Paper 22, 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 you will need 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. 1Choosing the true statement about water molecules in ice, liquid water and steam.[1]
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   1. Think of the three states in order. In a solid the particles are packed close and held in place, but they are not still: they vibrate about fixed positions.
   2. In a gas the particles are far apart and moving fastest. So steam has the molecules furthest apart and with the highest average speed, which rules out three of the statements.

   AnswerD: the molecules in ice can vibrate.

   "Fixed in position" does not mean "not moving". Particles in a solid always vibrate, and that is exactly what this question is checking.

   Revise this: Solids, liquids and gases
2. 2What cooling does to the energy of gas molecules and to the pressure in a sealed container.[1]
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   1. Temperature is a measure of the average kinetic energy of the particles. Cool the gas and the molecules slow down, so their average kinetic energy goes down.
   2. The container is sealed and rigid, so the same molecules are in the same space. Slower molecules hit the walls less often and less hard, so the pressure falls too.

   AnswerA: both decrease.

   The volume cannot change here, because the glass container is sealed. So cooling shows up as a drop in pressure.

   Revise this: Solids, liquids and gases
3. 3Picking the particle diagram that shows a mixture of compounds.[1]
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   1. A compound is made of molecules in which different kinds of atom are joined. So look for boxes where every particle has both a white and a black circle in it.
   2. A mixture of compounds needs two different kinds of such molecule in the same box.
   3. One box has only pairs of white atoms: that is a single element. One has all its molecules the same, each with both colours: a single compound. One has white pairs and black pairs: a mixture of elements. The remaining box has two different molecules, each containing both colours.

   AnswerC: two different compounds mixed together.

   Read the word after "mixture of". A mixture of elements and a mixture of compounds are both in the diagrams, and they are easy to swap.

   Revise this: Elements, compounds and mixtures
4. 4Why water counts as a compound.[1]
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   1. In a compound the elements are chemically bonded, in a fixed ratio by mass, and the compound's properties are different from those of its elements.
   2. Because they are bonded, they cannot be pulled apart by physical means such as filtering or distilling. It takes a chemical change, such as electrolysis.

   AnswerA: the hydrogen and oxygen can only be separated by a chemical change.

   The other three statements describe a mixture: separated physically, any proportions, properties of its parts kept.

   Revise this: Elements, compounds and mixtures
5. 5Spotting which two particles behave the same chemically.[1]
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   1. Chemical properties come from the electrons, and especially from how many are in the outer shell. Atoms of the same element have the same number of protons and so the same number of electrons.
   2. W and Y both have 17 protons and 17 electrons. They are atoms of the same element, chlorine, with different numbers of neutrons: isotopes.
   3. X has 16 protons and 18 electrons, so it is an ion of a different element, and Z has 18 protons, a third element.

   AnswerB: W and Y.

   Neutrons change the mass, not the chemistry. X and Z have the same number of electrons, but they are different elements, and one of them is an ion.

   Revise this: Isotopes and relative atomic mass
6. 6Why sodium chloride melts at such a high temperature.[1]
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   1. Sodium is a metal and chlorine is a non-metal, so sodium chloride is ionic: a giant lattice of positive sodium ions and negative chloride ions.
   2. Oppositely charged ions attract each other strongly in every direction. Melting means breaking those attractions throughout the lattice, which takes a lot of energy.

   AnswerA: it contains strong ionic bonds.

   Covalent bonds are between non-metals, metallic bonds are in metals, and intermolecular forces need molecules. Sodium chloride has none of those.

   Revise this: Ions and ionic bonding
7. 7Working out the structure of an element used both as a lubricant and in cutting tools.[1]
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   1. One element with two very different forms and those two uses is carbon. Graphite is slippery, so it lubricates. Diamond is extremely hard, so it tips cutting tools.
   2. Both are giant covalent structures: every atom is joined to others by covalent bonds in a network that goes on and on.

   AnswerA: giant covalent.

   Graphite conducts electricity and feels soft, which tempts people towards "metallic" or "simple covalent". It is still a giant covalent structure, arranged in layers.

