October/November 2025 Paper 11 Worked Answers (IGCSE Chemistry 0620 Core)

40 questions · 40 marks · 45 minutes

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1. Question 1

   1 marksDiffusion
   Step 1: Diffusion is the net movement of particles from a high concentration to a low concentration. Step 2: Particles move more slowly through a liquid than through a gas because liquid particles are closer together. Step 3: So the very slow movement described is a substance diffusing through a liquid.
   Method:

   Identify the process as diffusion, then pick the medium where particles move very slowly.
   Examiner tips

   • Diffusion is fastest in gases and slowest in liquids; solids do not diffuse noticeably.

2. Question 2

   1 marksSubatomic particles in ions
   Step 1: The proton number is 19, so there are 19 protons. Step 2: Neutrons = nucleon number − proton number = 39 − 19 = 20. Step 3: The $\text{K}^+$ ion has lost one electron, so it has 19 − 1 = 18 electrons.
   Method:

   Read the proton number for protons, subtract it from the nucleon number for neutrons, then remove one electron for the + charge.
   Examiner tips

   • For a positive ion, subtract the charge from the number of electrons.

3. Question 3

   1 marksIon charge and electronic configuration
   Step 1: Group II atoms have 2 outer electrons and lose both to form a 2+ ion. Step 2: A magnesium atom is 2,8,2; losing the 2 outer electrons leaves 2,8. Step 3: So the magnesium ion has a charge of +2 and the electronic configuration 2,8.
   Method:

   Use the group number for the ion charge, then remove the outer electrons to find the ion configuration.
   Examiner tips

   • Group number gives the outer electrons; metals lose them to leave a full shell.

4. Question 4

   1 marksElectron transfer in ionic bonding
   Step 1: Sodium is a metal with 1 outer electron, which it loses to form a positive ion. Step 2: Chlorine is a non-metal with 7 outer electrons, which gains 1 electron to form a negative ion. Step 3: So sodium loses electrons and chlorine gains electrons.
   Method:

   Classify each element as metal or non-metal, then decide which loses and which gains electrons.
   Examiner tips

   • In ionic bonding electrons transfer from the metal to the non-metal.

5. Question 5

   1 marksProperties of ionic and covalent compounds
   Step 1: Ionic strontium nitrate conducts electricity when molten or dissolved because the ions become free to move, and it has a high melting point. Step 2: Simple covalent cyclohexane has weak forces between molecules, so it has a low boiling point and does not conduct electricity. Step 3: The matching pair is strontium nitrate conducting in aqueous solution and cyclohexane having a low boiling point.
   Method:

   Assign each substance its bonding type, then match the property that fits ionic and covalent.
   Examiner tips

   • Decide ionic vs covalent first, then recall the typical melting point, solubility and conductivity.

6. Question 6

   1 marksCovalent bonding
   Step 1: Covalent bonds form between non-metal atoms that share electrons. Step 2: Nitrogen is a non-metal and shares electrons with hydrogen to form ammonia. Step 3: Magnesium and sodium are metals (they form ionic bonds) and neon is an unreactive noble gas, so nitrogen is correct.
   Method:

   Pick the reactive non-metal, since only non-metals share electrons with hydrogen.
   Examiner tips

   • Covalent compounds with hydrogen come from non-metals such as nitrogen, oxygen and carbon.

7. Question 7

   1 marksGraphite and diamond
   Step 1: Graphite and diamond are both forms of carbon with giant covalent structures, so both have high melting points. Step 2: Only graphite conducts electricity, because it has free electrons; diamond does not. Step 3: Both are covalent, so the correct statement is that both have giant structures.
   Method:

   Recall that both are giant covalent carbon structures and rule out the false statements.
   Examiner tips

   • Both allotropes of carbon are giant covalent; graphite conducts because of delocalised electrons.

8. Question 8

   1 marksBalanced equations and state symbols
   Step 1: Calcium hydroxide is $\text{Ca(OH)}_2$ and calcium carbonate is an insoluble solid, $\text{CaCO}_3(\text{s})$. Step 2: The balanced equation is $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$. Step 3: The correct formula and the (s) state symbol for the precipitate make the first equation right.
   Method:

   Check the formulae, balance the equation, then confirm the state symbol of the carbonate.
   Examiner tips

   • Write each correct formula first, then balance and add the right state symbols.

9. Question 9

   1 marksReacting masses
   Step 1: Moles of magnesium = 9.6 ÷ 24 = 0.4 mol. Step 2: The equation shows 2 mol of Mg form 2 mol of MgO, a 1:1 ratio, so 0.4 mol of MgO forms. Step 3: Mass of MgO = 0.4 × 40 = 16.0 g.
   Method:

   Find moles of Mg, use the 1:1 ratio for MgO, then convert moles back to mass.
   Examiner tips

   • Always work in moles, then multiply by the relative formula mass to get a mass.

10. Question 10

    1 marksElectrolysis of molten compounds
    Step 1: In molten zinc chloride the only ions are zinc ions and chloride ions. Step 2: Positive chloride ions are not present; chloride ions are negative and move to the positive electrode, where chlorine forms. Step 3: Zinc ions are positive and move to the negative electrode, where zinc metal forms.
    Method:

    List the ions present, then send each to the correct electrode to find the products.
    Examiner tips

    • For a molten binary salt: metal at the cathode (negative), non-metal at the anode (positive).

