Here in this post we provided NCERT Exemplar Problem Class 6 Science Changes Around Us Chapter 6 Full Explanation by group of Science Experts. We have provided here each questions answer with reason behind the answer.
Hope it help you!
Changes Around Us Class 6 Exemplar full Explanation:
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Multiple Choice Questions
1) Pick the change that can be reversed from the following
a) Cuttingof trees
b) Melting of ghee
c) Burning of candle
d) Blooming of flower
Answer:-b) melting of ghee
Melting of ghee is reversible. It can form solid ghee on cooling.
2) Which of the following change cannot be reversed?
a) Hardening of cement
b) Freezing of ice cream
c) Opening a door
d) Melting of chocolate
Answer:-a) hardening of cement
Hardening of cement is irreversible change. Cement Harden cannot change as before.
3) An iron ring is heated, which of the following statement about it is incorrect?
a) The ring expands.
b) The ring almost comes to the same size on cooling.
c) The changes in this case is reversed.
d) The ring changes its shape and the change cannot be reversed.
Answer:- d) the ring changes its shape and the change cannot be reversed.
The ring changes its shape and these change is reversible .
4) While lighting a candle, paheli observed the following changes
I) Wax was melting.
II) Candle was burning.
III) Size of the candle was reducing.
IV) Melted wax was getting solidified.
Of the above, the changes that can be reversed are
a) I) and ii)
b) Ii) and iii)
c) Iii) and iv)
d) I) and iv)
Answer:- d) I) and IV)
After lighting a candle due to heat wax melts and melted wax due to condensation process getscoveredinto solid again.
5) Salt can be separated from its solution ( salt dissolved in water ) because
a) Mixing of salt in water is a change that can be reversed by heating and melting of salt.
b) Mixing of salt in water is a change that cannot be reversed.
c) Mixing of salt in water is a permanent change.
d) Mixing of salt in water is a change that can be reversed by evaporation.
Answer:- d) mixing of salt in water is a change that can be reversed by evaporation.
By the process of Evaporation Water formed the vapour and salt remains as it is.
6) Rolling of chapati and baking of chapati are the changes that
a) Can be reversed
b) Cannot be reversed
c) Can be reversed and cannot be reversed, respectively.
d) Cannot be reversed and can be reversed, respectively.
Answer:- c) can be reversed and cannot be reversed, respectively.
Rolling of chapati can be reversed and baked chapati cannot be reversed.
7) Iron rim is made slightly smaller than the wooden wheel. The rim is usually heated before fixing into the wooden wheel. Because on heating the iron rim
a) expands and fits onto the wooden wheel.
b) Contracts and fits onto the wooden wheel.
c) No change in the size takes place.
d) Expands first, then on cooling contracts and fits onto the wooden wheel.
Answer:- d) Expands first , then on cooling contracts and fits onto the wooden wheel.
Iron is metal on heating expands and slightly becomes larger in size . On cooling it contracts and becomes smaller in size thus fits onto the wooden wheel.
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Very Short Answer Questions
(8) Look at fig. 6.1 which shows three situation (a) a burning candle (b) an extinguished candle (c) melting wax.
Which of these shows a reversible change and why ?
Answer:- As per the given three situations, figure (c ) which is melting of wax is a reversible change.
Because melted wax can be reversed into solid again on cooling. Thus the change is reversible.
But in other given figures, i.e a burning candle and an extinguished candle the changes are not reversible, these are irreversible changes.
9) A piece of iron is heated till it becomes red- hot . It then becomes soft and is beaten to a desired shape. What kind of changes are observed in this process – reversible or irreversible ?
Answer:-
In this question, the given change is reversible.
Because, a piece of iron which is can be heated, melted and moulded into any shape after it becomes soft. This change can be reversed. Thus the change is reversible.
10) Paheli had bought a new bottle of pickle from the market. She tried to open the metal cap to taste it but could not do so. She then took a bowl of hot water and immersed the upper end of the bottle in it for five minutes. She could easily open the bottle now. Can you give the reason for this?
Answer:- When paheli kept the upper end of pickle bottle into the hot water , the metal cap or upper end of bottle got heated. On heating it expands and becomes slightly larger in size. Thus due to expansion of the metal cap of bottle, paheli could open the upper end of bottle after heating in five minutes.
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Short Answer Questions
11) Can we reverse the following changes? If yes, suggest the name of the method.
a) Water into water vapour.
Answer:- yes, these is reversible change.
The method is condensation.
b) Water vapour into water.
Answer:- yes, these is a reversible change.
The method is Evaporation.
c) Ice into water.
