Physical and Chemical Changes - Revision Notes

 CBSE Class 7 Science

Revision Notes 
Chapter – 6
Physical and Chemical Changes

• Changes can be of two types, physical and chemical.

1. Physical changes are changes in the physical properties of substances.  No new substances are formed in these changes.These changes may be reversible.
   Example: crushing a can, glowing of an electric bulb, tearing of paper, mixing of sand and water.
    
2. Chemical Changes are changes in which the composition and chemical properties of the substance get changed. In chemical changes new substances are produced. This change is permanent and irreversible.
   Example: burning of a candle, formation of curd from milk, ripening of fruits.

• ·  Chemical Reactions in daily life:

1. Rusting of Iron: Rusting is the process in which iron turns into iron oxide. It happens when iron comes into contact with water and oxygen. The process is a type of corrosion that occurs easily under natural conditions.
   
   Prevention of Rusting:
   (i) By Painting
   (ii) By Oiling and greasing
   (iii) By Chromium plating
   (iv) By Galvanizing
   (v) By Alloying
    
2. Cooking of food: Cooking causes breakdown of complex molecules of carbohydrates, fats and proteins into smaller molecules.
   It is regarded as a decomposition reaction. Cooked food is easier to digest than uncooked food.
    
3. Decay of Organic Substances: Microorganisms like fungi and bacteria produce enzymes which break down complex organic compounds into smaller substances.
   It is also regarded as a decomposition reaction.

• Some substances can be obtained in pure state from their solutions by crystallization.