Changes Around Us: science class six
NCERT Exercise and Answer
Question (1) To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?
Answer: Yes this change can be reversed.
Question (2) You accidentally dropped your favourite toy and broke it, This is a change you did not want, Can this change be reversed?
Answer: No this change cannot reversed.
Question (3) Some change are listed in the following table, For each change, write in the blank column, whether the change can be reversed or not.
| Question | ||
|---|---|---|
| S.No | Change | Can be reversed (Yes/No) |
| 1. | The sawing of a piece of wood | |
| 2 | The melting of ice candy | |
| 3 | Dissolving sugar in water | |
| 4 | The cooking of food | |
| 5 | The ripening of a mango | |
| 6 | Souring of milk | |
| Answer | ||
|---|---|---|
| S.No | Change | Can be reversed (Yes/No) |
| 1. | The sawing of a piece of wood | No |
| 2 | The melting of ice candy | Yes |
| 3 | Dissolving sugar in water | Yes |
| 4 | The cooking of food | No |
| 5 | The ripening of a mango | No |
| 6 | Souring of milk | No |
Question (4) A drawing sheet changes when you draw a picture on it. Can you reverse this change
Answer: If you draw a picture on a drawing sheet with a pencil, it can be reversed by erasing the drawing. But if you draw a picture on a drawing sheet with any colours like water colour, wax colour or oil colours then it cannot be reversed.
Question (5) Give examples to explain the difference between changes that can or cannot be reversed.
Answer: Tearing of paper, cooking of food, growth of plants, etc. are irreversible changes. This means these changes cannot be reversed.
Freezing of water, glowing of bulb, melting of ice, etc. are reversible change. These changes can be reversed.
Question (6) A thick coating of a paste of plaster of paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immoblised. Can the change in POP be reversed?
Answer : After hardening of plaster of paris (POP), It cannot be reversed.
Question (7) A bag of cement lying in the open gets wet due to rain during the night the next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?
Answer: Hardening of cement cannot be reversed.
Extra Questions and Answers
(1)Write "T" for true and "F" for false statement given follows-
(i)Burning of wax is reversible change.
Answer : F
Correct statement: Burning of wax is irreversible change.
(2) Burning of wood is a physical change.
Answer: F
Correct statement: Burning of wood is a chemical change. Because new substances are formed after burning.
(3)Change of milk into curd is an example of irreversible change.
Answer: T
(4) Eruption of volcano is natural change.
Answer: True
(5) Phenomenon of day and night is a desirable change.
Answer: T
(2) Choose the right answer for the following questions -
(i) Which is the physical change?
(a) Burning of wax
(b)Melting of wax
(c) Making of paneer.
(d)Rusting of iron
Answer: (b)Melting of wax
(ii)Which is the reversible change?
(a) Burning of wax
(b)Melting of wax
(c) Making of paneer.
(d) Rusting of iron
Answer: (b)Melting of wax
(iii) Which is man-made change?
(a) Bursting of a volcano
(b) Formation of cloud
(c)Evaporation of water by gas burner.
(d)Evaporation of water by sunlight.
Answer (c) Evaporation of water by gas burner.
(iv) Which is a chemical change?
(a)Rusting of iron
(b) Formation of cloud
(c) Evaporation of water by gas burner.
(d) Evaporation of water by sunlight.
Answer: (a) Rusting of iron
(v) Separation of butter from milk is an example of which type of change?
(a) Physical change
(b) Chemical change
(c ) Undesirable change
(d) None of these
Answer: (b) Chemical change
(vi) Which of the following can be considered as a fast change?
(a) Growth of a child
(b) Germination of seeds.
(c) Bursting of a fire cracker
(d) cooking of food
Answer: (c) Bursting of a fire cracker
(vii) Evaporation of water is which type of change?
(a) Physical change
(b) Fast change
(c) Irreversible change
(d) Chemical change
Answer: (a) Physical change
(viii) Which one of these is an irreversible change?
(a) Growth of a child
(b) Stretching a rubber band
(c ) evaporation of water
(d) Formation of ice from water
Answer: (a) Growth of a child
(ix) A piece of paper undergoes a chemical change, when it is
(a) cut
(b) folded
(c) reshaped into a paper toy
(d) burnt
Answer: (d) burnt
(x) Which one of these changes is a reversible change?
(a) burning of a candle
(b) inflating a balloon
(c) baking of chapati
(d) grinding of wheat grains
Answer: (b) inflating a balloon
(3) Answer the following questions:
Question (a) Phenomenon of day and night is which type of change?
Answer: Phenomenon of day and night is a natural changes.
Question (b) Give an example of fast change.
Answer: Bursting of a cracker
Question (c) Can we get milk back from butter?
Answer: No, we cannot get milk back from butter.
Question (d) Which type of change is the bursting of a cycle tube?
Answer: Bursting of a cycle tube is fast and undesirable change.
Question (e) What is the cause of bursting of a cycle tube?
Answer: More pressure is the cause of bursting of a cycle tube.
Question (f) What happen when you mix salt in the water and which type of change it is?
Answer: When we mix salt in water, salt dissolves in water. Dissolving of salt into water is a chemical change.
Question (g) Give few examples of man made changes.
Answer: Heating water on stove, making of curd from milk, making of ice from water, etc are some examples of man made changes.
Question (h) Why we cannot control over the natural changes?
Answer: Because nature is beyond the control of man.
Question (i) When we light a candle how many type of changes occur there?
Answer: When we light a candle two type of changes take place. One melting of wax is a physical change and burning of wax is a chemical change.
Question (j) What do you understand by chemical changes?
Answer: A change in which new substance is formed; is called a chemical changes. Cooking food, burning of fuels, respiration of living, etc. are the examples of chemical changes.
Question (k) What do you understand by fast changes?
Answer: A change that takes place very quickly is called fast changes. Bursting of a fire cracker, lighting a matchstick, etc. are some examples of fast change.
Question (l) What do you understand by slow changes?
Answer: A change which take place a longer duration or time is called slow change. Growing a child, growing a plant, are some examples of slow changes.
Question (m) What is physical changes?
Answer: A change in which no new substance is formed and only physical nature of matter is changed, is called a physical change. For example: tearing of paper, cutting a cauliflower, freezing of water, etc. are physical change.
Question (n) What are the differences between physical and chemical changes.
Answer: The differences between physical and chemical changes are
(a) In a physical change no substance is formed while in chemical change new substance is formed.
(b) In physical change only physical nature of substance changes while in chemical change chemical nature of substance changes.
(c) Most of the physical changes are reversible while chemical changes are irreversible.
Question (o) What are causes of change?
Answer: There are many causes behind a change. For example change in temperature, change in pressure, burning of anything, etc. are some causes of changes.
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