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Explain changes around us.
Change means to become different. Changes are of different types, and they are:-
1. Desirable change are those changes which we want to occur.
These changes are useful to us.
For example ripening of fruits, germination of seeds, change in season
2. Undesirable change: Those changes which we do not want to take place are called undesirable changes.
Examples: rusting of iron, the souring of milk, rotting of fruits etc.
3. Reversible Changes are those changes which can be reversed easily to form the 'original substance'.
Example: Ice is a solid substance. When we heat ice, it melts to form liquid water. when we freeze water it again forms ice.
Ice – water – ice.
4. Periodic changes are changes which occur periodically or at regular intervals. Example: Change of seasons. Day and night, changing phases of the moon, beating of heart, the clock striking an hour.
5. Physical change is the change of that matter where only appearance or molecular arrangement of matter changes without changing its composition.
Examples: Boiling and freezing of water, melting of wax, mixing sand and grains, crumpling a paper bag, chopping a wood etc.
Properties of physical change are as follows:
• Most of the physical changes are reversible. Original form of the matter can be attained.
• It is a temporary change and no new substance is formed.
• During a physical change, no energy is absorbed or released
• Physical change causes a change in physical properties such as shape, size, etc.
6. Chemical change: The change in which chemical properties of the matter change is known as chemical change.
During a chemical reaction, the chemical bond between the molecules broken, rearranged, and new bonds are formed resulting in the formation of the new compound.
For example, photosynthesis reaction is a chemical change as carbon dioxide and water in the presence of sunlight and chlorophyll to give sugar molecule and oxygen as a product. Here, sugar once formed cannot be converted back to carbon dioxide and water easily. As the new compound is formed from the reaction between water and carbon dioxide, it is a chemical change.
Other Examples: Digestion of food, burning of petrol and diesel, rusting, preparation of tea and coffee, etc.
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