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Changes around us Notes

Answer the following questions:

1. Classify the following as slow or fast changes:
(i) ripening of fruits
(ii) curdling of milk by adding lemon juice
(iii) change of season
(iv) bursting of a cracker
(v) the spinning of a top
(vi) rusting of a bicycle rim
vii) the burning of paper
(viii) Weathering of a rock

Answers:

1. Classify the following as slow or fast changes:
(i) ripening of fruits — slow change
(ii) curdling of milk by adding lemon juice— fast change
(iii) change of season — slow change
(iv) the bursting of a cracker — fast change
(v) the spinning of a top — fast change
(vi) rusting of a bicycle rim — slow change
(vii) the burning of paper — fast change
(viii) weathering of a rock — slow change

2. Give two examples each of:
(i) slow and fast changes
(ii) desirable and undesirable changes
(iii) periodic and non-periodic changes
(iv) reversible and irreversible changes
(v) physical and chemical changes

Ans: 
(i) the spinning of a top — fast change
the burning of paper — fast change
rusting of a bicycle rim — slow change
weathering of a rock — slow change
(ii) Desirable changes — formation of curd from milk formation of manure from cow-dung and dead plants.
Undesirable changes — spoiling of foodstuff, burning of a building
(iii) Periodic changes — the change of seasons the phases of the moon
Non-periodic changes — the growth of a plant, falling of leaves, rusting of an iron nail etc.
(iv) Reversible changes — The change from ice to water and water to ice and folding of leaves of touching me not plant.
Irreversible changes — Milk can be changed into curd but curd cannot be changed back into the milk. The growth of a child into an adult.
(v) Physical changes — Sublimation, Stretching of a rubber band.
Chemical changes — Burning of fuels, Rusting of iron.

 

3. State two changes which are desirable as well as undesirable.

Ans: (i) Change of seasons
(ii) Clothes getting worn out
4. What are periodic and non-periodic changes?

Ans: The changes that do not repeat themselves at regular intervals of time are known as non-periodic changes.
The changes that occur again and again after a fixed interval of time are known as periodic changes.
5. Classify the following changes as periodic or non-periodic:
(i) high and low tides in the sea
(ii) phases of the moon
(iii) the eruption of volcanoes
(iv) the falling of tree leaves

Ans: Classify the following changes as periodic or non-periodic:
(i) high and low tides in the sea — periodic change
(ii) phases of the moon — periodic change
(iii) the eruption of volcanoes — non-periodic change
(iv) the falling of tree leaves — non-periodic change
6. What are reversible and irreversible changes?

Ans: A change which can be reversed to the original form of a substance is called a reversible change. A change which cannot be reversed to form the original substance is known as an irreversible change.
7. Classify the following changes as reversible or irreversible:
(i) the breaking of a glass tumbler
(ii) cooking of rice
(iii) melting of Wax
iv) germination of seeds
(v) ploughing of a field
vi) stretching of a rubber band

Ans: 

(i) the breaking of a glass tumbler — irreversible change
(ii) cooking of rice — irreversible change
(iii) melting of wax — reversible change
(iv) germination of seeds — irreversible change
(v) ploughing of a field — irreversible change
(vi) stretching of a rubber band — reversible change
8. What are the physical and chemical changes?

Ans:

A physical change is a temporary change in which the physical appearance of a substance may change but its chemical composition remains unchanged. A chemical change is a permanent change in which the composition of the substance changes giving rise to one or more new substances with different properties.
9. Classify the following cases as physical or chemical:
(i) cooking of food
(ii) curdling of milk
(iii) formation of clouds
(iv) the burning of a candle
(v) drying of wet hair
(vi) ironing of clothes
(vii) the souring of milk
(viii) rusting of iron
(ix) wool being knitted into a sweater
(x) coal burning in air
Ans:

(i) cooking of food — chemical
(ii) curdling of milk — chemical
(iii) formation of clouds — physical change
(iv) the burning of a candle — physical change
(v) drying of wet hair — physical change
(vi) ironing of clothes — physical change
(vii) the souring of milk — a chemical change
(viii) rusting of iron — a chemical change
(ix) wool being knitted into a sweater — physical change
(x) coal burning in the air — a chemical change

Q- State whether the following statements are ‘True’ or ‘False’
(i) Curdling of milk is a desirable change.
(ii) Cooking of food is a physical change.
(iii) The occurrence of an earthquake is a periodic change.
(iv) The bursting of crackers is a fast as well as a chemical change.
(v) Dissolution of quicklime in water is an exothermic change.
(vi) The burning of paper is a reversible change.

Ans:

(i) True, (ii) False, (iii) False, (iv) True, (v) True, (vi) False.

C. Pick the correct answer
(i) Which of the following is a chemical change?
(a) the breaking of a brick with a hammer
(b) melting of sulphur
(c) curdling of milk
(d) the beating of the heart
(ii) Which of the following is not a periodic change?
(a) The occurrence of day and night
(b) High and low tides in the sea
(c) Change of seasons
(d) landslides
(iii) Ripening of fruits cannot be called as a/an
(a) slow change
(b) irreversible change
(c) desirable change
(d) physical change
(iv) Which of the following is a physical change?
(a) Burning of paper
(b) Formation of clouds
(c) Cooking of food
(d) Rusting of iron nails

Ans:

(i) (c), (ii) (d), (iii) (d), (iv) (b)

 


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