Students compare changes that can be reversed with changes that create something entirely new, discovering the difference between physical and chemical change.
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Teach: reversible, irreversible, physical change, chemical change, reaction, product, evidence. The key question: 'Can you get back exactly what you started with?' If yes, it is physical.
Focus on the reversible/irreversible distinction only before introducing the four signs of chemical change.
Can students correctly classify five changes as physical or chemical with a reason? Can they name three signs that indicate a chemical change has occurred?
Tearing paper and dissolving salt are free. Burning is possible over a small fire. Vinegar and bicarbonate of soda are very cheap.
Students often classify melting as a chemical change because it looks dramatic. Melting is physical — the substance is unchanged, just in a different state. You can freeze it and get the original substance back.
The physical/chemical change distinction is fundamental to chemistry. It introduces the idea that chemistry is about creating new substances, not just rearranging them.
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