CHANGES AROUND US


Changes Around Us

1. Many changes are taking place around us on their own.

2. We can bring about a change in a substance by doing one or more of the following.
     
  • Heating.
  •  
  • Applying force
  • Mixing it with something else.
 
3. Changes caused by heating:
When an object is heated, it gets affected in one or many possible ways.
  • Some objects get hot but do not change in any other way.
  • Some objects get hot and also expand in size.
  • Some objects get hot and begin to burn.
  • Some objects get hot and change their state.
 
4. Changes by applying pressure : When we apply force to an object,
  • We can change its shape and size.
  • Air can be compressed.
  • Metals can be hammered into thin sheets.
  • Elastic can be stretched.
  • Cotton can be spun into thin threads.
 
5. Changes by mixing a substance with other: We can bring about a change in a substance by mixing it with another, e.g.,Making solution by mixing water soluble substances in water.
 
6. Expansion: Metals expand on heating and contract on cooling.

7. Chemical changes:These are the changes in which chemical properties of a substance changes, and a new substance is formed. For example: cooking of food. 

8. Physical changes:
These are the changes in which only physical property of a substance changes and no new substance is formed.
 
9. Characteristic of physical changes :
  • No new substances are formed.
  • Products are identical to the reactant.
  • These changes are reversible.
 10. Characteristics of chemical changes :
  • Properties of products are different from the properties of reactants.
  • Most of the chemical changes are irreversible.
  • These changes always result in energy changes.
 
11. Reversible changes: These are the changes that can be reversed. For example stretching of rubber.

 12. Irreversible changes:These are the changes which cannot be brought back to its stretching of rubber.

 13. Contraction: A process in which an object becomes smaller is called contraction.

 14. Evaporation: A process in which liquid changes into vapor on heating is called evaporation.

 15. Melting: A process in which solid changes into liquid on heating is called melting or fusion.

 16. Melting point: A constant temperature at which solid starts melting. This temperature is called the melting point of that solid.

 17. Freezing: A process in which liquid changes into solid is called freezing.

 18. Force: A push or pull acting on a body which tends to change its state of rest or motion is called a force.

 19. Natural Changes:The changes which occur in nature on their own are called natural changes. For example, change of day and night, change of season.

20. Slow changes:
The changes which take longer time to occur are called slow changes. For example, rusting of iron, tooth decay.
 
Q.1. to walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?

Ans. Yes. This can be reversed by unfolding the folded clothes.

Q.2. You accidentally dropped your favorite toy and broke it. This is a change you did not want. Can this change be reversed?

Ans. No. This changecannot be reversed; Breaking of a toy is an irreversible change.

Q.3. 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.ChangeCan 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 
 
Ans.
S.No.ChangeCan be reversed (Yes/No)
 
1.
 
The sawing of a piece of woodNo
2.
 
The melting of ice candyYes
3.
 
Dissolving sugar in waterYes
4.
 
The cooking of foodNo
5.
 
The ripening of a mangoNo
6.
 
Souring of milkNo
 
Q.4. A drawing sheet changes when you draw a picture on it. Can you reverse this change?

Ans. No. We cannot reverse this change. Because we cannot get fresh drawing sheet once a picture is drawn on it. But if the pencil is used to draw the picture we can reverse the change.

Q.5. Give examples to explain the difference between changes that can or cannot be reversed.

Ans. One way we can group changes is to see if they can be reversed or not.

1. Changes that can be reversed : In such changes we can return back to original position if the cause of change is withdrawn for example :
  • Heating of milk.
  • Drying of clothes.
  • Knitting a sweater.
  • Stretching a rubber band.
  • Melting of ice.
2.  Changes that cannot be reversed : These are the changes in which we cannot be back to original position, even if we withdraw the cause of change, for example :
  • boiling of egg.
  • making curd from milk.
  • making flour from grain.
  • flowering of a bud.
  • producing biogas from cow dung.

Q.6. A thick coating of a paste of Plaster of Pairs (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilized. Can the change in POP be reversed?

Ans. No. Because it is an irreversible (i.e., chemical change) change.

Q.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?


Ans.
No. Because these are irreversible chemical changes.

 

ADDITIONAL IMPORTANT QUESTIONS


Q.1. Can you obtain wood from sawdust?

Ans. No, because it is an irreversible change.

Q.2. Can we say that ironing of a cloth is a reversible change? Give reasons.

Ans. Ironing removes the wrinkles of the clothes, which can be come back in the same condition. Hence, ironing of a cloth is a reversible change.

Q.3. Can deforestation be considered as a reversible change?

Ans. No, deforestation can’t consider as a reversible change because no same tree can be planted after, falling down or cutting.

Q.4. Is printing a reversible or an irreversible change?

Ans. Printing is an irreversible change. Because we cannot separate and collect the printing ink after printing.

