Physical and chemical changes
How to tell a chemical change from a physical one.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.1.1Identify physical and chemical changes and describe the differences between them.
5 minute read
Physical and chemical changes
A physical change alters the form or appearance of a substance, but no new substance is formed — the same particles are present before and after, just arranged or moving differently. Examples: melting, freezing, boiling, condensing, dissolving. Physical changes are usually easy to reverse.
A chemical change (a chemical reaction) always produces at least one new substance, with different properties from the starting materials. Examples: burning, rusting, a reaction producing a gas or precipitate. Chemical changes are usually difficult or impossible to reverse by simple physical means.
Signs that a chemical change has happened
- A colour change
- A gas is produced (bubbles, or a gas that can be tested for)
- A precipitate (insoluble solid) forms in a solution
- A temperature change occurs that is not simply due to heating or cooling from outside
- A new smell appears
Not every one of these has to occur — even one is usually enough evidence — but a genuinely new substance being formed is the defining test.
Think of it like this
A physical change is like folding a piece of paper into an origami shape — it looks completely different, but it is still the exact same paper, and you can unfold it back to how it started. A chemical change is like burning that same paper — the ash and smoke are genuinely new substances, and there is no way to "unburn" it back into paper.
Common misconceptions
- Assuming any dramatic-looking change (a big colour change, a lot of bubbles) must be chemical, and anything gentle must be physical. The correct test is always whether a new substance has formed, not how visually dramatic the change looks.
- Believing dissolving is a chemical change because the solid "disappears". The dissolved substance is still chemically the same, just spread throughout the solvent — it can be recovered unchanged by evaporating the solvent.
- Thinking all chemical changes are impossible to reverse under any circumstances. Some can be reversed, but only by another chemical reaction, not simply by a physical method like cooling or evaporating.
In the exam
- When justifying that a change is chemical, name the specific piece of evidence observed (e.g. "a gas was produced" or "the colour changed from blue to colourless") rather than just asserting "a new substance formed" with nothing to support it.
- For "physical or chemical?" questions, always check first whether the change can be reversed by a simple physical method (like cooling) — if yes, that is strong evidence it is physical.