Chapter 6 – changes around us
ACTIVITY-1: To understand reversible change.
Question 1-
Before preparing roti your mother makes a ball from dough. Can the ball be reversed into dough?
Answer- Yes.
Question 2- You
make a toy boat from a piece of paper. Can you obtain back the paper in original form?
Answer- Yes.
Question 3-
With what kind of fabric your dupatta is made up of?
Answer- Light and smooth cotton.
ACTIVITY-2: To
understand irreversible change.
Question 1-
Observe the height of students of class 5th and of class 8th . Is the change in height a reversible or
irreversible change?
Answer- Irreversible change.
Question 2- Is
burning of candle reversible or irreversible change?
Answer- Irreversible change.
ACTIVITY- 3 : To
understand physical change.
Question 1-
Which type of change is involved in tearing of paper?
Answer- Physical and irreversible change.
Question 2-
Which type of change is involved in melting of ice to water?
Answer- Physical and reversible change.
ACTIVITY- 5 : To
demonstrate chemical change.
Question 1-
What type of change is observed in formation of cheese (paneer) from milk?
Answer- Chemical and Irreversible change.
Question 2-
Name the change involved in formation of wax from burning of candles?
Answer- Physical and reversible change.
ACTIVITY- 6 : To
study expansion and contraction.
Question 1-
Have you seen Bunsen Burner in the shop of goldsmith? What is its purpose there?
Answer- Yes, it is used to melt metals like gold and
silver.
Question 2-
When you put clinical thermometer in your mouth, why do Mercury in the thermometer rises and why it
falls down when you take it out? (note that Mercury is a metal which is liquid at room temperature)
Answer- Due to high body temperature, Mercury in the
thermometer expand so it rises. When we take thermometer out from the mouth,
due to low room temperature Mercury contracts and falls down in the thermometer.
EXERCISE
Question 1- Fill In
The Blanks.
a. A new substance is always formed in chemical
change..
b. Melting of ice is physical and reversible change.
c. Burning of paper is chemical change.
d. On heating metals expand.
e. Change that is repeated after regular interval of
time is known as periodic change.
Question 2- Write
True Or False.
a. Converting milk into paneer is a reversible change.
(False)
b. Rusting of iron is a slow change. (True)
c. Metals contract on heating. (False)
d. Melting of snow from hills is a natural change. (True)
e. Burning of cracker is a fast change. (True)
Question 3- Match
The Column ‘A’ With Column ‘B’.
Column A Column B
a. Freezing of water
a. Non periodic (d)
b. Milk to curd b. Physical and reversible (a)
c. Burning of a matchstick c.
Periodic (e)
d. Earthquake d. Fast (c)
e. Change of season e. Chemical (b)
Question 4. Choose
The Correct Answer.
(i). Cooking of
food is which change:
a) Physical
b) Fast
c) Reversible
d) Irreversible (✓)
(ii). Which of
the following change is non periodic?
a) Heart beat
b) Earthquake (✓)
c) Occurrence of day and night
d) Motion of pendulum
(iii). Which of
the following expand on heating?
a) Wood
b) Paper
c) Metal (✓)
d) Cloth
(iv). Rusting
of iron is which type of change?
a) Reversible
b) Slow (✓)
c) Periodic
d) Fast
(v). Growth in
plants and animals is which type of change?
a) Slow (✓)
b) Reversible
c) Chemical
d) Periodic
Question 5- Very
Short Answer Type Questions.
(i) What are
changes?
Answer- Changes refers to the differences which occur
in color, shape, size, or position of things around us.
(ii) Define
slow and fast changes with examples.
Answer- Slow
changes- Slow changes are those which takes more time to happen. For
example: growing of tree, rusting of iron.
Fast
changes- Fast changes are those that happen very
fast. For example: burning of matchstick, bursting of crackers.
(iii) Give two
examples of reversible change.
Answer- Making boat with a piece of paper and melting
of ice.
(iv) Why iron
rim is made slightly smaller than the wooden wheel?
Answer- Because on heating the metal rim expands and
can easily lie on the wheel. When the rim cools, it contracts and fits tightly
onto the wheel.
(v) Give two
examples of chemical change.
Answer- Converting milk into curd and burning of
paper.
Question 6- Short
Answer Type Questions.
(i) Give
difference between periodic and non-periodic changes with examples.
Answer-
|
Periodic changes |
Non-Periodic changes |
|
1.
Changes that are repeated after regular interval of time are called periodic
changes. |
1.
Changes that are not repeated after regular interval of time are called
non-periodic changes. |
|
2.
Examples – Beating of heart, change of day and night. |
3.
Examples – occurrence of earthquake and occurrence of rain. |
(ii) Give
difference between reversible and irreversible changes with examples.
Answer-
|
Reversible changes |
Irreversible changes |
|
1.
This type of change can be reversed. |
1.
This type of change cannot be reversed. |
|
2.
Paper can be folded to make different shapes.This is reversible change as
shapes of paper can be unfolded back into paper sheet |
2.
When paper is burnt and turned into ash, it can't be reversed |
|
3.
If we fill balloon with air, the shape and size of the balloon changes. This
change can be reversed |
3.
If balloon burst while inflating then this change can't be reversed. |
(iii) Why a
candle reduces in size on burning?
Answer- Because on burning candle some wax melts and
some burns.
(iv) Give difference
between physical and chemical changes with examples.
Answer-
|
Physical
changes |
Chemical
changes |
|
1.
In physical change not any new substance is produced. |
1.
In chemical change new substance is produced. |
|
2.
Example – Melting of ice into water. |
3.
Example – Converting milk into curd. |
Question 7- Long
Answer Type Questions.
(i) What is
expansion? Define thermal expansion. Explain with two examples.
Answer- Expansion-
Expansion is the increase in the dimension of a body when subjected to high temperature
and high pressure.
Thermal
expansion- When expansion is due to increase in
temperature, it is known as thermal expansion.
Examples-Mercury
expands in thermometer on increasing temperature; A balloon expands on filling
air in it.
