What fun if you suddenly get some magical powers to change anything around you! What are the things you would want to change? We do not have magical powers, of course. But, we can still change a few things around us, perhaps many things. Many changes are taking place around us on their own. In the fields, the crops change from time to time. Sometimes, leaves fall from trees, change colour and dry out. The flowers bloom and then wither away. Are any changes happening in your body? Your nails grow, your hair grows, you grow taller and your weight increases as you grow. Did you realise earlier that so many changes are taking place around you all the time?
How can we group various changes? It might help, if we find some similarities between them.
Take a balloon and blow it. Take care that it does not burst. The shape and size of the balloon have changed . Now, let the air escape the balloon.
Take some dough and make a ball. Try to roll out a roti. May be you are not happy with its shape and wish to change it back into a ball of dough again.
Now, think about the the changes you observed in Activity 1 and 3. What do they have in common?
Was it possible to get the balloon back to its original shape and size? Was it possible to get back the ball of dough again?
What do you conclude? In each of the these activities, is it possible to get back to the material with which we started our activity? If the answer is yes, it means that the changes occurring in these activities can be reversed. Now, let us repeat the same activities with a difference.
Take the same balloon, which you used in Activity 1. Blow it to its full size and tie its mouth with a string tightly. Prick it with the pointed tip of your pencil. Oops! It burst.
Similarly, roll out a roti from the ball of dough again and bake it on a tawa.
Suppose, you are asked the same questions which you answered. What would your answers be, now? We see that, the changes which have occurred in the Activity 3 and 4 can not be reversed.
You use a pencil and an eraser. With repeated use, their shape and size changes. Can we reverse this change?
Some common changes are given in Table -1. Which of these changes, do you think can be reversed?
We find that one way we can group changes is to see if they can be reversed.
Table -1 : Some common changes
Can be Reversed
|Raw egg to boiled egg||Yes/No|
|Batter to idli|
|Wet clothes to dry clothes|
|Woollen yarn to knitted sweater|
|Grain to its flour|
|Cold milk to hot milk|
|Straight string to coil string|
|Bud to flower|
|Milk to paneer|
|Cow dung to biogas|
|Stretched rubber band to its normal size|
|Solid ice cream to molten ice cream|
We all have seen the tools which are used to dig the soil. Have you ever seen how the iron blade in these tools is fixed to the wooden handle?
The iron blade of these tools has a ring in which the wooden handle is fixed. Normally, the ring is slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it becomes slightly larger in size (expands). Now, the handle easily fits into the ring. When the ring cools down it contracts and fits tightly on to the handle.
Such a change is also used for fixing the metal rim on a wooden wheel of a cart . Again 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.
When we heat water in a pan, it begins to boil after some time. If we continue to heat further, the quantity of water in the pan begins to decrease.The water changes into its vapour. You have observed that water vapour gets changed into liquid water when it is cooled. We all have noticed melting of ice. Ice melts when it is heated. What does it change into? Is it possible to change this water back into ice?
Let us observe some more changes.
Take a small candle and measure its length with a scale. Now, fix it at a suitable place and light it. Let it burn for some time. Now blow out the candle and measure its length again.
Can the change in the length of the candle be reversed? If we were to take some wax in a pan and heat it, can this change be reversed (Fig. 1)?
Repeat Activity 5 with an incense stick. Wait till it burns away completely. What are the changes that occur in the incense stick? The stick burns to produce some new material. These are ash and some gases. We cannot see these gases but can sense them due to their pleasant smell. Can this change be reversed? And what about the change, which occurred in the matchstick you used for lighting the candle or incense stick?
So far we have discussed the changes occurring in a given object or its material. What about the changes that occur when two substances are mixed together?
When we dissolve salt in water. Do you think a change occured in salt or in water? Is it possible to reverse this change? In previous lessons, we learnt how to separate salt from its solution in water. So, can we say that the change due to dissolving salt in water be reversed?
A small quantity of curd is added to warm milk. The milk is stirred and is set aside for a few hours at a warm place. In a few hours, the milk changes into curd. Can this change be reversed?
We find that a few ways to bring about a change in a substance could be, by heating it or by mixing it with some other substance. We also find that some changes can be reversed, while some others cannot be reversed. There must be many other ways of changing things around us. It is possible that some of them could be reversed. Thus, changes around us could be grouped as those that can be reversed or cannot be reversed. In higher classes, you will learn more about the ways in which changes can be made and the way these can be grouped.
Some changes can be reversed and some cannot be reversed.
A change may occur by heating a substance or by mixing it with some other.
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