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Class VI - Science: Changes Around Us
One Word Answer Questions:
Q) Name the change in which the final substance can be converted back to the original substance.
    
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Q) What change is a permanent change that cannot be undone?
    
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Q) Name the element which expands on heating and contracts on cooling?
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Q) Dissolving, Evaporation, Melting and Freezing are examples of Irreversible changes? Yes or No.
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Q) Heating of limestone is an example of which change?
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Short Answer Questions:
Q) What is Reversible change?
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Q) Define Irreversible Change?
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Q) Explain about the working of a blacksmith?
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Q) Define slow changes and fast changes. State few examples.
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Long Answer Questions:
Q) Explain about expansion and contraction of metals?
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Q) Explain about the fixing of a metallic rim on a wooden wheel?
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Q) Explain briefly about the Reversible and Irreversible changes with examples.
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Q) Briefly explain about any two factors affecting change.
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Q) Give examples to explain the difference between changes that can or cannot be reversed.
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Q) Explain briefly about physical change and chemical change.
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Changes Around Us

Introduction
  • 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.
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Changing seasons
  • Every year we observe that seasons changes.
  • We go from rains to winter and winter to summer and so on.
  • What changes do you observe from winter season to summer season?
Is there any change in the clothes we wear?

If the winter season changes into summer, we observe change in our clothes.

For example, wearing of woollen clothes in winter changes to wearing of cotton clothes in summer.

Is there any change in coldness and hotness of the air around us?

We observe that the winter season is cool and summer season is hot.

Is there any change in duration of day and duration of night?

In winter, duration of night is longer than in summer.

Is there any change in the food that we eat or drink?

We take cold drinks in summer but prefer hot tea, coffee or milk in winter.

Compare the change of milk to curd with change of seasons.
  • If we compare the two changes i.e. the "change from milk to curd" and "change of seasons", we notice that the change of seasons is slow when compared to change of milk to curd.
  • But if we compare change of milk to curd and change in electric bulb due to the switch being on or off, the change of milk to curd is a slow change.
  • Thus the change of milk to curd is a fast change when compared with change of season but it is a slow change when compared with change in electric bulb being put on or off.
  • Therefore, whether a change is slow or fast is relative.
  • Similarly, the comparison of above two changes explains that change of seasons takes place naturally, but to change milk into curd we need to add some curd to the warm milk and keep it in such a way that it is not shaken and remains warm. Thus we need some initiation and intervention from human beings to bring a change in the milk.
  • Seasonal changes are temporary as these changes from winter to summer and summer to rains then rains to winter are continuous. Thus we get winter again.
  • Change of milk into curd is permanent because we cannot get back milk from curd.
  • The comparison shows that it is possible to classify certain changes as slow or fast, natural or man-made and temporary or permanent.
Some of the examples of changes around us
  • Change of ice into water and water into ice
  • Rusting of iron
  • Growth in plants
  • Rice to cooked rice
  • Melting of ice-cream
  • Boiling an egg in water
  • Electric bulb on and off
  • Changes in Atti-Patti Plant
Changes observed around us can be classified as
  • Reversible change
  • Irreversible change
Reversible and Irreversible Changes

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.
changes around us
Folding a Paper
changes around us
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.
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Cooking Is An Example Of Irreversible Change

Mixing substances can also cause an irreversible change
  • 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 your original substances.
  • changes around us
    Irreversible Change
  • 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.
changes around us
Human Growth 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.
changes around us
Piece Of Iron Is Turned Into Desired Shape After Heating
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.
changes around us
Before Fixing Wooden Handles Tools Are Heated
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 onto the wheels.
changes around us
Fitting An Iron Rim On A Wooden Wheel Cart
changes around us
Wheel With A Metal Rim
Activity:1
Does the sun rise exactly in the east in all seasons?
  • Take a magnetic compass find the North-South directions with its help.
  • We know that the East-West direction is exactly perpendicular to North-South direction.
  • Mark East-West direction with the help of magnetic compass and compare it with the direction in which the sun rises during the winter season.
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  • Observe the direction of sunrise three to four times in winter and in summer.
  • Compare it with the exact east direction marked with the help of the compass.
  • Do you find any change in direction of sunrise between winter and summer season?
  • What difference do we notice?
  • Did the sun rise exactly in the east?
  • Observing the changes in shadow during winter and summer seasons
  • Teja likes photos very much. His father took photos in the months of December and May .
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  • Observe shows the shadow of a boy, standing on the doorstep of an east facing house, at the time of sunrise. This is on a day in the month of December.
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  • Shadow formed at the time of sunrise on a day in the month of May.
  • What difference do you notice in length and direction of the two shadows?
  • What does it say about the change in the direction of sunrise in December and in May?
  • You may also ask some elders about the change in the direction of the sun rays coming through windows or doors facing east during winter and summer.
  • You can also observe shadows formed by the sun rays through windows and doors in your house or in a neighbour's house.
  • You will notice that the sun does not exactly rise in the east.
  • Around 20th December, the direction of sunrise is a little south of east.
  • Around 15th May, the direction of sunrise is very close to the east.
  • Because of this, we find that the shadow of the boy is right behind him in May and shifts towards his left in December.
  • This slight change in the direction of the sunrise is one of the reasons for changes in seasons.
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