CHANGES AROUND US.pptx for class 7 , 8,9 and 10 for cbse students

Changes around us geography lesson for classes 10,9,8,7 and 6 for cbse and matriculation students

1. Changes Around Us All these changes are not same . These changes are of different types and can be classified as Follows: Reversible and irreversible changes Physical and chemical changes Reversible and irreversible changes A change which can be undone or reversed is called a reversible change .( A change that can be easily changed back to its original form).

2. When we heat ice cubes, they melt to form water. But, when this water is cooled in a refrigerator, it changes back into ice cubes. Similarly, when water is heated, it changes into water vapour and when this water vapour comes in contact with some cold surface, it again changes into liquid water. From the above discussion, we conclude that the changes can be undone or reversed. Therefore, we can say that a reversible change is a temporary change. Thus we conclude that:

3. Folding is also an example of a reversible change. The size of the paper remained the same before and after you made the boat. Melting is an example of a reversible change. Ice cubes melt on heating but they can be changed back into solid ice cubes on cooling. Water changes into water vapour on heating but it can be changed back into liquid water by cooling. Similarly, freezing is also an example of a reversible change. But, all the changes are not reversible. There are many changes that are not reversible. When cement is mixed with water, we cannot get

4. i t back. A change which cannot be undone or reversed is called an irreversible change.An Irreversible change is a permanent change. ( A change which cannot be reversed). What happens when these things are heated? Can you get these things back by cooling or by any other method?

5. What happens when these things are burnt? Therefore, we can conclude that: Mixing of substances can cause an irreversible change.The mixing of cement with water causes a permanent change. This cannot be turned back into cement. Heating of a substance can cause an irreversible change, for example,

6. The cooked eggs cannot be changed into raw eggs The baked cake cannot be changed back into the cake mixture and The cooked chapati cannot be changed back into rolled chapati . Burning of a substance can cause an irreversible change, for example, when we burn paper, coal a nd wood, smoke and ash are formed.Here , we c annot change smoke and ash back to either p aper, coal or wood. When any substance burns, t he original substance changes into ash and some g ases.

7. D ifferences between reversible and irreversible Changes : Reversible change Irreversible change A change which can A change which cannot be undone or reversed be undone or reversed is called a reversible is called an irreversible change. c hange. 2. A reversible change is An irreversible change is a temporary change. a permanent change. 3. No new substance is New substance is formed. formed. 4. Dissolving , melting and Burning of a substance is an folding are examples of example of an irreversible reversible change. Change.

8. PHYSICAL CHANGES A change in which shape, size, appearance or state Of the substance alter or change is called physical change. In this change, no new substance is formed. A physical change is uaually reversible and temporary. Consider the following changes: When we freeze water, it changes into ice. Here, the physical state of liquid water changes into solid ice but no new substance is formed. When we stretch a rubber band, its shape changes. Here also, no new substance is formed.

9. When we inflate a balloon, its size changes but no new substance is formed. Similarly, dissolving of salt or sugar in water, folding a paper, evaporation of water, melting of butter, etc., are some more examples of physical changes.

10. CHEMICAL CHANGES: A chemical change in which new substance is formed with different properties. It is usually i rreversible .

11. Burning is an example of a chemical change. When Something burns, the original substance changes into ash and some gases. Burning of coal, candle, paper, wood, dry leaves, Rusting of iron, bursting of a cracker, cooking of food, rotting of food, igniting a matchstick, etc., are few more examples of chemical changes. Physical and chemical changes can occur together. For example, on burning a candle, the following changes are observed: On heating, the wax under the wick melts and flows down. Also, the length of the candle gets reduced.This is a chemical change.Also , this change

12. is irreversible. Thus, when a candle burns, both physical and chemical changes take place together. Melting of wax is physical change. In melting of wax, only the state of wax changes from solid to liquid.It is a reversible reaction and no new substance is formed. Burning of candle is both physical and chemical change.  When the candle burns, the wax slowly melts which freezes after some time (Physical change). Burning of wick is a chemical change as it involves combustion. The molten wax back to solid wax. Changing of molten wax back to solid wax is a physical change

13. D ifferences between physical and chemical changes Physical change Chemical change A physical change is one in A chemical change is one in which no new substance is which a new substance is formed. formed. 2. Most physical changes are Most chemical changes are reversible. Irreversible. 3. The mass of individual The mass of individual substance remains always substance changes. unchanged. 4. Evaporation and Burning and cooking are condensation are examples examples of chemical changes. of physical changes.

14. EXPANSION AND CONTRACTION Solids, liquids and gases increase in size (expand) on heating. They also come back to their original size(contract) on cooling. This process of heating and cooking is used in fixing wooden handles into the iron blade of tools which are used to dig soil and for other purposes. The process of heating and cooling is also used in fixing iron rim on the bullock cart wheel. Bullock carts have wooden wheels which are fitted with iron rims. The iron rim is made slightly smaller than the diameter of the wooden wheel. It is heated so that it becomes

15. slightly larger in size, i.e.,expands . The hot iron rim is then fixed on the wooden wheel and then cooled quickly by pouring water on it . On cooling, the iron rim contracts and fixes firmly on the wooden wheel. HIGHER ORDER THINKING SKILLS (HOTS) Why small gaps are left between the railway tracks? 2. Why is mercury used in thermometers?

16. Activity : Aim: To separate a mixture of dry sand and bits of waste paper by winnowing. Materials Required: Bits of waste paper, dry sand, a stool and a cardboard Procedure: Mix some bits of waste paper with dry sand. Place this mixture on a cardboard. Now, Stand on a stool and allow the mixture to fall down slowly. Keep shaking the cardboard continuously. Observation: The bits of paper are carried away by wind and dry sand falls down vertically and forms a heap.

17. Conclusion: A mixture of dry sand and bits of paper can be separated by winnowing.

18. Activiy : Aim: To observe change in a paper due to folding. Materials Required: A sheet of paper Procedure: Take a sheet of paper and fold it to make a boat.Now , unfold the paper. Observation: The size of paper remains the same. Conclusion: Folding of paper can be undone. This i s a reversible change.