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Detailed notes on Chemical Reactions for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.
Physical: same substance, different state. Chemical: NEW substance with new properties. Tell them apart by looking for evidence of new substances.
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
Physical: nothing new. Chemical: new substance. Look for the give-aways.
Physical change. A change of state, shape, or form WITHOUT producing a new substance.
Chemical change (chemical reaction). Produces ONE OR MORE NEW SUBSTANCES with different properties from the starting materials.
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed? | No | Yes — one or more |
| Bonds | Not broken or formed (in most cases) | Broken and formed; atoms rearrange |
| Reversibility | Usually reversible | Often hard to reverse |
| Examples | Melting, boiling, dissolving sugar | Burning, rusting, baking, photosynthesis |
Evidence of a chemical change.
Tip. Even one of these signs strongly suggests a chemical change. Two or more is essentially conclusive.
Total mass before = total mass after. Energy moves between system and surroundings.
Mass conservation. In ANY chemical reaction (closed system), the total mass of reactants = total mass of products. Atoms are neither created nor destroyed — they're just rearranged.
Worked qualitative. A piece of magnesium ribbon with mass 0.24g is burned in air. The product (magnesium oxide) has mass ∼0.40g. Where did the extra mass come from? Oxygen from the air combined with magnesium → MgO. Total mass of MgO = mass of Mg + mass of O₂ that reacted.
| Mass | |
|---|---|
| Magnesium at the start | 0.24 g |
| Oxygen from the air that reacted | 0.40 − 0.24 = 0.16 g |
| Magnesium oxide formed | 0.40 g |
Worked qualitative (apparent loss). A candle burns and seems to lose mass. Where did the mass go? Mostly into CO₂ and H₂O vapour, which escape into the air. If you trapped them, the total mass before and after would balance.
Energy. Chemical reactions usually release or absorb energy. The system's internal energy changes; mass is unaffected (for IGCSE chemistry — relativistic mass-energy effects are negligible).
Cambridge tip. Conservation of mass is a foundational law. When asked to verify a balanced equation, count atoms on both sides AND check that the total mass calculations work out from Ar values.
Verbatim phrases and definitions Cambridge mark schemes credit.
Physical/chemical change is a 2-3 mark item on most Paper 2s (classify a list, identify evidence). Examiner reports flag students treating any colour or temperature change as automatic evidence of chemical change — dilution and warming are PHYSICAL.
Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (7.1); 0620/22 May/Jun 2024 — Q1 (physical vs chemical); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.
Step-by-step solutions to past-paper-style questions on physical and chemical changes, written exactly the way a tutor would explain them at the board.
Question
Classify each change as physical or chemical, giving a reason: (a) melting ice, (b) burning magnesium ribbon, (c) dissolving sugar in water, (d) rusting of an iron nail.
Step-by-step solution
Step 1
A physical change makes no new substance — only the state, shape or size changes — and is usually easily reversed. (a) Melting ice: solid water → liquid water, same substance, reversible by freezing → physical.
Step 2
(c) Dissolving sugar: the sugar is still sugar (recoverable by evaporating the water), no new substance → physical.
Step 3
A chemical change forms one or more new substances and is usually hard to reverse. (b) Burning magnesium: Mg + O2 → white MgO, a new substance, with light and heat → chemical.
Step 4
(d) Rusting: iron reacts with oxygen and water to make hydrated iron(III) oxide (rust), a new substance → chemical.
Answer
Physical: (a) melting ice, (c) dissolving sugar — no new substance, reversible. Chemical: (b) burning magnesium, (d) rusting — a new substance is formed.
Examiner tip
Always state the REASON, not just the label: 'no new substance / reversible' for physical, 'new substance formed' for chemical.
Question
List the five types of observation that suggest a chemical reaction has taken place, giving one example of each.
Step-by-step solution
Step 1
Colour change — e.g. green copper(II) carbonate turning black copper(II) oxide on heating.
Step 2
Temperature change — heat released (exothermic, e.g. combustion) or absorbed (endothermic, e.g. some neutralisations feeling cold).
Step 3
Gas produced (effervescence) — e.g. bubbles of CO2 when an acid reacts with a carbonate.
Step 4
Precipitate formed — an insoluble solid appears when two solutions are mixed, e.g. white BaSO4.
Step 5
Light given out — e.g. the bright white flame when magnesium burns.
Answer
Colour change; temperature change; gas (effervescence); precipitate; light. Each indicates a new substance has formed.
Examiner tip
Be specific — 'gas given off' or 'effervescence' scores; 'bubbles' alone is risky because boiling also bubbles.
Question
Green copper(II) carbonate is heated strongly. It turns black and a gas is given off that turns limewater milky. Explain why this is a chemical change and write the equation.
Step-by-step solution
Step 1
Two signs of a chemical reaction are seen: a permanent colour change (green → black) and a gas produced.
Step 2
Limewater turning milky identifies the gas as carbon dioxide, so a new substance has been made — the original carbonate has broken down.
