left in that state for some time.
Rust is not iron.  It is different from iron
on which it gets deposited.
Let us consider a few more changes
where new substances are formed.
Activity 6.6
(To be demonstrated by the teacher)
CAUTION
It is dangerous to look for long at the
burning magnesium ribbon.  The
teachers should advise children not
to stare at the burning ribbon.
Get a small piece of a thin strip or
ribbon of magnesium.  Clean its tip with
sandpaper.  Bring the tip near a candle
flame.  It burns with a brilliant white
Collect the ash and mix it with a
small amount of water. Stir the mixture
(aqueous solution) well. Test the mixture
with blue and red litmus papers.
Does the mixture turn red litmus
blue?
Does the mixture turn blue litmus
red?
On the basis of this test, how do you
classify the aqueous solution — acidic
or basic?
On dissolving the ash in water it
forms a new substance. This change can
be written in the form of the following
equation:
Magnesium oxide (MgO) + Water
(H
2
O) ? Magnesium hydroxide [Mg(OH)
2
]
As you have already learnt in
Chapter 5, magnesium hydroxide is a base.
So, magnesium oxide is a new substance
formed on burning of magnesium.
Magnesium hydroxide is another new
The equations here are different from
those in mathematics. In equations
of this kind, the arrow implies
‘becomes’. No attempt should be made
to balance chemical equations at this
stage.
© NCERT
not to be republished
Page 4


SCIENCE 58
Physical and Chemical
Changes
6
E
very day you come across many
changes in your surroundings.
These changes may involve one
or more substances.  For example, your
mother may ask you to dissolve sugar
in water to make a cold drink. Making a
sugar solution is a change. Similarly,
setting curd from milk is a change.
Sometimes milk becomes sour.  Souring
of milk is a change. Stretched rubber
band also represents a change.
Make a list of ten changes you have
noticed around you.
In this chapter we shall perform some
activities and study the nature of these
changes.  Broadly, these changes are of
two kinds, physical and chemical.
Fig. 6.1   Paper pieces
6.1  PHYSICAL CHANGES
Activity 6.1
Cut a piece of paper in four square
pieces. Cut each square piece further
into four square pieces.  Lay these pieces
on the floor or a table so that the pieces
acquire the shape of the original piece
of paper (Fig. 6.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 6.2
Collect the chalk dust lying on the floor
near the blackboard in your classroom.
Or, crush a small piece of chalk into
dust. Add a little water to the dust to
make a paste. Roll it into the shape of a
piece of chalk. Let it dry.
Did you recover chalk from the
dust?
Activity 6.3
Take some ice in a glass or plastic
tumbler. Melt a small portion of ice by
placing the tumbler in the sun. You have
now a mixture of ice and water. Now
place the tumbler in a freezing mixture
(ice plus common salt).
Does the water become solid ice once
again?
© NCERT
not to be republished
PHYSICAL AND CHEMICAL CHANGES 59
6.3 and 6.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 6.5, the hack-saw blade
changed colour on heating.
Properties such as shape, size, colour
and state of a substance are called its
physical properties. A change in which
a substance undergoes a change in its
physical properties is called a physical
change. A physical change is generally
reversible. In such a change no new
substance is formed.
Let us now consider the other kind
of change.
6.2  CHEMICAL CHANGE
A change with which you are quite
familiar is the rusting of iron.  If you
leave a piece of iron in the open for some
time, it acquires a film of brownish
substance.  This substance is called rust
and the process is called rusting
(Fig. 6.2). Iron gates of parks or
farmlands, iron benches kept in lawns
and gardens, almost every article of iron,
kept in the open gets rusted. At home
you must have seen shovels and spades
getting rusted when exposed to the
Activity 6.4
Boil some water in a container. Do you
see the steam rising from the surface of
water? Hold an inverted pan by its
handle over the steam at some distance
from the boiling water.  Observe the
inner surface of the pan.
Do you see any droplet of water
there?
Activity 6.5
Fig. 6.2   Rusting iron
CAUTION
Be careful while handling a flame.
