ThinkRankPhysical and Chemical Changes
A physical change may alter size, shape, or state without making a new substance, while a chemical change produces one or more new substances. Some changes can be reversed easily; others cannot. Observing evidence such as gas formation, colour change, temperature change, or a new solid can help distinguish them.
Learning Objectives
- •Distinguish physical and chemical changes
- •Identify evidence that a new substance may have formed
- •Explain rusting and conditions that support it
- •Describe ways to slow corrosion and reduce material loss
Key Concepts
A new substance is the key idea
Chemical change changes the composition and properties of matter, not merely its appearance.
Rusting needs conditions
Iron rusts when it reacts with oxygen in the presence of moisture; salt and pollutants can speed corrosion.
Changes can be useful or harmful
Baking and digestion are useful chemical changes, while corrosion damages structures and consumes resources.
Terminology
Historical Insight
Crystallisation
Salt or sugar crystals can form from a solution as the solvent evaporates, illustrating a physical separation process.
Protecting iron
Painting, oiling, galvanising, or using an alloy can reduce contact between iron and air or water.
Quick Check
Why is melting wax different from burning wax?
What evidence suggests a chemical change?
How can rusting be prevented?
How to use this lesson
Read the overview first, then explain the main idea in your own words before checking the key concepts. Use the quick-check questions without looking at the answer. For examination preparation, verify dates, definitions, and syllabus requirements against your official textbook and school or examination-board guidance.