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?
Answers
Answer:
1 .reversible changes mean that can be reverse .
example: evolution, melting and freezing
Answer:
1.A reversible change is a chemical change where no new materials are created and the original materialcan be recovered. Examples include freezing water to make ice or melting chocolate.
2.A change which cannot happen backward, that is, it cannot be reversed is called an irreversible change. When you burn a piece of paper, it turns to ash. It cannot become paper again.Some examples of irreversible changes are burning of paper, Burning of fuels (like Wood, Coal and LPG), Cooking of food, Rusting of iron
3.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.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.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.Chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. These processes are called chemical reactions and, in general, are not reversible except by further chemical reactions.