Saturday, June 23, 2012

CBSE Class 6 Science - CH6 - Changes Around Us

Changes Around Us

Trees buried under Earth for
million years changed into coal
NCERT Chapter Solutions and other Q & A

Q1: To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?

Answer: Yes, by unfolding the dress we can reverse this change.

Q2: What are the various types of changes?

Answer: Changes can be categorised as:
  1. Slow and fast changes.
  2. Reversible and irreversible changes.
  3. Natural and man-made changes.
  4. Periodic and non-periodic changes.
  5. Physical and chemical changes.

Q3: You accidentally dropped your favourite toy and broke it. This is a change you did not want. Can this change be reversed?

Answer: No, it cannot be reversed. It is an irreversible change.

Q4: Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not.


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

Q5: A drawing sheet changes when you draw a picture on it. Can you reverse this change?

Answer: By erasing the drawing we can undo the change. For example, if we draw with the help of pencil, we can erase the drawing by using an eraser. Then, it is a reversible change.

If we draw using a pen, we cannot erase the drawing. Then this change cannot be reversed.

Q6: Bursting of cracker is a/an
(a) reversible change
(b) irreversible change
(c) both reversible and irreversible
(d) None of these

Answer: (b) irreversible change

Q7: Give examples to explain the difference between changes that can or cannot be reversed.

Examples of Reversible Changes:
  1. Melting of Ice into water. By freezing the water we can obtain ice again.
  2. Folding a paper: By unfolding it, we can undo the change.
  3. Hot milk to cold milk: By boiling milk, we can make it warm.
Example of Irreversible Changes:
  1. Bursting a balloon.
  2. Bud to flower
  3. Cutting a paper into pieces
Q8:  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 immobilised. Can the change in POP be reversed?

Answer: When water is added to POP, it undergoes a chemical change. Its composition changes and is converted into another substance. Since it is a chemical change, it cannot be reversed.

Q9: 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: Due to water, cement hardens and its composition changes. Chemical changes, in general, are irreversible changes. Since it is a chemical change which cannot be reversed.

Q10: What is a change?

Answer: An object may undergo different  look, composition, colour, position, size or shape due to some factor (external or internal). It is called a change. Change may be permanent or temporary.

Q11: What are reversible changes? Give examples.

Answer: Those changes which can be reversed to obtain original form by removing the cause of change are called reversible changes.
Examples are:
  •  melting of ice
  • folding of paper
  • evaporation of water

Q12: What is irreversible change? Give examples of irreversible change.

Answer: The change in a substance which cannot be converted back to its original form is called irreversible change.
Examples are:
  • burning of candle
  • germination of seed
  • bursting a balloon
  • rusting of iron

Q13: What are slow and fast changes? Give examples also.

Slow Changes: Changes that take place in longer duration i.e. few hours, days, months or years are called slow changes. Rusting of iron, growth of a baby, cooking of food, formation of coal etc. are examples of slow changes.

Fast Changes: Changes that take place in a short duration (instant or nick of time) are called fast changes. Bursting of cracker, burning of paper etc. are examples of fast changes.

Q14: Identify from the following list of changes which are fast or slow. Also mention the duration that take place in happening the change.
(a) Curdling of milk
(b) Switching on light bulb in a dark room
(c) Formation of Petrol
(d) Bursting a balloon
(e) Turning of a child into an adult.
(f) Burning a matchstick.


ChangesFast or Slow         Duration
(a) Curdling of milk     SlowFew hours
(b) Switching on light bulb in a  
dark room
(c) Formation of PetrolSlowMillion years
(d) Bursting a balloonFastInstant
(e) Turning of a child into an adult.Slow20 years
(f) Burning a matchstick.Fastless than a minute

Q15: What are periodic changes? Give examples of periodic changes.

Answer: Those changes that occur at periodic intervals of time are called periodic changes. It is easy to periodic changes.
Examples are:
  • Occurence of day and night
  • Phases of moon
  • Heartbeat

Q16: What are non-periodic changes? Also list two examples of non-periodic changes.

Answer: Changes that do not occur at fixed intervals of time are termed as non-periodic changes. Such changes may be unpredictable.
Examples are:
  • Earthquake
  • Eruption of volcano
  • An accident
  • Sneezing

Q17: Differentiate  between physical changes and chemical changes.


S.No.Physical ChangesChemical Chnages
1.Composition of substance remains same
No new substances are formed.
Composition of substance changes.
New substance os formed.
2.Change in shape, size or physical state occurs. Chemical properties of substance remain same.  New substance have new shape, physical state, different physical and chemical properties.
3.Physical changes are reversible.Chemical changes are irreversible.
4.e.g.: Melting of ice, expansion of iron on heatingBurning of wood, rusting of iron

Q18: Explain how iron rim is fixed over a wooden wheel. Which type of change is associated with this activity?

Answer: The iron rim taken, has slightly smaller diameter than the wooden wheel The iron rim is fixed over the wooden wheel by heating the rim first. On heating, the iron expands and becomes slightly bigger than the wheel. Once mounted, water is poured on the rim to cool it down. On cooling the iron rim contracts to its original size. As a result it becomes fixed on the wooden wheel.

Iron expands on heating and contracts on cooling, this physical change is associated with this activity.