Q.5. Give two examples each of reversible and irreversible changes.

Ans. Reversible changes: Melting of wax and stretching of rubber band.
         Irreversible changes: Burning of paper and growth of plants.

Q.6. Formation of clouds is a physical change. Explain.

Ans. Formation of clouds is a physical change as it phases transformation cycle of natural water from liquid to gas and then, gas to liquid. Hence, the property of water never changes in clouds form.

Q.7. Explosion of a cracker is a chemical change. Explain.

Ans. Explosion of crackers is a chemical change because the explosive reactants are transformed into gaseous products along with heat and light and thus cannot be reversed. Hence, it is a chemical change.

Q.8. Give some examples of physical and chemical changes.

Ans.

Examples of physical changes

Tearing of sheet of paper into pieces, melting of ice, change of water into steam, breaking of glass tumbler, growing of electric bulb, dissolution of sugar or salt in water.

Examples of chemical changes

Burning of paper, wood, candle etc., and formation of curd from milk, cooking of food, rusting of iron and mixing of vinegar with baking soda, electrolysis of water.

Q.9. What is charring of sugar? Is it a chemical change?

Ans.
When sugar is continuously heated in the porcelain dish, it becomes foggy due to water vapor. A black powdery substance is left behind, which is charcoal. This process is also called charring of sugar and this is a chemical change.

 
Q.10. Classify the following changes in as many ways as you can:

  1. Breaking of a brick with a hammer.
  2. Beating of heart.
  3. Germination of a seed.
  4. Burning of an incense-stick (agarbatti).
  5. Occurrence of solar eclipse.
Ans. (i) Physical change, irreversible change.
         (ii) Periodic change, irreversible change.
         (iii) Irreversible change.
         (iv) Irreversible, chemical change.
         (v) Irreversible change.

Q.11. Most physical changes are reversible. Give reasons with two examples.

Ans . (i) Melting of ice : During this change the water changes from its solid form to liquid form. It can be solidified again. The water remains water in both cases hence reversible.
  1. Glowing of an electric bulb: During this change, electricity is passed through the filament which becomes white hot and glows, but when switch is off, the filament returns to its original shape and condition, hence totally reversible.
Q.12. Give some examples of changes which take place on their own.

Ans. Many changes are taking place around us on their own. For example :
  1. Ripening of crops in the fields.
  2. Leaves fall from tree.
  3. Flowers bloom and then wither away.
  4. Growing of hair and nail.
Q.13. State whether burning of a piece of paper is a reversible or an irreversible change.

Ans. When we burn a piece of paper, it changes into ash and smoke. We cannot combine the ash and smoke to form the original piece of paper. So the burning of a piece of paper is an irreversible change.

Q.14. Classify the following as reversible or irreversible changes.

  1. Growth of a plant
  2. Ploughing a field
  3. Melting of wax
  4. Falling of rain
  5. Pulling of rubber string
  6. Breaking of a glass rod
  7. Cooking of food.
Ans. Reversible changes: (ii), (iii), (v)
         Irreversible changes: (i), (iv), (vi), (vii)

Q.15. What are reversible and irreversible changes?

Ans. (i) Reversible changes: The changes which can be brought back to its original form are known as reversible changes. For example, Melting of wax and stretching of rubber band. (Fig. 6.2)
         (ii) Irreversible changes:  the changes in which the matter cannot be brought back to its original state are known as irreversible changes. For example, Burning of paper changes it into ash and smoke.        Paper cannot be obtained back from ash and smoke.

 
Q. 16. Give one example each for physical and chemical change.

Ans. Physical change (Melting of ice)
 
Chemical change (Burning of paper)
 
Q.17. Define physical and chemical changes. Give examples.

Ans. (i) Physical change: Physical change is a temporary change in which chemical composition of the substance do not change and no new substance is formed. During a physical change only the physical properties of a substance changes. It is a reversible change. For example, Melting of ice, during this change the water changes from its solid form to liquid form. It can be solidified again. The water remains water in both the cases.

         (ii) Chemical change:
A chemical change is a permanent change in which not only the physical properties but chemical properties also change. It is an irreversible change. For example, Formation of curd from milk, rusting of iron etc.

Q.18. Give some examples from daily life where expansion of metal by heating is used. Explain.

Ans. Fixing of a metal rim on a wooden wheel and fixing of wooden handles in iron blade in agricultural tools are such examples.
The iron blade of these tools has a ring in which the wooden handle is fixed. Normally, the ring is slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it becomes slightly larger in size (expands). Now, the handle easily fits into the ring. When the ring cools down it contracts and fits tightly on to the handle.
 
Such a change is also used for fixing the metal rim on a wooden wheel of a cart as shown in Figure. Again the metal rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits onto the wheel. Cold water is then poured over the rim, which contracts and fits tightly onto the wheel.
 
Q.19. In the following boxes, add a reverse arrow wherever the change is reversible.
 
