Paper 12, worked through. Cambridge IGCSE Chemistry, May/June 2026: worked solutions

Forty questions, one mark each, 45 minutes: just over a minute a question. The Periodic Table is on the back page, and a handful of answers come straight off it. If one is taking too long, put a mark beside it, guess, and come back at the end: a blank scores the same as a wrong answer.

1. 1Explaining why bromine vapour spreads through two gas jars once the plate between them is removed.[1]
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   1. Particles in a gas move about at random. Where there are many bromine particles, more of them wander out than wander in.
   2. So, overall, bromine moves from where it is concentrated to where it is not, until it is evenly spread. That is diffusion, and it is why both jars end up the same light brown.

   AnswerB: bromine particles move from an area of high concentration to an area of low concentration.

   Bromine is denser than air, and that is true, but it would keep the bromine at the bottom. A true statement that does not explain the change is still a wrong answer.

   Revise this: Diffusion
2. 2The relative charge and relative mass of an electron.[1]
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   1. An electron has a charge of −1. A proton is +1 and a neutron is 0.
   2. An electron is far lighter than a proton or a neutron: about 1/1840 of the mass of either.

   AnswerA: charge −1 and mass 1/1840.

   Revise this: Inside the atom
3. 3Working out what is in the nucleus and in the shells of a lithium-7 atom.[1]
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   1. The lower number, 3, is the proton number: 3 protons. A neutral atom has the same number of electrons: 3 electrons.
   2. The upper number, 7, is the nucleon number: protons plus neutrons. So there are 7 − 3 = 4 neutrons.
   3. Protons and neutrons are in the nucleus. Electrons are in the shells.

   AnswerA: 3 protons and 4 neutrons in the nucleus, and 3 electrons in the shells.

   Electrons are never in the nucleus, which removes the two rows that put them there.

   Revise this: Inside the atom
4. 4Choosing the diagram for the bond between a Group I element and a Group VII element.[1]
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   1. A Group I element is a metal with one outer electron. A Group VII element is a non-metal with seven. A metal and a non-metal form an ionic bond, so the two diagrams of overlapping shells, which show sharing, are out.
   2. The metal, M, gives its one outer electron to N. M becomes a positive ion and N becomes a negative ion with a full outer shell of eight.
   3. So you want M in the bracket marked +, and N in the bracket marked − with eight electrons, one of them drawn differently to show that it came from M.

   AnswerC

   Metals lose electrons and become positive. The diagram with N as the positive ion has it backwards.

   Revise this: Ions and ionic bonding
5. 5Counting the shared electrons in a molecule of ammonia.[1]
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   1. Ammonia is NH₃. The nitrogen atom is joined to each of the three hydrogen atoms by a single covalent bond.
   2. Each single bond is one shared pair, which is two electrons. 3 × 2 = 6.

   AnswerC: 6

   The question asks for electrons, not pairs. Nitrogen also has two electrons that are not shared, which is where 8 comes from.

   Revise this: Covalent bonding and simple molecules
6. 6The use of graphite that relies on its layers sliding.[1]
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   1. In graphite the carbon atoms are in flat layers, with only weak forces between one layer and the next, so the layers slide over each other easily.
   2. That makes graphite slippery, which is what a lubricant needs to be.

   AnswerD: lubricant.

   Electrodes use a different property of graphite: it conducts electricity. Cutting tools and jewellery are uses of diamond.

   Revise this: Diamond, graphite and giant structures
7. 7Reading a molecular formula from a diagram of a molecule.[1]
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   1. Count the atoms of each element in the drawing: 3 hydrogen, 1 phosphorus and 4 oxygen.
   2. Write them together: H₃PO₄.

   AnswerC: H₃PO₄

   Count every atom, including the oxygen that has a double bond and no hydrogen on it. Forget that one and you might settle for P(OH)₄, which also has four oxygens, but one hydrogen too many.

   Revise this: Formulae and equations
8. 8Finding the mass of oxygen needed, and of water made, when 160 g of methane burns.[1]
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   1. Find the moles of methane: 160 ÷ 16 = 10 mol.
   2. The equation says each mole of methane needs 2 moles of oxygen and makes 2 moles of water. So 20 mol of each.
   3. Oxygen: 20 × 32 = 640 g. Water: 20 × 18 = 360 g.

