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Question
Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these?
(i) Stitching cloth to a shirt
(ii) Twisting of straight string
(iii) Making idlis from a batter
(iv) Dissolving sugar in water
(v) Drawing water from a well
(vi) Ripening of fruits
(vii) Boiling water in an open pan
(viii) Rolling up a mat
(ix) Grinding wheat grains to flour
(x) Forming of soil from rocks
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Answer
| Change | Reversible? | Reason |
|---|---|---|
| (i) Stitching cloth to a shirt | Yes | The shirt can be unstitched and the cloth recovered. |
| (ii) Twisting of straight string | Yes | The string can be untwisted to its straight form. |
| (iii) Making idlis from a batter | No | Cooking is a chemical change — idlis cannot become batter again. |
| (iv) Dissolving sugar in water | Yes | Evaporate the water; sugar can be recovered. |
| (v) Drawing water from a well | Yes | The water can be poured back into the well. |
| (vi) Ripening of fruits | No | Chemical change — ripe fruit cannot become unripe. |
| (vii) Boiling water in an open pan | No (in practice) | The steam escapes into the air; we cannot recover that water. (If we trapped and condensed it, it would be reversible.) |
| (viii) Rolling up a mat | Yes | The mat can be unrolled. |
| (ix) Grinding wheat grains to flour | No | Whole grains cannot be reassembled from flour. |
| (x) Forming of soil from rocks | No | Weathering takes thousands of years and is irreversible at any human timescale. |
Cases where you might be unsure:
- (iv) Dissolving sugar — Looks irreversible (you can't see the sugar), but evaporation recovers it.
- (vii) Boiling water — Reversible in principle (condense the steam) but not in an open pan in practice.
