Physical and Chemical Changes
How to decide whether matter only changed its form or whether its particles were rearranged into new substances
Big idea
A physical change changes a substance's state, shape, size, or distribution without changing which substances are present. A chemical change, also called a chemical reaction, rearranges atoms and produces one or more new substances. The most reliable question is not “Does it look different?” but “Did the identity and composition of the particles change?”
By the end of this lesson, you can:
- distinguish physical and chemical changes at the macroscopic and particle levels;
- interpret gas, colour, precipitate, temperature, light, and odour observations cautiously;
- explain why reversibility alone is not a valid definition;
- identify simultaneous physical and chemical changes in everyday and industrial processes;
- predict that total mass is conserved in a closed system.
1. The decision is about substance identity
We observe changes with our senses or instruments, but we classify them by what happens to matter. Cutting copper wire makes shorter pieces, melting ice makes liquid water, and spreading perfume through air changes arrangement or state; the original substances remain. Rusting iron, burning fuel, and cooking an egg form substances with new properties and compositions.