Short Answer

Physical changes are changes in the physical appearance or state of a substance without forming a new substance. The chemical composition remains the same. Examples include melting of ice, breaking a glass, dissolving sugar in water, and boiling water. In these changes, the original substance can usually be recovered.

Chemical changes are changes in which a new substance is formed with different properties. These involve chemical reactions. Examples include rusting of iron, burning of wood, souring of milk, and digestion of food. In chemical changes, the original substance cannot be easily brought back.

Detailed Explanation :

Physical and Chemical Changes

Changes occur all around us in nature and daily life. When matter undergoes a change, it may or may not form a new substance. Based on this, changes are classified into physical changes and chemical changes. Understanding these types of changes helps us study how substances behave, how they react, and how new materials are produced. Both changes are important in chemistry because they help explain natural processes, industrial reactions, and daily experiences.

Physical Changes

A physical change is a change in which only the physical form, shape, size, or state of a substance changes, but its chemical composition remains the same. No new substance is formed during a physical change. These changes are usually reversible, meaning the original substance can be obtained again.

Key features of physical changes:

• No new substance is formed
• Only physical properties change (shape, size, colour, state)
• Chemical composition remains unchanged
• Usually reversible
• Low amount of energy is involved

Common examples:

• Melting of ice – Ice changes to water, but the substance remains H₂O.
• Boiling of water – Water becomes steam but is still water chemically.
• Dissolving sugar in water – Sugar particles spread, but no new substance forms.
• Breaking glass – Shape changes, but the chemical nature remains the same.
• Stretching a rubber band – Only the shape changes.
• Cutting paper – Only size changes, not composition.

In all these examples, the identity of the substance remains unchanged.

Chemical Changes

A chemical change is a change in which a new substance is formed with different properties. These changes involve chemical reactions where atoms rearrange to form new products. Chemical changes are usually irreversible, meaning the original substance cannot be brought back easily.

Key features of chemical changes:

• New substance is formed
• Chemical properties change
• Often irreversible
• Heat, light, sound, or gas may be produced
• Large energy changes may occur
• Colour change or formation of precipitate may be seen

Common examples:

• Rusting of iron – Iron reacts with oxygen and water to form rust.
• Burning of wood – Wood burns to form ash, smoke, and gases.
• Souring of milk – New substances like lactic acid form.
• Cooking of food – Raw food changes chemically to become cooked.
• Digestion of food – Food is broken down into new substances inside the body.
• Burning of a candle – Wax reacts with oxygen to form carbon dioxide and water.

These examples show that a new substance is always produced in chemical changes.

Differences Through Examples

Physical and chemical changes can be understood more clearly through examples:

1. Ice melting vs. Iron rusting
   • Ice melting: No new substance; reversible; physical change.
   • Iron rusting: New substance (rust); irreversible; chemical change.
2. Sugar dissolving vs. Milk souring
   • Sugar dissolving: Sugar remains sugar; physical change.
   • Milk souring: New chemical substances form; chemical change.
3. Breaking a chalk vs. Burning paper
   • Breaking a chalk: Only shape changes; physical change.
   • Burning paper: New substances like ash and smoke; chemical change.

These examples highlight how one change affects only physical appearance, while the other forms new substances.

Why These Changes Are Important

Understanding physical and chemical changes helps in:

• Identifying how materials behave
• Understanding environmental and biological processes
• Classifying natural and industrial reactions
• Knowing how to separate or recover substances
• Designing chemical experiments safely

Industries like food processing, metallurgy, chemistry labs, and manufacturing all depend on understanding these two types of changes.

Conclusion

Physical changes involve alterations in the form or state of matter without forming new substances, while chemical changes result in the formation of new substances with different properties. Physical changes are usually reversible, whereas chemical changes are mostly irreversible. Examples such as melting ice and dissolving sugar show physical changes, while rusting, burning, and souring milk demonstrate chemical changes. Understanding these changes helps us study chemical reactions and natural processes more effectively.