Melting ice and burning hydrogen both change what you see. Only one of them makes something new. The difference between a physical and a chemical change comes down to whether the atoms are rearranged into new substances.
A physical change does not produce a new substance: the particles are only rearranged or moved apart, as in melting, boiling, dissolving or separating a mixture, and it is usually easy to reverse. A chemical change produces one or more new substances with different properties, as in burning, rusting or neutralisation. Chemical changes are often accompanied by an energy change and are usually difficult to reverse.
Fig. 1: particles in a physical and a chemical change
Section 01
Two kinds of change.
Ask one question: is there a new substance at the end? If yes, it is a chemical change.
burning, rusting, cooking an egg, neutralisation, thermal decomposition
Stage 1 · Learn
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Which of these is a chemical change?
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Which statement describes a physical change?
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Salt dissolves in water. What kind of change is this?
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Which observation suggests that a chemical change has taken place?
Section 02
Signs of a chemical change.
In practice you infer a new substance from what you observe.
A gas is given off that was not there before: fizzing when magnesium is added to acid.
A colour change that does not reverse on cooling: copper turning black when heated in air (copper(II) oxide forms).
A precipitate forms when two solutions are mixed.
A temperature change without any heating or cooling from outside.
Examiner trap
Dissolving is a physical change: evaporate the water from salt solution and the salt is recovered unchanged. Do not confuse "it disappeared" with "it reacted". And a colour change alone is not proof: heating anhydrous copper(II) sulfate is a chemical change, but a solution simply getting paler when diluted is not.
Stage 2 · Exam
Exam-style questions
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Which list contains only chemical changes?
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Copper is heated in air and turns black. On cooling it stays black. What is the best explanation?
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Why is boiling water a physical change?
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A student mixes two colourless solutions and a white solid appears. What kind of change is this, and why?
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