   Revise this: Diamond, graphite and giant structures
8. 8Writing the formulas of two ionic compounds from the charges on their ions.[1]
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   1. The charges have to cancel. Iron(II) is Fe²⁺ and bromide is Br⁻, so one iron needs two bromides: FeBr₂.
   2. Zinc is Zn²⁺ and nitrate is NO₃⁻, so one zinc needs two nitrates: Zn(NO₃)₂.

   AnswerB: FeBr₂ and Zn(NO₃)₂.

   The Roman numeral gives the charge on the iron, not how many iron atoms there are. Iron(II) is 2+.

   When you need two of an ion made of several atoms, put it in brackets: (NO₃)₂.

   Revise this: Formulae and equations
9. 9Recognising the definition of relative molecular mass.[1]
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   1. Relative molecular mass, Mr, is found by adding up the relative atomic masses of all the atoms in the formula.

   AnswerA: the sum of the relative atomic masses.

   The last option is the definition of relative atomic mass, for a single element. One of the others is the empirical formula and one is the molecular formula.

   Revise this: Relative formula mass
10. 10Finding the largest volume of gas a reaction can make when one reactant runs out first.[1]
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    1. Work out the moles of each reactant. Magnesium: 0.10 mol. Acid: 100 cm³ is 0.100 dm³, and 0.100 × 1.0 = 0.10 mol of HCl.
    2. The equation needs 2 mol of HCl for every 1 mol of Mg. So 0.10 mol of Mg would need 0.20 mol of acid, and there is only 0.10. The acid runs out first: it is the limiting reactant.
    3. 2 mol of HCl make 1 mol of H₂, so 0.10 mol of HCl makes 0.05 mol of H₂.
    4. One mole of gas takes up 24 dm³ at room temperature and pressure: 0.05 × 24 = 1.2 dm³.

    AnswerB: 1.2 dm³.

    2.4 dm³ is what you get by using the magnesium and forgetting to check the acid. Always find the limiting reactant first.

    Change cm³ to dm³ by dividing by 1000 before multiplying by the concentration.

    Revise this: Concentrations and gas volumes
11. 11Setting up a cell to electroplate a spoon with silver.[1]
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    1. The object to be plated goes at the cathode, the negative electrode. Positive metal ions travel to it and are deposited as metal.
    2. The anode is made of the plating metal, silver. It dissolves, which keeps the solution supplied with silver ions.
    3. The electrolyte must contain ions of the plating metal: aqueous silver nitrate.

    AnswerA: silver anode, spoon as cathode, aqueous silver nitrate.

    The thing being coated is always the cathode. A copper compound as the electrolyte would plate copper, not silver.

    Revise this: Electrolysis
12. 12Naming the gas made at the positive electrode when dilute sulfuric acid is electrolysed.[1]
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    1. Dilute sulfuric acid contains hydrogen ions, sulfate ions and, from the water, hydroxide ions.
    2. Negative ions go to the anode. The hydroxide ions are discharged in preference to the sulfate ions, and they give oxygen.
    3. The hydrogen ions go the other way and give hydrogen at the cathode.

    AnswerB: oxygen.

    Sulfate ions stay in solution. Nothing containing sulfur comes off at either electrode.

    Hydrogen is the product at the cathode, so check which electrode is being asked about.

    Revise this: Electrolysis
13. 13What happens when copper(II) sulfate solution is electrolysed with copper electrodes, not inert ones.[1]
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    1. At the cathode, copper ions from the solution gain electrons and are deposited as copper, so the cathode gets heavier.
    2. At the anode, with a copper electrode, the copper itself loses electrons and goes into solution as ions, so the anode gets lighter.
    3. Copper ions leave the solution at one electrode exactly as fast as they enter it at the other. Their concentration stays the same, so the blue colour stays the same.

    AnswerD: cathode gains mass, anode loses mass, colour unchanged.

    The solution only goes paler with inert electrodes (carbon or platinum), because then nothing replaces the copper ions.

    Revise this: Electrolysis
14. 14Calculating an overall energy change from bond energies.[1]
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    1. Add up the energy taken in to break the bonds in the reactants. One N≡N is 950, and three F–F bonds are 3 × 150 = 450. Total in: 1400 kJ.
    2. Add up the energy given out when the bonds in the products form. Two NF₃ molecules contain six N–F bonds: 6 × 280 = 1680 kJ.
    3. Energy change = energy in − energy out = 1400 − 1680 = −280 kJ/mol.

