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Changes Around Us

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Changes Around Us - Lesson Summary

Every day, we see different types of changes around us, like day turning into night, the season changing from winter to summer, melting of ice to  water, cooking of food, etc.
Changes take place all around us all the time. All the changes are not of the same type we get back the original substance in some, and can’t get them back in others.

Changes observed around us can be classified as
     •  Reversible change
     •  Irreversible change

Reversible Change
A reversible change is a change that can be undone or reversed. These changes are observed in the physical state, size and shape of the substance. A reversible change does not create a new material.
Examples: Melting ice, Boiling, evaporation and condensation, etc are the examples of reversible changes.
If you could capture all the steam that is made when a kettle boils, you could turn it back into water by cooling it.
Some other examples of reversible changes are folding a paper and blowing a balloon.

Irreversible Change
An irreversible change is a permanent change that cannot be undone. In an irreversible change, new substances are formed.
Examples: Cake batter is made from eggs, flour, sugar and butter. Once the cake has been baked, you cannot get the ingredients back.
Cooking is another example. We cannot get back the substances that we originally started  with.
Mixing substances can also cause an irreversible change.
Examples: when vinegar is mixed with bicarbonate of soda, you can observe the release of carbon dioxide gas in the form of bubbles. Again, here, we cannot get back our original substances.

Some other examples of irreversible changes are the digestion of food and heating of lime stone. 
The burning of wood is also an irreversible change. Once wood is burned, it changes to ash, and we cannot get back the wood again.
Our growth itself, as you can see, is an irreversible change.

Relevance of Reversible changes in Daily Life
Working of Blacksmith
A blacksmith changes a piece of iron into different tools. For that, a piece of iron is heated to red hot. This also softens it. It is then beaten into the desired shape. It is a reversible change.

Expansion and contraction of metals
Metals expand on heating and contract on cooling. This expansion and contraction of a metal is a reversible change.
Small gaps are left between the rails that form the parallel lines of a railway track. These gaps are called expansion gaps. These gaps allow the rails expand in hot weather. If there are no gaps or gaps are too small the lines can bend.

Fixing wooden handles on tools
While making agricultural tools the metal ring is heated before fixing the wooden handle. On heating the metal ring expands. When the ring become bigger size the wooden handle fix easily. When the metal ring cools, it contract and the handle fits in tightly.

Fixing metallic rim on a wooden wheel
The fixing of metallic rim to the wooden base of the wheel of bullock cart can be done in the similar manner. The metallic rim used is slightly smaller in diameter than the wheels .On heating the rim expands. And can easily lie on the wheels. When the rim cools, it contract and press tightly on to the wheels. 


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