7th Class Science Physical and Chemical Changes Physical Chemical Changes

Physical and Chemical Changes

All the substances have certain properties such as state (solid, liquid or gas), size, color, smell, temperature, shape, composition and structure etc.

When properties of a substance become different, we say that a change has taken place in it. Changes are taking place all around us. Some changes are beneficial to us and some are harmful to us. As for example ripening of fruits is a beneficial change. On the other hand rusting of iron is a harmful change.

Type of Changes

1. Physical changes
2. Chemical changes

Physical Change

Change in the physical properties (such as state, shape, size and colour) of a substance is called physical change. For example, breaking of glass, melting of ice, expanding of balloon, etc.

Characteristics of Physical Change

• No new substances are formed.
• Changes are temporary and can be reversed.
• Very little amount of energy is involved.

Chemical Changes

The changes in which two or more than two substances react with each other to form a new substance with different properties are called chemical changes. For example, rusting of iron, burning of wax, turning milk sour, etc.

Characteristics of Chemical Changes

• New substances are formed.
• Changes are permanent and usually irreversible.
• Great amount of energy is involved.

Rusting of Iron

Rusting of iron is a chemical change. When iron is exposed to moist air, it reacts with oxygen and water present in air and formed iron oxide which is red-brown flaky substance called rust.

      \[\underset{Iron}{\mathop{Fe}}\,\]   +   \[\underset{Oxygen}{\mathop{{{O}_{2}}}}\,\]  +   \[\underset{Water}{\mathop{{{H}_{2}}O}}\,\]     \[\underset{Iron\,Oxide}{\mathop{F{{e}_{2}}{{O}_{3}}}}\,\]

Crystallization:

The process in which large crystals of pure substances are obtained from their solution is known as crystallization. Crystallization is a physical change. Crystal of sugar, salt and alum can be obtained from their solution.