    AnswerB: −280 kJ/mol.

    Count the bonds from the balanced equation: 2NF₃ has six N–F bonds, not three and not two.

    Breaking takes energy in, making gives energy out. More out than in means a negative answer: exothermic.

    Revise this: Energy changes in reactions
15. 15Why concentrated sulfuric acid is added when an alcohol and a carboxylic acid make an ester.[1]
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    1. Ethanol and ethanoic acid react slowly and reversibly to give the ester ethyl ethanoate and water.
    2. Concentrated sulfuric acid speeds the reaction up without being used up: it is the catalyst.

    AnswerD: it acts as a catalyst.

    A catalyst does not shift the position of equilibrium. It only gets the mixture there faster.

    Revise this: Esters
16. 16Why a more concentrated reactant reacts faster, in terms of particles.[1]
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    1. Higher concentration means more particles in the same volume.
    2. More particles in the same space collide more often, so there are more collisions every second, and more of them are successful every second.

    AnswerD: there are more collisions per second.

    Concentration changes how often particles collide, not how hard. The fraction of collisions with enough energy only goes up when the temperature does, and the activation energy only comes down with a catalyst.

    Revise this: Rates of reaction
17. 17Picking the true statements about how ammonia is made in industry.[1]
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    1. Nitrogen comes from the air, which is about 78% nitrogen. Statement 2 is true.
    2. Hydrogen is made from methane, in natural gas, reacted with steam. Statement 3 is true, and statement 1 is not how it is done on that scale.
    3. The conditions are about 450 °C, a pressure of about 200 atmospheres and an iron catalyst. Statement 4 has the catalyst right and the pressure wrong.

    AnswerD: 2 and 3 only.

    When a statement packs in two facts, both have to be right. Statement 4 is there to catch people who stop reading at "iron catalyst".

    Revise this: Making ammonia and sulfuric acid
18. 18Identifying what is oxidised and what does the oxidising in an ionic equation.[1]
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    1. Follow the oxidation numbers. Sulfur is +4 in SO₂ and +6 in SO₄²⁻. It has gone up, so sulfur dioxide is oxidised.
    2. Chromium is +6 in Cr₂O₇²⁻ and +3 in Cr³⁺. It has gone down, so the dichromate ion is reduced.
    3. The oxidising agent is the thing that oxidises something else, and is itself reduced: the dichromate ion.

    AnswerC: SO₂ is oxidised, and Cr₂O₇²⁻ is the oxidising agent.

    The oxidising agent is the one that gets reduced. It sounds backwards, so say it to yourself before you choose.

    The hydrogen ions are +1 on both sides. They are not oxidised or reduced.

    Revise this: Oxidation and reduction
19. 19Comparing a weak acid with a strong acid reacting with the same amount of magnesium.[1]
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    1. Hydrochloric acid is a strong acid: all of it is split into ions. Ethanoic acid is a weak acid: only a small part of it is, so at the same concentration it has far fewer hydrogen ions. Fewer hydrogen ions means a slower reaction.
    2. The amount of hydrogen depends on the reactant that runs out. The acid is in excess both times, so the magnesium decides it, and the mass of magnesium has not changed.

    AnswerD: slower, and still 70 cm³.

    Weak means slower, not less. Given time, the weak acid makes just as much gas, because it keeps releasing hydrogen ions as they are used up.

    Revise this: Acids, bases and neutralisation
20. 20Naming the products when a metal carbonate reacts with an acid.[1]
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    1. Acid + carbonate always gives a salt, water and carbon dioxide.
    2. The salt comes from the metal in the carbonate and the acid used: magnesium and hydrochloric acid give magnesium chloride.

    AnswerC: carbon dioxide, water and magnesium chloride.

    Hydrogen comes from an acid with a metal, not with a carbonate. A carbonate gives carbon dioxide.

