Physical change — no new substance formed
In a physical change no new substance is formed: the chemical composition and formula stay the same. The change can usually be reversed easily and the original substance recovered — by cooling, evaporating or filtering. Mass is conserved. Examples: melting ice and boiling water (the same in a different state), dissolving salt in water (recovered by evaporation), and stretching a rubber band. The key exam language is "no new substance" and "reversible".
Chemical change — a new substance forms
In a chemical change one or more new substances form as bonds are made and broken. It is usually hard or impossible to reverse, and an energy change (heat or light) usually occurs. Observable evidence: a colour change, a gas (bubbles), a precipitate, a temperature change, or light. Examples: burning magnesium, rusting iron, thermal decomposition of copper(II) carbonate. Mass is still conserved — atoms are only rearranged.
Key test — has a new substance formed?
The most reliable test is: has a new substance with new properties formed? Yes means chemical; no means physical. Physical changes reverse easily with little energy change; chemical changes are hard to reverse with a large one. Mass is conserved in both. Tricky cases: dissolving is physical (the salt is recovered by evaporation); combustion and thermal decomposition are chemical even though heat is involved; melting only changes state.
Drawn from real examiner reports.
Describe observations, not product names
Examiners penalise naming the product instead of describing the observable change. "Zinc chloride is formed" scores zero; "bubbles of gas form and the zinc dissolves" gains credit. Observable signs: a colour change, a gas (bubbles/fizzing), a precipitate, a temperature change, or light. Describe what is seen, not what the product is called.
W22 P31 Q6(a); W22 P42 Q4(b); S23 P31 Q4(d)(ii) — product names given where observable changes (fizzing/effervescence) are required score zero.
Dissolving is physical, not chemical
Seeing a solid disappear into water tempts students to call dissolving a chemical change, but it is physical. The substance keeps its formula and can be fully recovered by evaporating the water — no new substance is made. Rule of thumb: if a simple physical method (evaporation, filtration or condensation) recovers the starting material, the change is physical.
Reversibility alone is not the test
"Reversible = physical, irreversible = chemical" is an incomplete rule. Bending a wire permanently looks irreversible but is physical — no new substance forms. Some chemical changes can be partly reversed yet are still chemical. The primary test is always "has a new substance formed?"; use reversibility only as a secondary clue.
Thermal decomposition is chemical
Heating a substance to break it into simpler ones is always a chemical change, even though only heat is applied and nothing is added. Heating copper(II) carbonate turns the green solid black and gives off a gas — new substances form. Do not call it physical just because you only heated it. Melting also uses heat but makes no new substance, so it stays physical.
Mass is conserved in both changes
A change in mass is not the test for chemical versus physical — mass is conserved in both. When magnesium burns, the white ash weighs more than the magnesium only because oxygen from the air has combined with it; if that oxygen is counted, the total mass is unchanged. Never decide a change is chemical just because the mass appears to rise or fall.
One sign alone is not conclusive
A single observation does not always prove a chemical change. Bubbles can be boiling (physical), not a gas from a reaction. Dissolving some salts changes the temperature yet is physical, since the salt is recovered by evaporation. Check the primary criterion — a new substance with different properties — rather than relying on one clue.
Three-question classification scaffold
Classify with three questions. (1) New substance with new properties formed? Yes → chemical (primary test); no → physical. (2) Recoverable by a simple physical method? Then physical. (3) Significant heat or light change? Supports chemical. State the type AND the evidence.
State what you observe, not the product
Describe what is observed, not the product name — write "a gas that turns limewater milky", not "carbon dioxide is formed". State the colour before and after ("green to black"). A product name on its own earns no observation mark.
Structure: type, then evidence
Structure the answer: this is a [physical/chemical] change because a new substance [is/is not] formed, and the original [can/cannot] be recovered. For a 3-mark version, add one observable change — colour, gas, precipitate or temperature change.
Physical change — a change in which no new substance is formed, the original substance can be recovered, and the chemical composition is unchanged.
Chemical change — a change in which one or more new substances (with different chemical properties) are formed; usually accompanied by an energy change (heat or light); usually difficult or impossible to reverse.
The law of conservation of mass applies to both types of change: the total mass before equals the total mass after, because atoms are neither created nor destroyed — they are only rearranged (chemical change) or retain the same arrangement (physical change).
Define physical change.
Zinc reacts with dilute sulfuric acid to produce zinc sulfate solution and hydrogen gas. This is a chemical change.
(a) State TWO pieces of observable evidence that confirm this is a chemical change.
(b) Write a balanced symbol equation for the reaction, including state symbols.