   AnswerD: 640 g of oxygen and 360 g of water.

   The 2 in front of O₂ and H₂O is what doubles both answers. Leave it out and you get 320 and 180, which are in the table waiting.

   Revise this: Concentrations and gas volumes
9. 9Recognising units of concentration.[1]
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   1. Concentration is how much solute there is in each cubic decimetre of solution.
   2. "How much" can be a mass in grams, giving g/dm³, or an amount in moles, giving mol/dm³.

   AnswerC: 2 and 3.

   In a concentration unit the dm³ is underneath. The unit dm³/mol has it on top: that is the volume of one mole of gas. And g/mol is the unit of molar mass.

   Revise this: The mole and reacting masses
10. 10Setting up the electroplating of nickel with silver.[1]
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    1. The object to be plated is the cathode, the negative electrode. Here that is the nickel.
    2. The anode is the metal you are plating with: silver.
    3. The electrolyte is a solution that contains ions of the plating metal: aqueous silver nitrate.

    AnswerB: nickel cathode, silver anode, aqueous silver nitrate.

    Positive silver ions are attracted to the negative electrode, and that is where the silver is deposited. So the object always goes at the cathode.

    Revise this: Electrolysis
11. 11Working out what kind of substance X is, from three facts about it.[1]
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    1. It can be electrolysed, so it must contain ions that are free to move. That means an ionic compound, either molten or dissolved in water.
    2. It is insoluble, so it cannot be an aqueous solution. It must be molten.
    3. "Binary" means it is made of two elements, which is why electrolysis gives exactly two.

    AnswerA: X is a molten ionic compound.

    Covalent compounds do not conduct, and a solid ionic compound does not either, because its ions are held in place.

    Revise this: Electrolysis
12. 12The correct statement about a fuel cell in a car.[1]
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    1. A hydrogen–oxygen fuel cell uses the reaction between hydrogen and oxygen to produce electricity. The electricity runs an electric motor, which drives the car.
    2. The hydrogen is carried in a tank; only the oxygen comes from the air. The only chemical product is water.

    AnswerD: the fuel cell produces electricity, which powers an electric motor.

    Revise this: Hydrogen fuel cells
13. 13What an exothermic reaction does to its surroundings.[1]
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    1. Exothermic means energy goes out: thermal energy is transferred to the surroundings.
    2. The surroundings gain energy, so their temperature rises.

    AnswerB: thermal energy is transferred to the surroundings, and their temperature increases.

    Two of the options contradict themselves. If the surroundings receive energy they cannot get colder, and if they give energy away they cannot get warmer.

    Revise this: Energy changes in reactions
14. 14Choosing the graph for a reaction repeated with a larger surface area of solid.[1]
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    1. A larger surface area means more collisions every second, so the reaction is faster. The line for Y starts steeper than the line for X.
    2. Nothing else has changed, so the amounts of the reactants are the same. The same total volume of gas is made, and the two lines level off at the same height.

    AnswerB: Y is steeper at first, and both finish at the same volume.

    Two separate things to check on any rate graph: how steep, which shows the rate, and where it ends, which shows the amount. Changing the surface area only changes the first.

    Revise this: Rates of reaction
15. 15What the two-way arrow in an equation means.[1]
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    1. The sign ⇌ shows that the reaction can go in both directions: the products can react to give the reactants back.

    AnswerD: the chemical reaction is reversible.

    Revise this: Reversible reactions and equilibrium
16. 16The gas made when lime and an ammonium salt are mixed.[1]
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    1. Calcium oxide is a base. Any base, warmed with an ammonium salt, drives off ammonia gas.
    2. The farmer loses the nitrogen that the fertiliser was meant to supply, which is why the two are not added together.

    AnswerA: ammonia.

    This reaction is also the test for an ammonium ion: warm with sodium hydroxide, and the gas given off turns damp red litmus paper blue.