    Revise this: Acids, bases and neutralisation
21. 21Completing a description of an acid–alkali titration.[1]
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    1. The fixed volume of solution that goes into the conical flask is measured with a volumetric pipette, which delivers one exact volume.
    2. The other solution is run in from a burette. Near the end it is added a drop at a time, so that you stop at exactly the right point.
    3. The end-point is when the indicator just changes colour. Methyl orange goes from red in acid to yellow in alkali.

    AnswerC: volumetric pipette, dropwise, changes.

    The burette holds the solution being added, not the one in the flask. And an indicator changes colour at the end-point: it does not go colourless.

    Revise this: Titrations
22. 22Predicting which two solutions give a precipitate when mixed.[1]
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    1. A precipitate forms when two of the ions in the mixture can make an insoluble salt. So swap the partners and check each new pair.
    2. Ammonium carbonate with calcium sulfate would give calcium carbonate and ammonium sulfate. Calcium carbonate is insoluble, so it comes out as a solid.
    3. All sodium, potassium and ammonium salts are soluble, and so are all nitrates. Every other pairing only makes soluble salts.

    AnswerA: 1 and 2, which give a precipitate of calcium carbonate.

    Learn the short list of insoluble salts: most carbonates; silver and lead chlorides; barium, calcium and lead sulfates. (Calcium sulfate dissolves just enough to give the dilute solution used here.) Anything with sodium, potassium, ammonium or nitrate stays dissolved.

    Revise this: Making salts
23. 23Picking the true statements about how the Periodic Table is arranged.[1]
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    1. Rows across are periods, and columns down are groups. Statement 1 has them the wrong way round.
    2. The elements are in order of increasing proton number. Statement 2 is true.
    3. The second column is Group II. Its atoms have two outer electrons and lose both, giving ions with a 2+ charge. Statement 3 is true.
    4. Across a period the elements go from metals on the left to non-metals on the right, so they become less metallic. Statement 4 is false.

    AnswerC: 2 and 3.

    Groups go down, periods go across. Metals are on the left.

    Revise this: The Periodic Table
24. 24Choosing the true statement about the alkali metals.[1]
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    1. Going down Group I, melting points fall, reactivity rises and density generally rises.
    2. Sodium is above potassium, so sodium has the higher melting point.

    AnswerC: sodium has a higher melting point than potassium.

    Every Group I metal has one outer electron: that is why they are in Group I. And lithium, at the top, is the least dense and the least reactive of them.

    Revise this: Group 1: the alkali metals
25. 25How an alloy compares with a pure metal for being hammered into shape, and why.[1]
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    1. In a pure metal all the atoms are the same size and sit in neat layers. The layers slide over each other easily, which is why pure metals are soft and malleable.
    2. An alloy has atoms of a different size mixed in. They break up the regular layers, so the layers cannot slide as easily.
    3. Harder to slide means harder and less malleable.

    AnswerB: less malleable, because the layers of atoms slide less easily.

    Alloys are made to be harder and stronger than the pure metal. If your answer makes the alloy softer, it is the wrong way round.

    Revise this: Alloys
26. 26Explaining why aluminium seems unreactive with acid when zinc, a less reactive metal, reacts.[1]
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    1. Aluminium is above zinc in the reactivity series, so on reactivity alone it should react more readily, not less.
    2. The surface of aluminium is covered with a thin, tough layer of aluminium oxide. It sticks to the metal and keeps the acid away from it.
    3. Zinc has no such protective layer, so the acid reaches the metal and hydrogen is given off.

    AnswerB: only the aluminium is coated with a layer of its oxide.

    Two of the other statements are true facts about aluminium. But they would predict that aluminium reacts faster, so they cannot be the explanation.

    Revise this: The reactivity series
27. 27Recognising the reaction that produces iron in the blast furnace.[1]
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    1. Hematite is iron(III) oxide, Fe₂O₃.
    2. In the furnace, coke burns to carbon dioxide, which reacts with more coke to make carbon monoxide. The carbon monoxide takes the oxygen from the iron oxide, leaving iron and carbon dioxide.

    AnswerA: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

    Hematite is Fe₂O₃, not FeO. And iron oxide does not simply fall apart into iron and oxygen when heated: it needs a reducing agent.