 
 
OBJECTIVE TYPE QUESTIONS

Q.1. Match the following items given in Column A with that in Column B.
Column AColumn B
 
a. Irreversible

 

b. Reversible change

 

c. Sawing of piece of wood

 

d. Ripening of mango

 

f. Melting of wax

 

g. Formation of a sugar solution
i. Needs mixing of two substances

 

ii. Needs heating

 

iii.Chemical range

 

iv. A new substance is formed

 

v. Physical change

 

vi. No new substance is formed
 
Q.2. Fill in the blanks with appropriate words:
  1. Changes that lead to the formation of new substances are called ………… .
  2. If the removal of the cause of change leads to regain the original situation, then the change is known as ……….. .
  3. Burning of wood in a chulha is an ………… change.
  4. Melting of candle wax is a ………. Change.
  5. Heat is a form of ……….. .
  6. On cooling, liquid ………. Into solid.
  7. Conversion of a liquid to vapour by heating is called ………. .
  8. The changes which are controlled by man are called ………… .
  9. ………… is a reversible change.
  10. No new substance is formed in ………….
Q.3. State whether the statements given below are True or False :
  1. Cooking of rice is a physical change.
  2. Breaking of a china (porcelain) dish is a chemical change.
  3. Rusting of iron is an irreversible change.
  4. Eruption of a volcano is a reversible change.
  5. Formation of clouds is a irreversible change.
  6. Conversion of a solid to liquid by heating is called fusion.
  7. Change of seasons is a natural change.
  8. Pressure is equal to the force per unit second.
Q.4. Choose the correct option in the following questions:

1. Which of the following changes can be reversed?

(a)Raw egg to boiled egg                           (b)Wet clothes to dry clothes
(c)Bud to flower                                               (d)Cow dung to biogas

2. Which of the following changes cannot be reversed?

(a)Milk to panner                                             (b)Cold milk to hot milk
(c)Yarn to knitted sweater                              (d)Wet clothes to dry clothes

3. Metal rim is fixed to the wooden cart wheel by

(a)heating metal rim                                       (b)heating wooden wheel
(c)cooling metal rim                                        (d)cooling wooden wheel


4. On heating metal rim


(a)expands                                                       (b)contracts
(c)depends how much it is heated              (d)depends as the type of metal

5. Which one of the step while burning a candle is not reversible?

(a)Melting of solid wax                                   (b)Liquid wax changes into vapour
(c)Wax vapour burn into flame                     (d)All of these

6. Which is a way to make a change happen?
  1. Heating a substance
  2. Cooling a substance
  3. Mixing a substance with another substance
  4. All of these
7. Rusting of iron is an example of

    (a)slow change                                                (b)fast change
    (c)reversible change                                       (d)physical change

8. Conversion of a solid to liquid by heating is called

     (a)evaporation                                                 (b)melting
     (c)condensation                                             (d)freezing

9. Which of the following is not a man-made change?

(a)Change of day and night                        (b)Burning of fuels
(c)Drying of clothes                                       (d)Tearing of paper

10. Which of the following is a chemical change?

(a)Evaporation of water                                (b)Burning of fuels
(c)Moving of a wheel                                    (d)Breaking of a brick
 
 
 
ANSWERES TO OBJECTIVE TYPE QUESTIONS
Q.1. Match the items in Column A with Column B.
Column AColumn B
 
a. Irreversible

 

b. Reversible change

 

c. Sawing of piece of wood

 

d. Ripening of mango

 

f. Melting of wax

 

g. Formation of a sugar solution
(iv) A new substance is formed
 
  (vi) No new substance is formed
 
   (v) Physical change
 
   (iii) Chemical range
 
    (ii) Needs heating
 

(i) Needs mixing of two substances
 
 
Q.2. Fill in the blanks :
        (i)chemical                               (ii)reversible change               (iii)irreversible
        (iv)reversible                            (v)energy                                (vi)freezes
        (vii)evaporation                        (viii)man made changes
        (ix)Glowing of an electric bulb      (x)physical changes
Q.3. True/False :
        (i)False                (ii)False                  (iii)True                 (iv)False
        (v)False               (vi)True                  (vii)True                (viii)False
Q.4. Choose the correct option :
(i)      (b) All other changes are irreversible.
(ii)     (a) Paneer is a new substance and cannot be converted into milk.
(iii)    (a) On heating metal ring expands and fits around the wheel.
(iv)    (a) All the metals expand on heating.
(v)     (c) Burning of vapours is an irreversible process in which combustion of vapours occur.
(vi)       (d) A change can be make to happen by any of these means.
(vii)      (a) Rusting of iron is a slow change.
(viii)     (a) Solid change into liquid by heating is called melting.
(ix)       (a) Change of day and night is a natural change.
(x)        (b) burning of a fuel is an example of chemical change.