Step 3
This is thermal decomposition: one substance breaking down into two or more substances when heated.
CuCO3→CuO+CO2
Answer
It is a chemical change because new substances (black copper(II) oxide and carbon dioxide gas) are formed: CuCO3→CuO+CO2.
Examiner tip
Thermal decomposition is always chemical — heat is the energy that breaks the bonds, not just a state change.
Question
A student adds water to white anhydrous copper(II) sulfate and it turns blue. A second student dissolves blue copper(II) sulfate crystals in more water and the colour fades. Explain why neither of these is reliable evidence on its own of a chemical change.
Step-by-step solution
Step 1
Colour change is one possible sign of a reaction, but it can also happen in physical changes, so it is not proof on its own.
Step 2
Dissolving the blue crystals (fading) is physical — the copper(II) sulfate is unchanged and can be recovered by evaporating the water.
Step 3
Adding water to anhydrous copper(II) sulfate (white → blue) is a borderline case: water of crystallisation is taken in and the colour returns on heating, so for 0620 it is treated as easily reversible.
Step 4
To be confident of a chemical change, look for additional signs (gas, precipitate, large temperature change) and check whether the change is easily reversible.
Answer
Colour change alone is not proof — dissolving is physical and reversible. Look for other signs (gas, precipitate, energy change) and whether the change can be easily reversed before deciding it is chemical.
Examiner tip
Examiners reward candidates who note that one observation is rarely conclusive — physical changes can also change colour.
Question
In a sealed flask, 4.0g of calcium carbonate reacts completely with hydrochloric acid containing 5.5g of dissolved HCl in 40.5g of water. The flask and contents weigh the same before and after the reaction. Explain this using conservation of mass, and state the total mass of products.
Step-by-step solution
Step 1
In a chemical reaction atoms are only rearranged — they are not created or destroyed — so the total number and type of atoms is unchanged.
Step 2
Therefore the total mass of products equals the total mass of reactants in a closed (sealed) system. The flask is sealed, so no gas can escape.
CaCO3+2HCl→CaCl2+H2O+CO2
Step 3
Total reactant mass = 4.0+5.5+40.5=50.0g, so the products (including the trapped CO2) must also weigh 50.0g.
Answer
Atoms are conserved (rearranged, not created or destroyed), so total mass of products = total mass of reactants = 50.0g. The mass is unchanged because the flask is sealed and the carbon dioxide cannot escape.
Examiner tip
The marks are for 'atoms conserved / rearranged' AND 'closed system so no gas lost'. State both.
Question
(a) When magnesium ribbon is burned in an open crucible, the mass of the white solid product is GREATER than the mass of the magnesium. (b) When a candle burns in the open, its mass DECREASES. Explain both observations, showing that mass is still conserved overall.
Step-by-step solution
Step 1
(a) Magnesium combines with oxygen from the air: 2Mg+O2→2MgO. The oxygen atoms become part of the solid product.
Step 2
So the product mass = mass of magnesium + mass of oxygen gained. The solid is heavier because matter (oxygen) was added from outside the crucible.
Step 3
(b) A candle (hydrocarbon wax) burns to make CO2 and H2O, which are gases that escape into the air, so the solid/liquid wax remaining loses mass.
Step 4
In both cases mass is still conserved overall: if you also weighed the gases entering or leaving, total mass of reactants would equal total mass of products. The apparent change is only because the system is open.
Answer
(a) The magnesium gains mass because oxygen from the air is added to form MgO. (b) The candle loses mass because gaseous products (CO2 and water vapour) escape. Mass is conserved overall — the change is only apparent because the system is open and gases enter or leave.
Examiner tip
Top answers say mass is conserved overall and pin the apparent change on gas being gained from / lost to the surroundings.
High-scoring sample answers for physical and chemical changes on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State one observation that shows a chemical reaction has taken place. (1 mark)
Model answer
Any one of: a permanent colour change; a temperature change; a gas given off (effervescence); a precipitate forming; or light being given out.
Why this scores
One mark for any single valid sign. 'Bubbles' on its own is risky — say 'gas given off' or 'effervescence' to be safe, since boiling also bubbles.
State two differences between a physical change and a chemical change. (2 marks)
Model answer
In a physical change no new substance is formed (only the state, shape or size changes), whereas in a chemical change one or more new substances are formed. A physical change is usually easily reversed (e.g. by cooling or evaporating), whereas a chemical change is usually not easily reversed.
Why this scores
Two marks for two clear differences: (1) new substance vs no new substance; (2) easily reversible vs not easily reversible. The first difference is the most important one.
Classify each of the following as a physical or a chemical change and give a reason in each case: melting candle wax, the rusting of iron, the boiling of water. (3 marks)
Model answer
Melting candle wax — physical: only the state changes (solid to liquid), no new substance is formed and it is reversed by cooling. Rusting of iron — chemical: iron reacts with oxygen and water to form a new substance (rust, hydrated iron(III) oxide), and it is not easily reversed. Boiling water — physical: liquid water changes to steam, the same substance, and it condenses back to water on cooling.
