Example 1
A glass breaks into pieces. Is this chemical because it is hard to undo?
No. It is a physical change because the pieces are still the same glass material.
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Year 8
Physical or chemical change?
Use whether new substances form to classify common changes.
Method and worked examples
A physical change alters state, shape or arrangement without forming a new substance. Examples include melting ice, cutting paper and dissolving sugar without reaction. A chemical change forms new substances with different chemical identities, such as rust forming from iron or products forming when a fuel burns.
Reversibility is not the definition. Cutting paper is hard to undo but remains physical; some chemical reactions can be reversed under suitable conditions. Read what happens to the substance. Sugar recovered unchanged after water removal has undergone physical dissolving. A change in particle motion or arrangement does not automatically mean a new substance formed.
Example 1
Example 2
Iron reacts with oxygen and water to form rust. Classify the change.
Chemical: rust is a new substance, rather than simply iron in a different shape or state.
Watch out: The defining test is whether new substances form. Breaking glass is physical even though restoring it is difficult.