Chapter Overview
Changes can alter the form of a substance or produce new substances. Evidence such as gas formation, a lasting colour change, a precipitate or energy transfer can help distinguish chemical change from physical change.
The chapter follows an enquiry-based approach: observe carefully, distinguish evidence from explanation, use appropriate units and communicate conclusions honestly.
Learning Objectives
After studying this chapter, you should be able to:
- explain physical change in clear scientific language;
- explain chemical change in clear scientific language;
- explain reversible and irreversible change in clear scientific language;
- explain rusting and prevention in clear scientific language;
- connect scientific ideas with familiar situations;
- plan or evaluate a safe and fair investigation.
Key Concepts
| Concept | Explanation | |---|---| | Physical change | The substance’s identity remains the same, although state, size or shape may change. | | Chemical change | One or more new substances with new properties are formed. | | Reversible and irreversible change | Reversibility is useful evidence but is not the definition: some physical changes are difficult to reverse. | | Rusting and prevention | Iron rusts in the presence of oxygen and water. Paint, oil, galvanisation and suitable alloys can reduce contact with these conditions. |
Detailed Explanation
Physical change
The substance’s identity remains the same, although state, size or shape may change.
Chemical change
One or more new substances with new properties are formed.
Reversible and irreversible change
Reversibility is useful evidence but is not the definition: some physical changes are difficult to reverse.
Rusting and prevention
Iron rusts in the presence of oxygen and water. Paint, oil, galvanisation and suitable alloys can reduce contact with these conditions.
A scientific statement should be supported by observations, measurements or a tested explanation. Record unexpected results instead of hiding them.
Investigation
Observe ice melting and compare it with an iron nail rusting over several days. Record evidence, identify variables and do not taste any product.
For a fair test, identify what is changed, what is measured, and which important conditions are kept the same. Record results before interpreting them.
Important Definitions
- Physical change: The substance’s identity remains the same, although state, size or shape may change.
- Chemical change: One or more new substances with new properties are formed.
- Reversible and irreversible change: Reversibility is useful evidence but is not the definition: some physical changes are difficult to reverse.
- Rusting and prevention: Iron rusts in the presence of oxygen and water. Paint, oil, galvanisation and suitable alloys can reduce contact with these conditions.
Use precise terms, labelled diagrams where relevant, and complete cause-and-effect statements. A correct scientific reason is more useful than a one-word answer.
Common Mistakes
- Calling every irreversible change chemical.
- Assuming bubbles always prove a new gas; boiling also produces bubbles.
- Forgetting that both oxygen and water support ordinary rusting.
Quick Revision
- Physical change: The substance’s identity remains the same, although state, size or shape may change.
- Chemical change: One or more new substances with new properties are formed.
- Reversible and irreversible change: Reversibility is useful evidence but is not the definition: some physical changes are difficult to reverse.
- Rusting and prevention: Iron rusts in the presence of oxygen and water.
Practice Questions
- Is melting wax a physical or chemical change?
- Why is burning paper chemical?
- Name two ways to prevent rusting.
- What is crystallisation?
Answers and Explanations
- Melting is a physical change because wax remains wax.
- New substances such as gases and ash are formed.
- Any two of painting, oiling, greasing, galvanising or alloying.
- A method of obtaining solid crystals of a substance from its solution.
Apply Your Learning
Find one example of this chapter’s science in your home, school or neighbourhood. Describe what you observe, propose an explanation, and state what additional evidence would test that explanation.