    Revise this: Acids, bases and neutralisation
17. 17Matching a solution to its indicator colour, pH and hydrogen ion concentration.[1]
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    1. A strong acid has a very low pH, about 1, and turns universal indicator red. It is acidic because it has a high concentration of hydrogen ions.
    2. Check the other rows. A strong alkali has a pH near 14, not 8, and is purple. Blue goes with an alkali, not an acid. A pH of 5 is acidic, not alkaline.

    AnswerC: strong acid, red, pH 1, high concentration of hydrogen ions.

    The lower the pH, the higher the concentration of hydrogen ions. All three entries in a row must agree with the description, and only one row passes.

    Revise this: Acids, bases and neutralisation
18. 18The method for making a soluble salt from an insoluble base and an acid.[1]
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    1. Add the solid, magnesium oxide, to the acid until no more will react. Using an excess of the solid makes sure all the acid is used up.
    2. Filter. The unreacted magnesium oxide stays in the filter paper, and the filtrate is magnesium sulfate solution with nothing else in it.
    3. Evaporate the water from the filtrate to leave the solid salt.

    AnswerC: react the acid with excess magnesium oxide, filter, and evaporate the filtrate.

    The excess must be the thing you can filter off. Excess acid cannot be filtered out, so it would end up in the product. And the salt here is soluble, so it is never a precipitate.

    Revise this: Making salts
19. 19The charge on a strontium ion.[1]
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    1. Find strontium in the Periodic Table: it is in Group II, so its atoms have two outer electrons.
    2. A metal loses its outer electrons when it forms an ion. Losing two electrons leaves a charge of 2+.

    AnswerB: 2+

    The group number tells you: Group I metals form 1+ ions, Group II form 2+, and Group VII non-metals form 1− ions.

    Revise this: The Periodic Table
20. 20Identifying three Group I metals from a clue about density and a clue about melting point.[1]
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    1. Going down Group I, the density increases and the melting point decreases.
    2. P is less dense than Q, so P is above Q in the group.
    3. Q has a higher melting point than R, so Q is above R.
    4. The order down the group is P, Q, R: lithium, sodium, potassium.

    AnswerA: lithium, sodium, potassium.

    Turn each clue into "above" or "below" before you look at the table. Two short facts then put the three elements in order.

    Revise this: Group 1: the alkali metals
21. 21Deciding which statements about fluorine are true, using the trends in Group VII.[1]
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    1. Going down Group VII, the elements get denser and their colour darkens, and they go from gases to a liquid to a solid. Reactivity goes the other way: it falls as you go down.
    2. Fluorine is at the top. So it is a gas, not a solid, and it is less dense than chlorine.
    3. It is the most reactive halogen, so nothing lower in the group, such as iodine, can displace it.
    4. Like all the halogens, it is made of molecules of two atoms, F₂.

    AnswerD: 3 and 4.

    Revise this: Group 7: the halogens
22. 22A property of the transition element cobalt.[1]
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    1. Transition elements have high densities and high melting points, they are good conductors, and they form coloured compounds.

    AnswerA: it forms coloured compounds.

    The other three options are the opposites of what transition elements are like, so one fact settles the question.

    Revise this: The transition elements
23. 23Which property makes aluminium suitable for food containers.[1]
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    1. A food container must not react with the food or the moisture in it. Aluminium resists corrosion, because it is covered by a thin layer of aluminium oxide that protects the metal underneath.
    2. Low density and good electrical conductivity are true of aluminium, but they are the reasons for other uses: aircraft and overhead cables. They are not why it is used for food.

    AnswerD: high resistance to corrosion only.

    The table is asking why it suits this use, not what is true about aluminium. Ticking every true property picks the wrong row.

    Revise this: Uses of metals
24. 24Choosing the alkali and acid whose salt supplies a fertiliser element in both of its ions.[1]
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    1. Fertilisers supply nitrogen, phosphorus and potassium.
    2. Ammonia and nitric acid make ammonium nitrate. The ammonium ion contains nitrogen and so does the nitrate ion.
    3. A salt made from sodium hydroxide or from hydrochloric acid has a sodium ion or a chloride ion, and neither is one of the three elements.

    AnswerB: ammonia and nitric acid.

