CHANGES AROUND US
1. What are reversible changes? Give 3 examples.
2. What are irreversible changes? Give 3 examples.
3. How can we fix the iron blade of tools to the wooden handle? Explain.
4. How can we fix the metal rim on a wooden wheel of a cart?
5. What are the changes that occur in an incense stick, when it is burnt?
6. How can we bring about a change in a substance?
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1) Reversible changes are changes that can be undone or reversed. Melting, freezing, boiling, evaporating, condensing, dissolving and also, changing the shape of a substance are examples of reversible changes.
2)Which can undone or reverse to his back state. Some examples of irreversible changes are burning of paper, Burning of fuels (like Wood, Coal and LPG), Cooking of food, Rusting of iron , Grinding of wheat grains into flour, Baking of chapatti (roti), Growth of a plant, Formation of flower from bud, Falling of leaves from a tree, Ripening of fruits, Ageing of man and ...
3) Answer: To fix the handle, the ring is heated and it becomes slightly large in size. Then, the handle easily fits into the ring. When the ring cools down, it contracts and fits tightly onto the handle
4)Answer: For fixing the metal rim on a wooden wheel of a cart, the metal rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits onto the wheel. Cold water is then poured over the rim, which contracts and fits tightly onto the wheel.
5)Explanation: When an incense stick is burnt new substances ash and smoke are produced. The incense stick cannot be produced from the residues again. Hence it is a chemical change because it goes through a chemical reaction and is irreversible while physical changes are reversible.
6)Changes by mixing a substance with other: We can bring about a change in a substance by mixing it with another, e.g.,Making solution by mixing water soluble substances in water.
Expansion: Metals expand on heating and contract on cooling.
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