Answer:- yes, the change is reversible.
The method is freezing.
d) Curd into milk.
Answer:- No. These is not reversible change. It is irreversible.
12) Which of the following changes cannot be reversed?
a) Blowing of a balloon
Answer:- it is reversible change.
b) Folding a paper to make a toy aeroplane
Answer:- it is reversible change.
c) Rolling a ball of dough to make roti.
Answer:- it is reversible change.
d) Baking cake in an oven
Answer:- It is not reversible. It is irreversible.
e) Drying a wet cloth
Answer:- it is reversible change.
f) Making biogas from cow dung
Answer:- it is not reversible.
g) Burning of a candle
Answer:- it is not reversible.
13) Boojho’s sister broke a white dove, a symbol of peace, made of plaster of paris (POP). Boojho tried to reconstruct the toy by making a powder of the broken pieces and then making a paste by mixing water. Will he be successful in his effort? Justify your answer.
Answer:- No, boojho will not be successful in his efforts. Because the toy which was made up of plaster of Paris is a irreversible change. These change cannot be reversed. Therefore will not be able to make the toy again of used Plaster of paris.
14) Tearing of paper is said to be a change that cannot be reversed. What about paper recycling?
Answer:-
Tearing of a paper is a irreversible change as we don’t get the paper after tearing. But if we think about the recycling of the paper then we will get the paper the quality will not be same as before. The colour and texture of the paper will be different.
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Long Answer Questions
15) Give one example in each case.
a) Change which occurs on heating but can be reversed.
Answer:- heating of an iron piece.
b) Change which occurs on heating but cannot be reversed.
Answer:- baking of chapati, roti or rice.
c) Change which occurs on cooling but can be reversed.
Answer:- formation of ice from water on cooling.
d) Change which occurs on mixing two substances, but can be reversed.
Answer:- formation of salt solution. Salt in water.
e) Change which occurs on mixing two substances, but cannot be reversed.
Answer:- mixing of cement and water.
16) A Potter working on his wheel shaped a lump of clay into a pot. He then baked the pot in an oven. Do these two acts lead to the same kind of changes or different? Give your opinion and justify your answer.
Answer:-
No. These two act don’t leadsto the same kind of change.
In the first condition, when the Potter shaped a lump of clay into a pot, that change can be reversible . The pot made up of lump of clay can be reversed and form again lump of clay.
But in second condition, when the pot is baked in oven , then these change is irreversible. As the baked pot cannot be reversed into lump of clay.
17) Conversion of ice into water and water into ice is an example of change that can be reversed. Give four more examples where you can say that changes can be reversed.
Answer:-
Melting of wax
Folding of paper
Knitting of a sweater
Inflating of a tyre
18) Change of a bud into a flower is a change which cannot be reversed. Give four more such example.
Answer:-
Example of the irreversible change are as follows.
1) Milk into curd – curd cannot be changed into milk.
2) Burning of wood–burned wood cannot be reversed into wood.
3) Ripening of fruits–ripened fruit cannot be reversed into raw fruit.
4) Digestion of food–digested food cannot be reversed into food.
19) Paheli mixed flour and water and (i) made a dough, (ii) rolled the dough to make a chapati, (iii) baked the chapati on a pan (iv) dried the chapati and ground it in a grinder to make powder. Identify the changes (i) to (iv) as the changes that can be reversed or that cannot be reversed.
Answer:- changes I, iii, and iv cannot be reversed, change (ii) can be reversed.
I) Made a dough
III)Baked the chapati on a pan.
IV) dried the chapati and ground it in grinder to make powder.
These changes are irreversible changes these cannot be reversed.
And II) rolled the dough to make chapati is a reversible change.
20) It was paheli’s birthday, her brought simba was helping her to decorate the house for he birthday party and their parents were also busy making other arrangements. Following were the activities going on at paheli’s home.
Identify the Activities at paheli’s home as those that can be reversed and those which cannot be reversed.
i) Simba blew balloons and put them on the wall.
Answer:- this process irreversible.
ii)Some of the balloons got brust.
Answer:- this process is irreversible.
iii) paheli cut colourful strips of paper and put them on the wall with the help of tape.
Answer:- this process is irreversible.
iv) she also made some flowers by origami ( paper folding) to decorate the house.
Answer:- This process irreversible.
v) her father made dough balls.
Answer:- this process is reversible.
vi) mother rolled the dough balls to make puries.
Answer:- this process is reversible.
vii) mother heated oil in a pan.
Answer:- this process irreversible.
viii) father fried the puriesin hot oil.
Answer:- this process is irreversible.