Properties such as shape, size, colour and state of a substance are called its physical properties. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
A change with which you are quite familiar is the rusting of iron. If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. The substance is called rust and the process is called rusting.
A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction.
Chemical changes are very important in our lives. All new substances are formed as a result of chemical changes. For example, digestion of food in our body, ripening of fruits, fermentation of grapes, etc., happen due to series of chemical changes.
New materials, such as plastics and detergents, are produced by chemical reactions.
The process of rusting can be represented by the following equation:
Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3)
For rusting, the presence of both oxygen and water (or water vapour) is essential.
If the content of moisture in air is high, which means if it is more humid, rusting becomes faster.
Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. In fact, these coats should be applied regularly to prevent rusting. Another way is to deposit a layer of a metal like chromium or zinc on iron.
The process of depositing a layer of zinc on iron is called galvanisation. The iron pipes we use in our homes to carry water are galvanised to prevent rusting.
Large crystals of pure substances can be formed from their solutions. This process is called crystallization. It is an example of a physical change.