NCERT Solutions for Class 7 science Chapter 5: Changes Around Us Physical and Chemical Class 7

Chapter-wiseChapter 5 Changes Around Us Physical and Chemical Class 7 Questions and Answers are useful for focused study.

Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Question Answer

Science Class 7 Chapter 5 Question Answer Changes Around Us Physical and Chemical

Changes Around Us Physical and Chemical Class 7 Question Answer (InText)

These students are describing some changes. What kinds of changes are they talking about?
Answer:

NCERT Class 7 Science Chapter 5 Question Answer Changes Around Us Physical and Chemical (Exercise)

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(ii) Twisting of straight string
Answer:
Twisting of straight string – Can be reversed: The string can be untwisted.

(iii) Making idlis from a batter
Answer:
Making idlis from a batter – Cannot be reversed: The batter cannot be returned to its original form after steaming.

(iv) Dissolving sugar in water
Answer:
Dissolving sugar in water – Can be reversed: Sugar can be recovered by evaporating water.

(v) Drawing water from a well
Answer:
Drawing water from a well – Can be reversed: Water can be returned to the well.

(vi) Ripening of fruits
Answer:
Ripening of fruits – Cannot be reversed: Once ripe, fruits cannot go back to unripe.

(vii) Boiling water in an open pan
Answer:
Boiling water in an open pan – Can be reversed: Evaporated water can be obtained back by condensation.

(viii) Rolling up a mat
Answer:
Rolling up a mat – Can be reversed: The mat can be unrolled.

(ix) Grinding wheat grains to flour
Answer:
Grinding wheat grains to flour – Cannot be reversed: Flour cannot be turned back into grains.

(x) Forming of soil from rocks
Answer:
Forming of soil from rocks – Cannot be reversed: Soil formation is a slow process and cannot form rock back.

(ii) Folding a handkerchief is a ______ change and can be ______.
Answer:
physical, reversed

(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called ______, and this is a ______
change.
Answer:
combustion, chemical

(iv) Magnesium, when burnt in air, produces a substance called ______. The substance formed is ______ in nature. Burning of magnesium is a ______ change.
Answer:
magnesium oxide, basic, chemical

Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/ chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change). His efforts are appreciated by all his family members.
Answer:
Prithvi’s Green Kitchen

Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.

Answer:

Class 7 Changes Around Us Physical and Chemical Question Answer (Activities)

Activity 5.1 Let Us Think and Reflect (Page 58)

Conclusion: We can categorise these changes in two types of changes, one in which only shape, size or state of a substance changes or other in which new substance(s) is/ are formed.

Activity 5.2: Let Us Create and Discuss (Page 59)

Changes listed in A, B and C are as follows:
A. Creating some objects with paper: When we create different objects like aeroplane, boat, bird, etc. by folding a sheet of paper, only shape and size changes. We can get back the original sheet by unfolding it again.

B. Playing with a balloon: When we loosen the grip of the inflated balloon we get the original shape of balloon back, and we can inflate it again but when we prick the inflated balloon with a pin, it gets deflated with a hole in it, so we cannot able to inflate it again.

C. Crushing a piece of chalk: When we crushed a piece of chalk to powder form, we cannot can get back the original piece of a chalk.

Observation and conclusion: In all these types of changes, material (piece of paper, balloon, chalk) remains the same but their size and shape changes only. Thus, these are examples of physical changes.

Activity 5.3: Let Us Create and Discuss (Page 60)

Observation and conclusion: When we blow air (carbon dioxide) from mouth to tumbler A containing water, we do not observe any change in the colour of water. But when we blow air into tumbler B containing lime water (calcium hydroxide), lime water turns milky and white-coloured insoluble substance (calcium carbonate) gets settle down. Thus, this is classified as chemical change as new substance is being formed.

Activity 5.4: Let Us Experiment (Page 61)

Observation and conclusion: When we add a pinch of baking soda (sodium hydrogen carbonate) to vinegar or lemon juice, bubbles of carbon dioxide gas are produced with a fizzing sound. When we pass this gas to lime water, it turns milky. Thus, in this process, new substances are formed, it is a chemical change.

Activity 5.5: Let Us Investigate (Pages 62-63)

Observation and conclusion: In figure (a), candle keeps on burning as it is not covered and supply of air (oxygen) is available. But in figure (b), as candle is covered with a glass tumbler, does not get a continuous supply of air, so, it gets extinguished. This concludes oxygen is required for combustion.

Activity 5.6: Let Us Investigate (Page 64)

Observation and conclusion: The substance catches fire only when its ignition temperature is reached. Ignition temperature is the minimum temperature at which the substance catches fire. When a lighted matchstick is brought near to the paper it catches fire immediately because the temperature of the matchstick is already higher that the ignition temperature of the paper. In another case, when sunrays are focused using a magnifying glass on a piece of paper, it gets heated up and the temperature of paper increases and it attains its ignition temperature and starts burning and emit smoke.

Requirements for combustion process to occur:

Activity 5.7: Think, Pair, and Share (Page 65)

Conclusion: Burning of a candle involves both physical and chemical change. Melting of wax, evaporation of wax to wax vapour and solidification of melted wax to solid wax are all physical changes. While burning of wax to produce new substance, carbon dioxide is a chemical change.

Activity 5.8: Let Us Think (Page 66)

Table 5.2; Can changes be reversed?

Conclusion: Changes around us can be grouped into reversible (original substance can be brought back) and irreversible (original substance cannot be brought back) changes.