    Revise this: The blast furnace and aluminium
28. 28Deciding what is reduced, and where, when aluminium oxide is electrolysed with carbon electrodes.[1]
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    1. At the cathode, aluminium ions gain electrons and become aluminium. That is the reduction of aluminium ions, which is not one of the choices.
    2. At the anode, oxide ions lose electrons and become oxygen gas. Losing electrons is oxidation, so the oxide ions are oxidised, not reduced.
    3. The clue is the carbon dioxide. The hot oxygen made at the anode reacts with the carbon the anode is made of: C + O₂ → CO₂. In that reaction the carbon is oxidised and the oxygen gas is reduced.

    AnswerB: oxygen gas is reduced at the carbon anode.

    This is the hardest question on the paper. Oxide ions are oxidised at the anode, so both statements about "reduction of oxide ions" are out straight away.

    That reaction is also why the carbon anodes burn away and have to be replaced.

    Revise this: The blast furnace and aluminium
29. 29Recognising the equation for photosynthesis.[1]
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    1. Photosynthesis takes in carbon dioxide and water, and makes glucose and oxygen, using energy from light.
    2. So carbon dioxide and water are on the left, and glucose, C₆H₁₂O₆, and oxygen are on the right.

    AnswerB: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

    The same equation written backwards is respiration, and it is one of the choices. Another is fermentation: glucose to ethanol and carbon dioxide.

    Revise this: Air pollution and the climate
30. 30What the catalyst in a car's catalytic converter does.[1]
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    1. In the hot engine, nitrogen and oxygen from the air combine to make oxides of nitrogen, which cause acid rain and breathing problems.
    2. The catalytic converter in the exhaust turns those oxides back into harmless nitrogen. It also turns carbon monoxide into carbon dioxide.

    AnswerC: they remove oxides of nitrogen made in the engine.

    A catalyst is not used up, so it does not need regular replacing. And the converter is in the exhaust: it deals with the oxides after they have formed, it does not stop them forming.

    Revise this: Air pollution and the climate
31. 31Matching one way of cutting global warming and one way of cutting acid rain.[1]
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    1. Global warming is caused by greenhouse gases, chiefly carbon dioxide and methane. Livestock give off methane, so farming fewer animals reduces it.
    2. Acid rain is caused by sulfur dioxide and oxides of nitrogen. Catalytic converters remove oxides of nitrogen from car exhausts.
    3. Check each row needs both halves to be right, in the right column.

    AnswerC: less livestock farming for global warming, catalytic converters for acid rain.

    Flue gas desulfurisation and low-sulfur fuels do help with acid rain, so look at which column they have been put in. And burning more wood or coal makes things worse, not better.

    Revise this: Air pollution and the climate
32. 32Comparing two hydrocarbons drawn out in full.[1]
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    1. Count the atoms in each. The first has six carbon atoms (five in the chain and one in the CH₃ branch) and only single bonds: C₆H₁₄, an alkane. The second has six carbon atoms and a C=C double bond: C₆H₁₂, an alkene.
    2. An alkene is unsaturated, so they are not both saturated. Their molecular formulas are different, so they are not isomers. Alkanes are CₙH₂ₙ₊₂ and alkenes are CₙH₂ₙ, so the general formulas differ.
    3. Each carbon atom ends up in one molecule of carbon dioxide when the compound burns completely. Six carbons each means the same amount of carbon dioxide from a mole of either.

    AnswerD: they give the same volume of carbon dioxide on complete combustion.

    Don't forget the carbon in the side branch when you count. Missing it makes the first molecule look like a five-carbon compound.

    Isomers must have exactly the same molecular formula. Same number of carbons is not enough.

    Revise this: Families of organic compounds
33. 33Finding the one structure that matches its name.[1]
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    1. Check that every carbon has four bonds and that the functional group is right.
    2. Propan-1-ol is three carbons with an –OH on the end one: CH₃CH₂CH₂OH. That one is correct as written.
    3. The propanoic acid is a hydrogen short on its middle carbon. The propene has too many hydrogens on its middle carbon. The propan-2-ol has been given two –OH groups.

    AnswerC: propan-1-ol.

