Physical and Chemical

Overview

Changes happen all around us. Some can be reversed and form no new substance, while others form new substances. This chapter explains physical and chemical changes.

Key concepts

• In a physical change, no new substance forms; it is often reversible.
• In a chemical change, one or more new substances form; usually irreversible.
• Melting, freezing and dissolving are physical changes.
• Burning, rusting and cooking are chemical changes.
• Chemical changes may release heat, light or gas.

Important terms / formulae

• Physical change: change in form without new substance.
• Chemical change: change producing a new substance.
• Rusting: slow reaction of iron with air and moisture.
• Reversible / irreversible: can or cannot be changed back.

Solved example or key process

Melting ice into water is a physical change because cooling the water makes ice again. Burning a piece of paper is a chemical change because ash and gases form and the paper cannot be recovered.

Important questions

1. Differentiate between physical and chemical changes.
2. Why is rusting called a chemical change?
3. Give two examples each of physical and chemical changes.
4. How can we prevent iron from rusting?

Quick revision

Physical changes form no new substance and are usually reversible. Chemical changes form new substances and are usually irreversible. Rusting and burning are common chemical changes.