Why this scores
One mark per correct classification WITH a valid reason. A label with no reason, or a wrong reason, does not score full marks.
Hydrated copper(II) sulfate crystals are blue. When heated, they turn white and a colourless liquid collects at the cool end of the tube; on adding water the white solid turns blue again. Explain whether the change described is physical or chemical. (4 marks)
Model answer
Heating drives off the water of crystallisation (the colourless liquid collected), turning the blue hydrated salt into white anhydrous copper(II) sulfate, so a different substance is produced and there is a colour change — features of a chemical change. However, adding water reverses the change (white turns blue again), which is characteristic of a physical change. Because it is easily reversed and only water is gained or lost, for IGCSE this is best described as a reversible chemical change (hydration/dehydration) rather than a simple physical change — the key point is that the colour change alone is not enough to decide, and the easy reversibility must be considered.
Why this scores
Marks for: identifying water of crystallisation lost; colour change observed; the change being reversible; and a reasoned conclusion. Recognising that reversibility is what makes it borderline scores highly.
When a piece of magnesium ribbon is burned in a crucible, the mass of the white powder formed is greater than the mass of the magnesium used. Use ideas about conservation of mass to explain this observation. (5 marks)
Model answer
Burning is a chemical reaction, and in a chemical reaction atoms are not created or destroyed — they are only rearranged, so mass is always conserved overall. When magnesium burns it reacts with oxygen from the air: 2Mg+O2→2MgO. The oxygen atoms become part of the white magnesium oxide powder, so the mass of the product equals the mass of the original magnesium plus the mass of oxygen added from the air. This is why the powder is heavier than the ribbon. Mass appears to increase only because the crucible is an open system, so oxygen can enter from the surroundings; if the oxygen taken in were also weighed, the total mass of reactants would equal the total mass of products.
Why this scores
Five marks across: chemical reaction / atoms conserved; magnesium reacts with oxygen from the air; correct equation or word equation; product mass = magnesium + oxygen; and the open-system explanation that mass is still conserved overall.
Rusting, combustion and thermal decomposition are all chemical changes. For each one, name a new substance that is formed and state one observation that shows a chemical reaction has occurred. (6 marks)
Model answer
Rusting: iron reacts slowly with oxygen and water to form hydrated iron(III) oxide (rust) as a new substance; the observation is a colour change as the shiny grey iron becomes orange-brown and flaky. Combustion: when a fuel such as carbon or a hydrocarbon burns in oxygen, new substances such as carbon dioxide (and water) are formed; the observations are a flame / light and heat given out (it is strongly exothermic). Thermal decomposition: when a carbonate such as copper(II) carbonate is heated it breaks down to form copper(II) oxide and carbon dioxide as new substances (CuCO3→CuO+CO2); the observations are a colour change (green to black) and a gas given off. In every case a new substance is formed, which is what makes each one a chemical change.
Why this scores
Six marks: for each of the three processes (1) a correctly named new substance AND (1) a valid observation of a chemical reaction. Linking each back to 'a new substance is formed' secures the classification.
Definitions to memorise and the exact keywords mark schemes credit for physical and chemical changes answers — sharpened from recent examiner reports for the 2026 0620 sitting.
A change in which no new chemical substance is formed — only the state, shape or size of the substance changes. Usually easily reversible (e.g. melting, boiling, dissolving, evaporation).
A change in which one or more new substances are formed. Usually not easily reversible and often accompanied by an energy change, colour change, gas, precipitate or light.
The breaking down of a single compound into two or more substances when it is heated — a chemical change (e.g. metal carbonate → metal oxide + carbon dioxide).
The total mass of the products of a chemical reaction equals the total mass of the reactants, because atoms are rearranged and not created or destroyed (in a closed system).
The production of bubbles of gas during a reaction — a common observation indicating that a chemical change is taking place.
The traps other students keep falling into on physical and chemical changes questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
0620/42 — recurring
Why it happens
Colour change is taught as a sign of reaction, so students treat it as proof.
How to avoid it
Some physical changes also change colour (e.g. dissolving coloured crystals, hydration of anhydrous copper(II) sulfate). Look for OTHER signs and check whether the change is easily reversible.
Why it happens
The solid disappears, which looks dramatic and final.
How to avoid it
Dissolving is usually physical — no new substance forms and the solute is recovered by evaporating the solvent.
Why it happens
In open systems the measured mass appears to rise (Mg burning) or fall (candle burning).
How to avoid it
Atoms are conserved, so mass is conserved overall. Apparent changes happen only because gas enters from, or escapes to, the surroundings in an open system.
Why it happens
Students remember that 'a change' indicates reaction but do not name it precisely.
How to avoid it
Be specific: gas given off / effervescence, precipitate formed, permanent colour change, temperature change, or light given out.
The things students keep getting wrong in this sub-topic, answered.