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
Cannot be reversed easily. The cloth is cut and stitched into a specific shape which is difficult to reverse completely.
Can be reversed. The string can be untwisted back to its original straight form.
Cannot be reversed. Cooking the batter causes chemical changes that transform it into idlis.
Can be reversed. The water can be evaporated to recover the sugar crystals.
Can be reversed. The water can be poured back into the well.
Cannot be reversed. Ripening involves chemical changes in the fruit's composition.
Cannot be reversed. The water evaporates and escapes into the air as steam.
Can be reversed. The mat can be unrolled back to its original flat position.
Cannot be reversed. The grains are crushed and cannot be reformed into whole grains.
Cannot be reversed. Weathering of rocks to form soil takes thousands of years and cannot be reversed.
Explanation
(i) Stitching involves cutting and joining cloth, which alters the original material permanently. (ii) Twisting is a physical change that only alters the shape temporarily. (iii) Cooking involves chemical changes that cannot be undone. (iv) Dissolving is a physical change; evaporation separates the components. (v) This is a physical displacement that does not change the water's composition. (vi) The chapter mentions ripening as an example of an irreversible chemical change. (vii) In an open pan, the water vapour escapes and cannot be recovered. (viii) Rolling is a physical change that only alters the shape temporarily. (ix) Grinding is a physical change but the original form cannot be restored. (x) The chapter explains that weathering is a slow natural change that cannot be reversed.