13- Changes –
Physical And Chemical
Q.
1. Distinguish between the following:
(1)
Physical change and chemical change.
Ans. Physical change Chemical
change
1.
The properties of the original substances 1. The properties of the original
substance
remain
the same in the physical change. alter in the chemical change.
2.
Their original composition remains 2. Their original composition
changes.
unchanged.
3. The physical changes are reversible. 3. The chemical changes are
irreversible.
e.g.,
Evaporation, boiling, dissolution etc. e.g., Photosynthesis, corrosion.
(2) Periodic change and non-periodic change.
Ans.
Periodic change Non-periodic
change
1.
Changes occurring again and again after a 1. Changes not occurring again and again
definite
interval of time are called periodic
after a definite interval of time
are called changes.
non-periodic changes.
2.
There is constancy in periodic changes. 2. There is no constancy in
non-periodic
e.g.,
High and low tide of the sea. changes
movement
of hands of clock.
e.g., Breaking of storm
(3) Natural change and man-made change.
Ans. Natural change Man-made
change
1.
The change occurring automatically by 1. The changes induced by man are
called
natural
cause is called natural change. man-made changes.
2.
Most of the natural events are the 2. Man-made changes are due to
man's wish examples of natural changes. to satisfy his
needs.
e.g.,
Ripening of fruits. e.g.,
Cutting down the trees.
Q. 2. Under which
different types can the following changes be classified?
Questions :
Ans.
(1)
Conversion of milk into yoghurt (1) Chemical, irreversible, slow.
(2)
Bursting of a cracker.
(2) Chemical, irreversible,
fast.
(3)
Occurrence of an earthquake. (3)
Physical, Chemical, irreversible, natural
(4) Revolution of the earth around the (4) Periodic, slow, natural.
sun.
(5)
Stretching of a spring
(5) Physical, reversible.
Q
3. Give reasons :
(1) While purchasing canned food, its expiry
date should be checked.
Ans.
Every canned item has a limited shelf life. This is the time limit during which
food item is safe to eat. This is mentioned on the can. But later, after the
expiry date is over, the food may not remain in a good condition due to
irreversible chemical changes in it. Consuming such items can harm our health.
Therefore, while purchasing canned food, it expiry date should be checked.
(2)
An iron article should be given a coat of paint.
Ans.
An iron article rusts naturally which is called corrosion. Corrosion takes
place due oxygen, moisture, vapours of chemicals in the air. Corrosion makes
articles weak and sable. Therefore, to keep the iron articles in better
condition, it should be given a coat of paint.
(3)
A wooden article should be polished.
Ans.
A wood is an organic substance that is affected by termites or fungus. This
natural change can result into irreversible damage. If polish is applied to
such wooden articles, the rate of natural changes slows down. Thus to prevent
this harmful change, a wooden article should be polished.
(4)
Copper and brass utensils should be tinned.
Ans. The metals like copper and brass gather a
greenish layer due to slow chemical changes. This is corrosion which happens
due to oxygen, moisture and vapours of chemical in the air. For the prevention
of such corrosion, copper and brass utensils are tinned. Moreover, the tinned
utensils are safer for cooking.
(5)
A dry handkerchief gets wet at once on dipping in water, but it takes long for
a wet handkerchief to dry.
Ans.
A dry handkerchief absorbs the water, once it is dipped into the water. This is
a change. But drying such moist handkerchief is a slow process which occurs due
to evaporation. Evaporation is the physical change which depends upon
atmospheric temperature and humidity. It may therefore take long for drying a
wet handkerchief.
Q
4.What will you take into account while identifying the following?
(1)
A
physical change in a substance.
Ans
It will be seen that whether the substance has changed its properties and on.
If both of these have remained unchanged then it is a physical change.
(2)
A
chemical change in a substance.
will
be seen that whether, the substance is changed in its properties. Whether the
consumption of the substance is changed, whether the change is irreversible? If
all these re taking place then it will be a chemical change.
Q.
5. Read the paragraph and note down the various types of changes it mentions.
It
was nearing six o'clock in the evening. The sun was setting. A breeze was
blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard,
rolling balls of wet soil and shaping them into various toys. Then he felt
hungry and went into the house. Mother made a dough from wheat flour and fried
purees. While eating hot purees, his attention was drawn outside the window. It
had started raining. There was lightning, too. Sahil was enjoying his dinner in
the dim light.
Ans.
(1) It was nearing six o'clock in the evening : (Time-reversible, periodic)
(2)
The sun was setting : (Sunset Periodic, reversible)
(3)
A breeze was blowing: (Blowing of wind-Physical, non-periodic)
(4) Leaves on the tree were shaking :
(Movements due to wind. Physical, periodic)
(5)
Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them
into various toys. (Making toys of
soil-Physical)
(6)
Then he felt hungry and went into the house. (Hunger-Periodic)
(7)
Mother made a dough from wheat flour and fried purees. (Frying-Physical, irreversible)
(8)
While eating hot purees, his attention was drawn outside the window. (Seeing randomly-Non-periodic)
(9)
It had started raining. (Raining-Physical, Periodic if raining in specific
season)
(10)
There was lightning. (Lightning-Physical. Chemical. Fast)