Hold a used hack-saw blade with a
pair of tongs. Keep the tip of the free
end on the flame of a gas stove. Wait for
a few minutes.
Does the colour of the tip of the blade
change?
Remove the blade from the flame.
Observe the tip once again after some
time.
Does it get back its original colour?
In Activities 6.1 and 6.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
© NCERT
not to be republished
SCIENCE 60
light (Fig. 6.3). When it is completely
burnt it leaves behind a powdery ash.
Does the ash look like the
magnesium ribbon?
The change can be represented by
the following equation:
Magnesium (Mg) + Oxygen (O
2
) ?
Magnesium oxide (MgO)
Fig. 6.3  Magnesium ribbon burning
atmosphere for some time. In the
kitchen, a wet iron pan (tawa) often gets
rusted if left in that state for some time.
Rust is not iron.  It is different from iron
on which it gets deposited.
Let us consider a few more changes
where new substances are formed.
Activity 6.6
(To be demonstrated by the teacher)
CAUTION
It is dangerous to look for long at the
burning magnesium ribbon.  The
teachers should advise children not
to stare at the burning ribbon.
Get a small piece of a thin strip or
ribbon of magnesium.  Clean its tip with
sandpaper.  Bring the tip near a candle
flame.  It burns with a brilliant white
Collect the ash and mix it with a
small amount of water. Stir the mixture
(aqueous solution) well. Test the mixture
with blue and red litmus papers.
Does the mixture turn red litmus
blue?
Does the mixture turn blue litmus
red?
On the basis of this test, how do you
classify the aqueous solution — acidic
or basic?
On dissolving the ash in water it
forms a new substance. This change can
be written in the form of the following
equation:
Magnesium oxide (MgO) + Water
(H
2
O) ? Magnesium hydroxide [Mg(OH)
2
]
As you have already learnt in
Chapter 5, magnesium hydroxide is a base.
So, magnesium oxide is a new substance
formed on burning of magnesium.
Magnesium hydroxide is another new
The equations here are different from
those in mathematics. In equations
of this kind, the arrow implies
‘becomes’. No attempt should be made
to balance chemical equations at this
stage.
© NCERT
not to be republished
PHYSICAL AND CHEMICAL CHANGES 61
substance formed by mixing magnesium
oxide with water.
Activity 6.7
(To be demonstrated by the teacher)
Dissolve about a teaspoonful of copper
sulphate (blue vitriol or neela thotha) in
about half a cup of water in a glass
tumbler or a beaker. Add a few drops of
dilute sulphuric acid to the solution.
You should get a blue coloured solution.
Save a small sample of the solution in a
test tube or a small glass bottle. Drop a
nail or a used shaving blade into the
remaining solution. Wait for half an
hour or so. Observe the colour of the
solution. Compare it with the colour of
the sample solution saved separately
(Fig. 6.4).
colour of the solution from blue to
green is due to the formation of iron
sulphate, a new substance. The brown
deposit on the iron nail is copper,
another new substance. We can write
the reaction as:
Copper sulphate solution (blue) + Iron
? Iron sulphate solution (green)
+ Copper (brown deposit)
Activity 6.8
Take about a teaspoonful of vinegar in
a test tube. Add a pinch of baking soda
to it. You would hear a hissing sound
and see bubbles of a gas coming out.
Pass this gas through freshly prepared
lime water as shown in Fig. 6.5.
What happens to the lime water?
Do you see any change in the colour
of the solution?
Take out the nail or the blade.
Has it changed in any way?
The changes that you notice are
due to a reaction between copper
sulphate and iron. The change of
The change in the test tube is as
follows:
Vinegar (Acetic acid) + Baking soda
(Sodium hydrogencarbonate) ?
Carbon dioxide + other substances
The reaction between carbon dioxide
and lime water is as follows:
Fig. 6.4   Change in colour of the copper sulphate
solution due to reaction with iron
Copper sulphate
Iron sulphate
© NCERT
not to be republished
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NCERT Textbook - Physical and Chemical Changes Class 7 Notes | EduRev

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