NCERT 6TH CLASS SCIENCE CHAPTER CHANGES AROUND US
Change is a permanent part of our lives. A change could alter physical properties or chemical properties or both of the concerned matter. These changes may be observe by us at school, at home, in the playground in the garden or any other place. A change can bring about different kinds of effect in the things around us. Some of the effect brought about are permanent in nature, and hence, cannot be reversed . However, some effects are about in position, shape, size or state of the things. These effects are temporary in nature, and hence can be reversed.
REVERSIBLE AND IRREVERSIBLE CHANGES
A change which can be reversed by the conditions is called a reversible change.
Eg. Melting of ice, dissolving of salt in water
A change that cannot be reversed by reversing the condition is called an irreversible change.
Eg. Burning of paper, curdling of milk
PERIODIC AND NON-PERIODIC CHANGES
A change which is repeated after regular intervals of time is called a periodic change. Swinging of a pendulum, rising and setting of the sun at a given place, and change of seasons are some examples of periodic changes. A changes which does not occur after regular intervals of time is called a non-periodic change. Most of the changes happening around us are non-periodic changes, occurrence of floods and cyclones are some of them.
NATURAL AND MAN-MADE CHANGES
A change that occurs naturally on its own is called a natural change. Eg. changing of season, occurence of tides, eclipses. A man-made change is the one that occurs because of human activities.
e.g. Changing the direction of flow of river by building a dam.
SLOW AND FAST CHANGES
Some changes take place at a slow rate, such changes are called slow changes.
Eg. Growh of plant, rusting of iron Some change take place in a fraction of second, such changes are called fast changes
Eg. Occurance of lightning during thunderstrom, earthquakes.
CHANGES CAUSED BY HEATING
(1) Some objects simply get hot, but no other changes take place.
When you drink hot milk or coffee from a cup. you may feel that the cup however no change in the state of the cup take place.
(2) Some object get hot and expand.
Eg. Metal key on heating got hot as well as increased in size.
(3) Some object get hot and expand.
Eg. The candle is lighted it starts burning. Its shape and size also changed.
(4) Some of the change their state.
Eg. Ice is heated it change into water and on further heating changes into steam. All these changes can be reverse.
CHANGES CAUSED BY DISSOLVING
Substances, such as common salt, sugar, copper sulphate crystals etc. are soluble water. During the process of dissolving a change is brought in the soluble substance. Equal amounts of all soluble substances do not dissolve in any fixed amount of water. However with the rise in temperature of water, more amount of substance dissolves in it.
PHYSICAL AND CHEMICAL CHANGES
Physical Changes :
Definition: A change which alters some specific physical property of matter, like its state, texture, magnetic or electrical conditions or its colour, without causing any change in the composition of its molecules, is called physical change. provided it can be reversed , if the cause producing the change is removed.
1. Physical chemical changes are a result of
absorption of :
(A) heat energy only
(B) light energy only
(C) sound energy only
(D) some kind of energy
2. A chemical change involves : (A) change of state only
(B) change of colour only
(C) change of state as well as composition
3. A physical change is a/an :
(A) temporary change
(B) permanent change
(C) irreversible change
(D) None of these
4. An example of a physical change is:
(A) magnetisation of iron
(B) burning of wood
(C) photosynthesis by plants
(D) digestion of food
5. An example of a chemical change is :
(A) exposure of a photographic plate
(B) tearing of paper
(C) melting of sulphur
(D) formation of dew
6. An example of an undesirable change is :
(A) melting of snow on mountains
(B) evaporation of water
(D) flowering of plants
7. An example of a periodic change is :
(A) swinging of a clock pendulum
(B) landslides during rains
(C) rusting of iron
8. During physical and chemical changes :
(A) energy is always absorbed
(B) energy is always released
(C) no energy is absorbed or released
(D) energy is either absorbed or released
9. A change is an alteration in the physical and chemical properties of matter due to the effect of :
(C) some kind of energy
10. Physical changes are :
(C) temporary and irreversible
(D) temparary and can be reversed
1. What is change ? Explain giving example.
2. (i) Define a chemical change.
(ii) Give four examples of chemical change
3. Classify the following changes as fast or slow changes :
(i) Germination of seed
(ii) Milk changing to curd
(iii) Burning of cooking gas in a gas stove
4. Define and explain periodic changes.
1. D 2. C 3. A 4. A
5. A 6. C 7. A 8. D
9. C 10. D 11.C 12. A
MULTIPLE CHOICE QUESTIONS
1. Pick the change that can be reversed from the following
(a) Cutting of trees (b) Melting of ghee
(c) Burning of candle (d) Blooming of flower
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
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 change in this case is reversed.
(d) The ring changes its shape and the change cannot be reversed.
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)
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.
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.
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.
VERY SHORT ANSWER QUESTIONS
8. Look at Fig. 2.1which shows three situation (a) a burning candle (b) an extinguished candle (c) melting wax.
Which of these shows a reversible change and why?
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?
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?
SHORT ANSWER QUESTIONS
11. Can we reverse the following changes? If yes, suggest the name of the method.
(a) Water into water vapour
(b) Water vapour into water.“CHANGES AROUND US 35
(c) Ice into water.
(d) Curd into milk.
12. Which of the following changes cannot be reversed?
(a) Blowing of a balloon
(b) Folding a paper to make a toy aeroplane
(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
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.
14. Tearing of paper is said to be a change that cannot be reversed. What about paper recycling?
LONG ANSWER QUESTIONS
15. 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 on cooling but can be reversed.
(d) Change which occurs on mixing two substances, but can be reversed.
(e) Change which occurs on mixing two substances, but cannot be reversed.
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.
17. 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 that the changes can be reversed.
18. Change of a bud into a flower is a change which cannot be reversed. Give four more such example.
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.
20. 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 by origami (paper folding)to decorate the house.
(v) Her father made dough balls.
(vi) Mother rolled the dough balls to make puries.
(vii) Mother heated oil in a pan.
(viii) Father fried the puries in hot oil.
Identify the activities at Paheli’s home as those that can be reversed and those which cannot be reversed.
1. b 2. a 3. d 4. d 5. d 6. c 7. d