CHAPTER 3 PHYSICAL AND CHEMICAL CHANGES
Changes occurring around us can be broadly divided into physical and chemical 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.
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.
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.