Physical change: no new substance
A physical change forms no new substance -- the same particles are present afterwards, just rearranged, with no bonds inside them broken or made.
Examples: all changes of state (melting, freezing, boiling, condensing, subliming); dissolving; mixing without reaction; separating a mixture by filtration, evaporation or distillation.
Usually easy to reverse, and the substance keeps its chemical identity -- melted wax is still wax.
Chemical change: a new substance forms
A chemical change forms one or more new substances with different properties from the reactants. Bonds in the reacting particles break and new bonds form; an equation shows reactants on the left, products on the right.
Usually hard to reverse: cooling magnesium oxide does not give back magnesium and oxygen. Signs: an energy change (exothermic: hotter; endothermic: colder), a gas, a precipitate, or a permanent colour change. No single sign is proof.
Mass is conserved in both kinds of change
In a closed system (nothing can enter or escape) the total mass stays the same whether the change is physical or chemical, because atoms are never created or destroyed -- only rearranged. This is why a symbol equation must show the same number of each type of atom on both sides.
If the mass appears to fall in an open container, a gas has escaped; if it appears to rise, a gas such as oxygen has been taken in from the air.
Drawn from real examiner reports.
Colour change is not proof of a reaction
Copper(II) sulfate crystals stirred into water turn it blue, but no new substance forms -- the same particles are simply spread out among the water particles, and the crystals reappear unchanged when the water is evaporated. Only a permanent colour change to a substance with genuinely different properties (e.g. a coloured precipitate) is evidence of a chemical change.
Define chemical change precisely
Vague answers such as "the atoms change" or "the substance is altered" earn no credit. The mark-scheme form has two parts: a new substance is formed, with different properties, and the change is usually difficult to reverse. In an "explain the difference" question give both sides -- no new substance and easily reversed for the physical case.
Jun 2022 P11 Q15 (digest C6 -- candidates did not state that a NEW substance forming is the deciding feature).
A temperature change proves nothing
"The beaker got hot, so it is a chemical change" is unsafe. Dissolving some salts warms the water, and melting and boiling absorb a great deal of energy -- yet there is no temperature change at all while the state is changing, and no new substance is formed. Treat any energy change as a clue only; the deciding test stays whether a new substance has formed.
Jun 2022 P11 Q33; Nov 2023 P13 Q31 (digest C1/P2 -- no temperature change occurs during melting or boiling).
Bubbles are not always a reaction
Gas produced is usually good evidence of a chemical change, but not always. Gently warming a fizzy drink releases bubbles of carbon dioxide that were already dissolved in it, unreacted -- no new substance forms, so that is a physical change. Check the gas is a product of a reaction, not a dissolved gas coming back out of solution.
Mixing is not reacting
Iron filings stirred with sulfur powder stay a mixture -- a physical change, and a magnet still pulls the iron out. Heat them and they react to form iron(II) sulfide, a compound with new properties (not magnetic) that no physical method can separate. A compound is two or more elements chemically combined; calling it a "mixture of elements" loses the mark.
Jun 2022 P31 Q11b; Jun 2023 P11 Q15 (digest C2 -- "chemically combined" omitted, or a compound called a mixture of elements).
Reversible does not mean physical
"It can be undone, so it must be physical" is the commonest shortcut, and it misleads. Reversibility is only a clue: the thermal decomposition of calcium carbonate is a genuine chemical change that can be partly reversed under the right conditions, while burning cannot be reversed at all. Ask instead whether a new substance, with different properties, has been formed.
Jun 2022 P11 Q15 (digest C6 -- physical vs chemical change judged by whether a new substance forms / reversibility).
State the verdict before the evidence
Open with the decisive sentence: "This is a physical/chemical change because a new substance has/has not been formed." That one line answers "identify" and "explain" together. Only then add the supporting evidence from the scenario.
Describing is not explaining
Re-listing what was seen -- "bubbles appeared and the tube got hot" -- does not answer "explain why this is a chemical change". Name the new substances formed first, then quote the observations as evidence for them.
Choose evidence, do not list it all
Give ONE piece of evidence per mark, chosen because it points to a new substance having (or not having) formed -- "a gas was produced", "a precipitate formed", or for a physical change "the solid reappeared unchanged on evaporating". Match your number of points to the tariff.
Balance any equation you offer as proof
If you support a chemical change with a symbol equation, balance it. Oxygen and nitrogen are diatomic, so magnesium burning is 2Mg + O2 -> 2MgO, not Mg + O2 -> MgO. An unbalanced equation shows atoms being created or destroyed, which cannot happen.
Cambridge 0654 spec reference: Section C6 "Chemical reactions", sub-topic C6.1 "Physical and chemical changes". Core only (no Extended/Supplement statement for this leaf): identify physical and chemical changes, and understand the differences between them.
| Term | Mark-scheme-precise meaning |
|---|---|
| Physical change | A change in which no new substance is formed -- particles are only rearranged; usually easy to reverse |
| Chemical change (chemical reaction) | A change in which one or more new substances are formed, with different properties from the starting substances; usually difficult to reverse |
| Reactant | A starting substance in a chemical change, present before the reaction |
| Product | A new substance formed by a chemical change |
Signs that COULD indicate a chemical change has happened (none is proof by itself -- always check whether a new substance has actually formed):
Mass is conserved in both physical and chemical changes, in a closed system -- atoms are rearranged, never created or destroyed. An apparent mass change in an open container usually means a gas has escaped or been taken in from the air.
Define a physical change.
Magnesium ribbon is held in a Bunsen flame. It burns with a bright white light, and a white powder (magnesium oxide) is left, with a different mass from the original ribbon.
Write the balanced symbol equation for this reaction. (2 marks)