    Count bonds on each carbon in turn: four, every time. It finds the wrong structures faster than trying to remember the right ones.

    Revise this: Naming organic compounds
34. 34Recognising a substitution reaction from its equation.[1]
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    1. In a substitution, one atom in a molecule is swapped for another, so there are two products.
    2. In the first equation, chlorine replaces a hydrogen atom in chloroethane, and the hydrogen leaves as hydrogen chloride. One atom out, one atom in: a substitution.
    3. The others are different kinds of reaction. Ethanol losing water to give ethene is a dehydration. Ethanol and oxygen is combustion. Ethene and hydrogen bromide joining into one product is an addition.

    AnswerA: the reaction of chloroethane with chlorine.

    Addition gives one product. Substitution gives two. That alone separates the first equation from the last.

    In the copy on file, the organic product in that equation is printed with one chlorine too many. Swapping one hydrogen for one chlorine gives CH₃CHCl₂, and it does not change the answer.

    Revise this: Alkanes
35. 35Working back from a dibromo compound to the alkene that made it.[1]
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    1. Bromine adds quickly across a C=C double bond. One bromine atom joins each of the two carbons that were double-bonded.
    2. In the product, the bromines are on the second and third carbons of a four-carbon chain. So that is where the double bond was: CH₃CH=CHCH₃, but-2-ene.

    AnswerC: CH₃CH=CHCH₃.

    The bromines mark where the double bond used to be. But-1-ene would put them on the first and second carbons.

    Butane has no double bond and only reacts with bromine slowly, in ultraviolet light, by substitution.

    Revise this: Alkenes and cracking
36. 36Picking out the structure of the polyester PET.[1]
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    1. PET is a polyester. Its monomers are joined by ester links: a C=O with an oxygen atom next to it in the chain, –COO–.
    2. Look along each chain for what joins the boxes. Only one has C=O groups followed by –O– all the way along.
    3. Chains joined through –CO–NH– are polyamides, such as nylon and proteins, not polyesters.

    AnswerA: the chain with ester links.

    Ester link has O in the chain, amide link has N–H. That one difference answers this question and the next one.

    Revise this: Polymers and plastics
37. 37Picking out the structure of nylon.[1]
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    1. Nylon is a polyamide. Its monomers are joined by amide links: a C=O bonded directly to an N–H, –CO–NH–.
    2. It is made from two monomers, one with a –COOH group at each end and one with an –NH₂ group at each end, so the two kinds of box alternate along the chain.
    3. Check the nitrogen is attached straight to the C=O carbon. In one of the diagrams there is a CH₂ in between, which is not an amide link.

    AnswerD: two C=O groups each bonded to an N–H.

    The diagram with –O– between the boxes is a polyester. Look for the nitrogen.

    Revise this: Polymers and plastics
38. 38Choosing the correct way to set up paper chromatography.[1]
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    1. The bottom edge of the paper must dip into the solvent, so that the solvent can soak up the paper.
    2. The spot of ink must start above the level of the solvent. Then the rising solvent carries the dyes up the paper with it.
    3. If the spot is at or below the solvent's surface, the ink simply washes off into the beaker. If the paper does not reach the solvent, nothing moves at all.

    AnswerC: paper in the solvent, with the spot above the solvent level.

    Spot above, paper in. Both have to be true.

    The starting line is drawn in pencil, because pencil does not dissolve and run in the solvent.

    Revise this: Chromatography
39. 39Choosing the process that gets drinking water from sea water.[1]
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    1. Sea water is water with salts dissolved in it. Dissolved salts pass straight through a filter, so filtration cannot remove them.
    2. In distillation the water is boiled, the steam is led away and condensed, and the salts are left behind. The condensed water is pure.

    AnswerB: distillation.

    Crystallisation is for collecting the salt, not the water. Chlorination kills microbes but leaves the salt in.

    Revise this: Separating mixtures
40. 40Identifying a metal ion from its flame colour.[1]
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    1. The flame colours to know: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.
    2. Orange-red is calcium.

    AnswerD: Ca²⁺.

    Lithium is red and calcium is orange-red. They are the two that get mixed up, so learn them as a pair.

    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.