5. Changes Around Us: Physical and Chemical
Complete solutions to Let Us Enhance Our Learning. CBSE / NCERT Class 7 Science (Curiosity).
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Let Us Enhance Our Learning
Answer: (c) (i) and (iii)
In a physical change, the state may change, but no new substance is formed.
Can be reversed: (i) stitching cloth to a shirt, (ii) twisting of straight string, (iv) dissolving sugar in water, (v) drawing water from a well, (viii) rolling up a mat.
Cannot be reversed: (iii) making idlis from a batter, (vi) ripening of fruits, (ix) grinding wheat grains to flour, (x) forming of soil from rocks.
Not sure: (vii) boiling water in an open pan.
Why not sure about (vii)? Water vapour can be cooled to get water back. But in an open pan, the vapour escapes into the air, so it is difficult to collect all of it. Some students may also be unsure about (i), because the cloth may get small holes after removing the stitches.
(i) Melting of wax is necessary for burning a candle. — True
(ii) Collecting water vapour by condensing involves a chemical change. — False
Correct: Collecting water vapour by condensing involves a physical change.
(iii) The process of converting leaves into compost is a chemical change. — True
(iv) Mixing baking soda with lemon juice is a chemical change. — True
(i) The brown deposits on the handle of the cycle are due to rusting, and this is a chemical change.
(ii) Folding a handkerchief is a physical change and can be reversed.
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.
(iv) Magnesium, when burnt in air, produces a substance called magnesium oxide. The substance formed is basic in nature. Burning of magnesium is a chemical change.
Both are physical changes.
Only the state of water changes. Ice, water and steam are the same substance. No new substance is formed. Ice can be melted back to water, and steam can be cooled back to water.
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Curdling of milk is a chemical change.
Curd is a new substance. Its taste, smell and texture are different from milk. Curd cannot be changed back into milk.
It is both a physical and a chemical change.
- Physical: Wind, rain, heating and cooling break rocks into smaller pieces. No new substance is formed.
- Chemical: Rainwater, air and acids react with the minerals in rocks and form new substances.
This process is called weathering. It cannot be reversed.
- Chopping vegetables, peeling potatoes, cutting fruits — physical changes
- Collecting seeds, fruits and peels into a clay pot — physical change
- Decomposition of peels into compost by bacteria and fungi — chemical change
- Germination of seeds and growth of plants into flowers — chemical change
Suitable title: ‘From Kitchen Waste to Colourful Flowers’
| Area | Changes |
|---|---|
| A – Physical | Tearing of paper, Melting of ice, Folding of clothes |
| B – Chemical | Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar |
| C – Both | Process of burning a candle |
In a burning candle, wax melts (physical change) and the wax vapour burns to form new substances (chemical change).
Lime water turned milky in (a) and (d).
- (a) Vinegar and baking soda, and (d) lemon juice and baking soda: an acid reacts with baking soda. Carbon dioxide gas is formed. This gas turns lime water milky.
- (b) Lemon juice and vinegar are both acids. (c) Vinegar and common salt do not react. No carbon dioxide is formed, so lime water does not turn milky.
Exploratory Projects
Exploratory Projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.