Solution of Chapter 6. Changes Around Us (NCERT - Science Exemplar Book)

Pick the change that can be reversed from the following

Which of the following change cannot be reversed?

An iron ring is heated. Which of the following statement about it is incorrect?

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

Salt can be separated from its solution (salt dissolved in water), because

Rolling of chapatti and baking of chapatti are the changes that

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

Look at Fig. 6.1 which shows three situations (a) a burning candle (b) an extinguished candle (c) melting the wax.

Which of these shows a reversible change and why?

A piece of iron is heated till it becomes red-hot. It then becomes soft and is beaten to the desired shape. What kind of changes are observed in this process-reversible or irreversible?

Paheli had bought a new bottle of from the market. She tried to open the metal cap to taste it but could not do so. She then took a bowl 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?

Can we reverse the following changes? If yes, suggest the name of the method.

(a) Water into water vapour

(b) Water vapour into water.

(c) Ice into the water.

(d) Curd into milk.

Which of the following changes cannot be reversed?

(a) Blowing of a balloon

(b) Folding a paper to make a toy airplane

(c) Rolling a ball of dough to make roti

(d) Baking cake in an oven

(e) Drying a wet cloth

(f) Making biogas from cow dung

(g) Burning of a candle

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.

Tearing of paper is said to change that cannot be reversed.

What about paper recycling?

Give one example in each case

(a) Change which occurs on heating but can be reversed.

(b) Change which occurs on heating but cannot be reversed.

(c) Change which occurs by mixing two substances, but can be reversed.

(d) Change which occurs by mixing two substances, but cannot be reversed.

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.

Conversion of ice into water and water into ice is an example of change which can be reversed. Give four more examples where you can say four more examples where you can say that the changes can be reversed.

Change of a bud into a flower is a change which cannot be reversed. Give four more such example.

Paheli mixed flour and water and (i) made a dough, (ii) rolled the dough to make a chapatti, (iii) baked the chapatti on a pan, (iv) dried the chapatti 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.

It was Paheli ’s birthday, her brother Simba was helping her to decorate the house for the birthday party and their parents were also busy making other arrangements. Following were the activities going on at Paheli’s home :

(i) Simba blew balloons and put them on the wall.

(ii) Some of the balloons got burst.

(iii) Paheli cut colourful strips of paper and put them on the wall with the help of tape.

(iv) She also made some flowers with origami (paper folding) to decorate the house.

(v) Her father made dough balls.

(vi) Mother rolled the dough balls.

(vii) Mother heated oil in a pan.

(viii) Father fried the puries in hot oil.

Identify the activitie4s at Paheli’s home as those that can be reversed and those which cannot be reversed.