NCERT Textbook: Physical & Chemical Changes | Science Class 7 PDF Download

 Page 1


5
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. 5.1   Paper pieces
5.1  PHYSICAL CHANGES
Activity 5.1
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. 5.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 5.2
Collect the chalk dust lying on the floor
near the chalkboard 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 5.3
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?
Physical and
Chemical Changes
Reprint 2024-25
Page 2


5
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. 5.1   Paper pieces
5.1  PHYSICAL CHANGES
Activity 5.1
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. 5.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 5.2
Collect the chalk dust lying on the floor
near the chalkboard 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 5.3
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?
Physical and
Chemical Changes
Reprint 2024-25
SCIENCE 48
5.3 and 5.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 5.5, 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.
5.2  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. 5.2). 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 5.4
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 5.5
Fig. 5.2   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 of the blade on the 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 5.1 and 5.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
Reprint 2024-25
Page 3


5
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. 5.1   Paper pieces
5.1  PHYSICAL CHANGES
Activity 5.1
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. 5.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 5.2
Collect the chalk dust lying on the floor
near the chalkboard 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 5.3
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?
Physical and
Chemical Changes
Reprint 2024-25
SCIENCE 48
5.3 and 5.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 5.5, 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.
5.2  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. 5.2). 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 5.4
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 5.5
Fig. 5.2   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 of the blade on the 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 5.1 and 5.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
Reprint 2024-25
PHYSICAL AND CHEMICAL CHANGES 49
light (Fig. 5.3). 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. 5.3  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 5.6
(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 4, 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.
Reprint 2024-25
Page 4


5
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. 5.1   Paper pieces
5.1  PHYSICAL CHANGES
Activity 5.1
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. 5.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 5.2
Collect the chalk dust lying on the floor
near the chalkboard 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 5.3
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?
Physical and
Chemical Changes
Reprint 2024-25
SCIENCE 48
5.3 and 5.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 5.5, 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.
5.2  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. 5.2). 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 5.4
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 5.5
Fig. 5.2   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 of the blade on the 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 5.1 and 5.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
Reprint 2024-25
PHYSICAL AND CHEMICAL CHANGES 49
light (Fig. 5.3). 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. 5.3  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 5.6
(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 4, 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.
Reprint 2024-25
SCIENCE 50
substance formed by mixing magnesium
oxide with water.
Activity 5.7
(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. 5.4).
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 5.8
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. 5.5.
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. 5.4   Change in colour of the copper sulphate
solution due to reaction with iron
Copper sulphate
(blue)
Iron sulphate (greenish)
Reprint 2024-25
Page 5


5
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. 5.1   Paper pieces
5.1  PHYSICAL CHANGES
Activity 5.1
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. 5.1).
Obviously, you cannot join the pieces
back to make the original piece, but is
there a change in the property of the
paper?
Activity 5.2
Collect the chalk dust lying on the floor
near the chalkboard 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 5.3
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?
Physical and
Chemical Changes
Reprint 2024-25
SCIENCE 48
5.3 and 5.4, water changed its state (from
solid to liquid, or from gas to liquid). In
Activity 5.5, 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.
5.2  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. 5.2). 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 5.4
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 5.5
Fig. 5.2   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 of the blade on the 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 5.1 and 5.2 above, you
saw that paper and a piece of chalk
underwent changes in size. In Activities
Reprint 2024-25
PHYSICAL AND CHEMICAL CHANGES 49
light (Fig. 5.3). 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. 5.3  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 5.6
(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 4, 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.
Reprint 2024-25
SCIENCE 50
substance formed by mixing magnesium
oxide with water.
Activity 5.7
(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. 5.4).
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 5.8
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. 5.5.
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. 5.4   Change in colour of the copper sulphate
solution due to reaction with iron
Copper sulphate
(blue)
Iron sulphate (greenish)
Reprint 2024-25
PHYSICAL AND CHEMICAL CHANGES 51
Carbon dioxide (CO
2
) + Lime water
[Ca(OH)
2
] ??Calcium Carbonate
(CaCO
3
) + Water (H
2
O)
When carbon dioxide is passed
through lime water, calcium carbonate
is formed, which makes lime water milky.
The turning of lime water into milky is a
standard test of carbon dioxide. You will
use it in Chapter 6 to show that the air
we breathe out is rich in carbon dioxide.
In Activities 5.6–5.8, you saw that
in each change one or more new
substances were formed. In Activity 5.6,
the ash was the new substance formed
when magnesium was burnt in air. In
Activity 5.7, the reaction of copper
sulphate with iron produced iron
sulphate and copper.  Both of these are
new substances. Copper was deposited
on the shaving blade of iron. In Activity
5.8, vinegar and baking soda together
produced carbon dioxide, which turned
lime water milky. Can you name the new
substance formed in this reaction?
A change in which one or more
new substances are formed is called a
chemical change. A chemical change
is also called a chemical reaction.
Chemical changes are very important
in our lives.  All new substances are
formed as a result of chemical changes.
For example, digestion of food in our
body, ripening of fruits, fermentation of
grapes, etc., happen due to series of
chemical changes. A medicine is the end
product of a chain of chemical reactions.
Useful new materials, such as plastics
and detergents, are produced by
chemical reactions. Indeed, every new
material is discovered by studying
chemical changes.
We have seen that one or more new
substances are produced in a chemical
change.  In addition to new products,
the following may accompany a chemical
change:
? Heat, light or any other radiation
(ultraviolet, for example) may be given
off or absorbed.
? Sound may be produced.
? A change in smell may take place or
a new smell may be given off.
? A colour change may take place .
? A gas may be formed.
Let us look at some examples.
You saw that burning of magnesium
ribbon is a chemical change.  Burning
of coal, wood or leaves is also a chemical
change. In fact, burning of any
substance is a chemical change.
Burning is always accompanied by
production of heat.
Fig. 5.5  Set up to pass gas through lime water
Vinegar +
Baking soda
Carbon
dioxide
Lime
water
Reprint 2024-25