CBSE Class 7 Chemistry Notes- Chapter 3 Physical and Chemical Changes

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CHAPTER 3 PHYSICAL AND CHEMICAL CHANGES 
 
Changes occurring around us can be broadly divided into physical and chemical changes.
 
PHYSICAL CHANGES
When a substance experiences a change in its physical properties like shape, size, colour or state, it is called physical change. Physical change does not form any new substance.
 
CHEMICAL CHANGES
When a substance undergoes a change in their chemical properties, it is called a chemical change. A chemical change is also called a chemical reaction and leads to the formation of new substances.
E.g. (i) Rusting of iron
       The process of rusting is as follows:
       Iron + Water + Oxygen  Rust
         Fe  +   H2O   +     O2      Fe2O3
When iron is left out in the open, a brownish film is formed over it. The brown film is called rust and this process is called rusting of iron.
       To prevent rusting, iron is coated with a layer of zinc. This process is called galvanization.
       (ii) Crystallization
       A crystal of a substance is its pure solid form. Obtaining a substance in its pure form from its solution is called crystallization.
 
Depending on the changes, they may be classified into reversible or irreversible.

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