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.
- 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.
- 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:
- 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
- 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.
- 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.