In this chapter

1. Changes in our day to day life
2. Various types of changes such as :

• reversible and irreversible changes
• slow and fast changes

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# BEACONS OF SCIENCE KNOWLEDGE ZONE In 1810, a French candy maker Nicolas Appert discovered that food that had been heated and sealed in airtight jars stayed fresh for several months. Soon food packaging factories were opened in England.

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Everywhere around us many changes are taking place. Growing of a seed into a sapling into a plant, yellowing of leaves, growing of a baby into an adult etc are some of the changes that we observe. We also observe changes like formation of curd from milk, rolling of chapati from a ball of dough, melting of ice, burning of candle etc. In fact, changes take place around us all the time. Changes take place not only around us, but also within us. We cannot notice them. Our hair and nails grow, day by day, so that the length of the hair changes regularly.

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# PUTTING CHANGES INTO VARIOUS GROUPS Can we group together some of the changes? Yes,we can. To group various changes,we have to find out some similarities between than : 1. Reversible and irreversible changes 2. Slow and fast changes

Reversible and Irreversible Changes

A reversible change is one in which the changed substance can be obtained back in its original from on removing the cause of the change.
A Reversible change can take place in both the direction. It is shown with two opposite arrow(°). For example, ice changes into water on heating,whereas water changes into ice on cooling. Similarly,an electric bulb gives light, when we switch on the power supply. The bulb returns into its original position, when we swit
These example suggest that during a reversible change :

1. No new substance is formed.
2. The change is temporary.
3. The change can be reversed.
4. The change can take place in both the directions.

Irreversible

An irreversible change is one in which the substance changes permanently and we cannot obtain the original substance even after removing the cause. Similarly, cooking of rice and baking of a cake are Irreversible Changes.
All these example suggest that during an irreversible change :
1- A new substance is formed.
2- The change is permanent.

1. The change cannot be reversed.
   4- The change takes place in only one direction

Uses of Reversible Changes

Some reversible changes are very useful for us. For example,a metal melts when we heat it at a high temperature and regains it's solid form when we cool it. These changes in metals are reversible. Due to these changes, a piece of metal can be moulded in various forms and shapes.

Slow and Fast Changes

Changes that require a long time to take place are known as slow changes. These changes may take hours,days or years. Growing of a plant from a seedling,rusting of iron, ripening of fruits and changing o milk into curd are some examples of slow changes.
Changes that occur in a short period of time are known as fast changes. These changes take place within seconds or minutes.
Burning of paper, breaking of glass, dissolving salt or sugar in water, explosion of crackers and burning of cooking gas are some examples of fast changes.

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