Physical and Chemical Changes

A change in which a substance undergoes a change in its physical properties is called physical change, physical properties involves shape, size, colour and state of a substance. It is generally reversible; in physical change no new substance is formed. For example: evaporation, condensation etc.

A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed. Example—Burning of coal, photosynthesis

The difference between physical change and chemical change is as follows-Physical change
(1) No new substance is formed. A substance undergoes a change in its physical properties.
(2) Physical change is generally reversible.
Example—Melting of ice, lighting of bulb Chemical Change
(1) One or more new substances are formed by chemical reaction.
(2) Chemical change is irreversible.
Example—burning of coal, photosynthesis

When a piece of iron is left in open for some time, it acquires a film of brownish substance called rust. The process is known as rusting.

Iron (Fe) + Oxygen (O2, from the air) + water (H2O)→ Rust (iron oxide) (Fe2O3.H2O)

Both Oxygen and water or water vapours are essential for the process of rusting.

We can prevent rusting by following methods:

1. By painting or greasing: - by applying a coat of paint or grease regularly on iron articles.
2. By galvanizing:- Depositing a layer of zinc or chromium on iron. The process is known as galvanization.

1. Depositing a layer of zinc or chromium on iron to prevent it from rusting is known as galvanization. Doing this prevents direct contact of iron with air and oxygen and thus prevents it from rusting.
2. Some substances can be obtained in pure state from their solutions by the process of crystallisation