Category :
7th Class
Physical and Chemical Changes
Synopsis
- Most of the changes occurring around us can be classified into physical and chemical changes.
- Properties like the shape, size, colour and the state of a substance are called physical properties.
- A change in the physical properties of a substance is called a physical change.
- Physical changes are usually reversible and no new substances are formed.
- A change in which one or more new substances are formed is called a chemical change.
- Chemical changes take place because of chemical reactions.
- Exchange of heat, light, sound, smell or colour can be observed in chemical changes.
- Chemical changes are irreversible.
- The process of coating iron with a layer of zinc to protect it from rusting is called galvanization.
- The formation of large crystals of pure substances from their solutions is called crystallization. It is a physical change.
- Differences between physical and chemical changes:
Physical Changes
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Chemical Changes
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(i) Change is temporary
(ii) It can be reversed.
(iii)No new substances are formed. Only physical properties of the substances change.
(iv)No energy change takes place
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(i) Change is permanent.
(ii) It cannot be reversed by simple chemical or physical means.
(ii) New substances with di? erent chemical properties are formed
(iv) Energy changes take place.
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