    Revise this: Fertilisers
25. 25Two properties of zinc.[1]
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    1. Zinc is above hydrogen in the reactivity series, so it reacts with dilute acids, giving a salt and hydrogen.
    2. It is a metal, and metals are good conductors of heat.

    AnswerD: reacts with dilute acid, and a good thermal conductor.

    Revise this: Properties of metals
26. 26Comparing how much four nails rust in different conditions.[1]
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    1. Iron rusts when it is in contact with both water and oxygen.
    2. W is a bare iron nail with wet cotton wool and air: it rusts. Z is a bare iron nail in damp air: it rusts as well.
    3. X is coated in grease, which keeps water and oxygen away from the iron: no rust. Y is stainless steel, which does not rust.
    4. So the only true statement compares a nail that rusts with one that does not, the right way round.

    AnswerA: more rusting is shown in W than in X.

    Sort the four tubes into "rusts" and "doesn't rust" first. Three of the statements then fall apart at once.

    Revise this: Rusting and how to stop it
27. 27The ore of aluminium and how the metal is extracted.[1]
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    1. The ore of aluminium is bauxite. Hematite is the ore of iron.
    2. Aluminium is more reactive than carbon, so carbon cannot take the oxygen from it. It is extracted by electrolysis.

    AnswerB: bauxite, electrolysis.

    Metals above carbon in the reactivity series need electrolysis. Those below it, such as iron, can be reduced with carbon in a blast furnace.

    Revise this: The blast furnace and aluminium
28. 28The true statement about substances found in natural water.[1]
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    1. Some dissolved metal compounds are useful: they supply minerals that living things need.
    2. The others are wrong. Dissolved oxygen is essential for aquatic life. Phosphates lead to deoxygenation of the water. Plastics do not break down, and they harm aquatic life.

    AnswerA: some metal compounds provide essential minerals for life.

    Be wary of "always" and "most". Sweeping statements are usually the false ones.

    Revise this: Water
29. 29Matching a pollutant to where it comes from and what harm it does.[1]
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    1. Methane is given off when vegetation decomposes, and by animals as they digest. It is a greenhouse gas, so it adds to global warming.
    2. The other rows mix things up. Carbon monoxide comes from incomplete combustion and is toxic. Oxides of nitrogen come from car engines. Sulfur dioxide does come from burning fuels that contain sulfur, but its effect is acid rain.

    AnswerB: methane, from the decomposition of vegetation, increases global warming.

    A row is only right if both the source and the effect are right. The sulfur dioxide row has the correct source and the wrong effect.

    Revise this: Air pollution and the climate
30. 30Comparing Neptune's atmosphere with the Earth's.[1]
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    1. You need the Earth's figures from memory: about 78% nitrogen, 21% oxygen, and small amounts of noble gases and carbon dioxide (about 0.04%).
    2. Clean air on Earth has almost no hydrogen, and Neptune's atmosphere is 80% hydrogen. So there is less hydrogen on the Earth.
    3. Check the rest: 18% helium is less than 21% oxygen, 1.5% methane is more than 0.04% carbon dioxide, and the Earth has only a trace of helium.

    AnswerC: there is less hydrogen on the Earth than on Neptune.

    Revise this: What air is made of
31. 31Picking out the hydrocarbons from four organic compounds.[1]
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    1. A hydrocarbon contains hydrogen and carbon only.
    2. Ethane, C₂H₆, and ethene, C₂H₄, pass. Ethanol and ethanoic acid both contain oxygen, so they are not hydrocarbons.

    AnswerC: ethane and ethene only.

    "Only" in the definition is the word that matters. Anything with an O in its formula is out.

    Revise this: Families of organic compounds
32. 32The uses of two petroleum fractions.[1]
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    1. Gas oil, also called diesel oil, is the fuel for diesel engines.
    2. Naphtha is used as a starting material for making chemicals.

    AnswerC: gas oil for diesel engine fuel, naphtha for making chemicals.

    Jet fuel is kerosene, and roads are made with bitumen. Each fraction has one use to learn.

