Physical and Chemical Changes NCERT science class 8 Chapter 6 Summary Note

Physical properties
Shape, size, colour and state of a substance are called its physical properties.

Chemical properties
Chemical composition of a substance are called its chemical properties.

Kinds of changes- 
(i) Physical changes 
(ii) Chemical changes

           OR 
(i) Reversible changes
(ii) Irreversible changes

Physical changes 
Changes in physical properties of a substance, is called physical changes. 
Such change is reversible change. 
No new substance is formed in such change.
Example- Cutting of paper

Chemical changes 
Changes in chemical properties of a substance, is called chemical changes. 
Such change is irreversible change. 
New substance is formed in such change.
Example- Burning of paper 
                 Burning of magnesium

Few common Examples of Chemical changes 
(i) Burning of magnesium ribbon
   Magnesium + Oxygen -----> Magnesium oxide
(ii) Copper sulphate + Iron ----> Iron sulphate + Copper
(iii) Vinegar + Baking soda ----> Carbon dioxide + Other substance
(iv) Carbon dioxide + Lime water ----> Calcium carbonate + water 
When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky. 

How to Identify Chemical changes
Chemical changes are accompanied by
(i) Heat, light or radiation may be given off or absorbed. 
(ii) Sound may be produced. 
(iii) Change in smell 
(iv) Change in colour
(v) A gas may be formed

Rusting
The process of formation of rust(iron oxide), is called rusting. 
Iron + Oxygen + Water ----> Iron oxide(rust) 
It is a chemical change.

For rusting, the presence of both oxygen and water(or water vapour) is essential. 

If, content of moisture in air is high, rusting becomes faster. 

Methods to Prevent Rusting
(i) Painting
(ii) Greasing
(iii) Galvanisation 
(iv) Formation of alloys

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

Alloys
Mixture of metal with metal or non-metal, is called alloy. 
Example- Mixture of iron and carbon is stainless steel. 
Alloys don't rust. 

Crystallisation 
The process where a substance is purified. 











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