Class 7 | Science | Chapter- 6 | Physical and Chemical Changes Notes

Class 7           Science

Chapter- 6

Physical and Chemical

Changes Notes

Changes can be of two types, physical and chemical change

Physical Changes- Physical changes are changes in the physical properties of substances. No new substances are formed in the changes. These changes may be reverible. Examples- Tearing of paper, mixing of sand and water, etc.

Physical properties- Properties such as shape, size, colour and state of a substance are called its Physical properties.

Chemical Changes- A change in which one or more new substances are formed is called a chemical change. Chemical changes are changes in which the composition and chemical properties of the substance get changed. It is permanent and irreversible. Example- Rusting of iron, burning of a candle and ripening of fruits, etc.

Chemical properties- One or more new products are formed. Heat, light or other radiation(UV, For example) may be given off or absorbed. Sound may be produced. May be change in smell. May be change in colour and a gas may be formed.

Rusting of Iron- It is a process in which iron turns into iron oxide. Iron rust due to the presence of both oxygen and water(or water vapour) . The process is a type of corrosion that occurs easily under natural conditions. 
Iron(Fe) + Oxygen(O2 from the air) + water(H20) ➡ rust(iron oxide Fe2O3)

Prevention of Rusting:- 

• By painting
• By oiling and greasing
• By Chromium plating
• By Galvanisation
• By Alloying

Rusting Iron

Galvanisation:- The process of depositing a layer of zinc on iron is called galvanisation.

Crystallisation:- Large crystals of pure substances can be formed from their solutions. The process is called crystallisation.