    Revise this: Crude oil and fuels
33. 33Which of four fuels are fossil fuels.[1]
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    1. The fossil fuels are coal, natural gas and petroleum. They formed over millions of years from the remains of living things.
    2. Ethanol is made by fermentation or from ethene, and hydrogen is manufactured, so neither is a fossil fuel.

    AnswerD: 2 and 4.

    Revise this: Crude oil and fuels
34. 34Finding the compound that turns aqueous bromine colourless.[1]
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    1. Aqueous bromine loses its colour with an unsaturated compound: one with a carbon–carbon double bond, such as an alkene.
    2. Alkenes have the general formula CₙH₂ₙ. C₄H₈ fits it.
    3. C₄H₁₀ is an alkane, C₄H₉OH is an alcohol and C₄H₉COOH is a carboxylic acid. None of those has a carbon–carbon double bond.

    AnswerD: C₄H₈

    Twice as many hydrogens as carbons means an alkene. An alkane has two more than that.

    Revise this: Alkenes and cracking
35. 35Deciding which statements about ethanol are true.[1]
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    1. Statement 1 is true: ethanol is made by reacting ethene with steam at 300 °C, with an acid catalyst.
    2. Statement 2 is false: fermentation is carried out at 25 to 35 °C. At 55 °C the enzymes in yeast would be denatured.
    3. Statement 3 is true: ethanol burns to give carbon dioxide and water.
    4. Statement 4 is false: ethanol has only single bonds. Ethene is the one with a carbon–carbon double bond.

    AnswerB: 1 and 3.

    The temperature is the catch. Both ways of making ethanol are in the syllabus, each with its own conditions, and this question tests whether you can tell them apart.

    Revise this: Alcohols
36. 36What is made, along with the salt, when ethanoic acid reacts with potassium hydroxide.[1]
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    1. Ethanoic acid is an acid and potassium hydroxide is an alkali. Acid + alkali → salt + water, and nothing else.

    AnswerD: water only.

    Hydrogen would need a metal, and carbon dioxide would need a carbonate. With a hydroxide the only other product is water.

    Revise this: Carboxylic acids
37. 37How a long chain of carbon atoms with hydrogens attached is made.[1]
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    1. The structure is a long chain made of the same unit repeated: a polymer.
    2. It is made by joining many small molecules together. The small molecules are alkenes: their double bonds open up so that each one can join the next. That is addition polymerisation.

    AnswerB: addition polymerisation of an alkene.

    Cracking does the opposite: it breaks large molecules into smaller ones. Alkanes have no double bond to open, so they cannot polymerise.

    Revise this: Polymers and plastics
38. 38The steps needed to get sand out of a mixture of sand and salt.[1]
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    1. Add water. The sodium chloride dissolves and the sand does not.
    2. Filter. The sand stays in the filter paper, and the salt solution runs through.
    3. That is the sand separated. Distilling would only be needed if you wanted the water back.

    AnswerB: 1 and 2 only.

    Read which substance the question wants. To get the salt back you would go on and evaporate the filtrate, but that is not what was asked.

    Revise this: Separating mixtures
39. 39The test for chlorine.[1]
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    1. Chlorine bleaches damp litmus paper: the colour disappears and the paper turns white.

    AnswerA: it bleaches damp blue litmus paper.

    The other three are tests for different gases: ammonia turns damp red litmus blue, sulfur dioxide decolourises acidified potassium manganate(VII), and carbon dioxide turns limewater cloudy.

    Revise this: Tests for ions and gases
40. 40Identifying a metal ion from its reaction with ammonia, then giving its flame colour.[1]
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    1. A light blue precipitate with aqueous ammonia, which dissolves in excess to give a dark blue solution, is the test for copper(II) ions. So Z is a copper(II) compound.
    2. Copper(II) compounds colour a flame blue-green.

    AnswerA: blue-green.

    Two facts are chained together here. The flame colours to know: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.

    Revise this: Tests for ions and gases

These explanations are Papermunch’s own, written to teach the method. The answers have been checked against the exam board’s mark scheme, which has the final say: open it above. The question paper and mark scheme belong toCambridge University Press & Assessment: each question shown here is drawn from the paper itself as you read, and is not kept on this site.