11. Question 11

    1 marksHydrogen-oxygen fuel cell reaction
    Step 1: A fuel cell combines hydrogen and oxygen to release energy and form water. Step 2: Oxygen exists as $\text{O}_2$ molecules, not single O atoms. Step 3: The correctly balanced equation is $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$.
    Method:

    Identify that the fuel cell forms water and choose the balanced equation with O2.
    Examiner tips

    • In a fuel cell the reactants combine to form water; the reverse equation is electrolysis.

12. Question 12

    1 marksCombustion of fuels
    Step 1: Burning a fuel is combustion, which is an exothermic reaction. Step 2: Exothermic reactions always release thermal (heat) energy. Step 3: Carbon dioxide, carbon monoxide and oxides of nitrogen depend on the fuel and conditions, so only thermal energy is always produced.
    Method:

    Recognise combustion as exothermic and select the energy that is always given out.
    Examiner tips

    • Combustion always gives out heat; the products formed depend on the fuel.

13. Question 13

    1 marksReaction pathway diagrams
    Step 1: When the products are at a lower energy than the reactants, energy has been released. Step 2: Energy released to the surroundings means the reaction is exothermic. Step 3: So thermal energy is given out to the surroundings and the reaction is exothermic.
    Method:

    Read the relative energy levels, decide if energy is released or absorbed, then name the reaction type.
    Examiner tips

    • On a pathway diagram, products below reactants = exothermic = energy out.

14. Question 14

    1 marksFactors affecting rate of reaction
    Step 1: A reaction is fastest when the surface area and the temperature are both highest. Step 2: Powder has a larger surface area than ribbon, and 50 °C is hotter than 45 °C. Step 3: So powder at 50 °C reacts fastest and finishes in the shortest time.
    Method:

    Compare surface area and temperature across the experiments and choose the highest of both.
    Examiner tips

    • Increasing surface area and temperature both increase the rate of reaction.

15. Question 15

    1 marksReversible reactions
    Step 1: Heating drives off the water to give white anhydrous copper(II) sulfate, and adding water reforms the blue hydrated form. Step 2: A reaction that can go both forwards and backwards is reversible. Step 3: So the reaction is reversible.
    Method:

    Notice the change goes both ways, which is the definition of a reversible reaction.
    Examiner tips

    • The blue/white water-of-crystallisation change is the classic reversible reaction.

16. Question 16

    1 marksOxidation numbers in names
    Step 1: Roman numerals in a chemical name show the oxidation number of the metal. Step 2: In iron(III) chloride the iron has an oxidation number of +3. Step 3: So the (III) is the oxidation number of iron, not an energy, rate or atom count.
    Method:

    Recall what Roman numerals indicate in chemical names and pick the oxidation-number option.
    Examiner tips

    • Read Roman numerals in names as the oxidation number of the metal.

17. Question 17

    1 marksUnits of concentration
    The unit of the concentration of a solution measured in moles is $\text{mol} / \text{dm}^3$.
    Method:

    Recall that molar concentration is moles per cubic decimetre.
    Examiner tips

    • Concentration in moles always has the units mol/dm3.

18. Question 18

    1 marksPrecipitation of insoluble salts
    Step 1: Both lead(II) nitrate and potassium chloride are soluble in water. Step 2: When mixed they form insoluble lead(II) chloride, which is a precipitate. Step 3: The other pairs either include an insoluble reactant or form only soluble products, so no precipitate appears.
    Method:

    Check each pair is soluble, then test whether swapping ions gives an insoluble product.
    Examiner tips

    • Use solubility rules: most nitrates are soluble; lead(II) chloride is insoluble.

19. Question 19

    1 marksAcidic and basic oxides
    Step 1: Basic oxides are oxides of metals. Step 2: Lithium oxide (1) and sodium oxide (3) are metal oxides, so they are basic. Step 3: Phosphorus oxide (2) and sulfur dioxide (4) are non-metal oxides, which are acidic, so the basic oxides are 1 and 3.
    Method:

    Classify each oxide by metal or non-metal, then keep the metal oxides as the basic ones.
    Examiner tips

    • Sort oxides by whether the element is a metal (basic) or non-metal (acidic).

20. Question 20

    1 marksProperties of transition elements
    Step 1: Transition elements have high densities and high melting points, and form coloured compounds. Step 2: They are also often used as catalysts. Step 3: So the correct statement is that cobalt can act as a catalyst.
    Method:

    Match cobalt to the standard transition-element properties and pick the true statement.
    Examiner tips

    • Recall the four key transition-metal properties: high density, high melting point, coloured compounds, catalysts.

21. Question 21

    1 marksElectronic configuration of noble gases
    Step 1: Noble gases have a full (stable) outer shell of electrons. Step 2: Diagram 1 (2) is a full first shell like helium, and diagram 3 (2,8,8) is a full outer shell like argon. Step 3: Diagram 2 (2,8,7) has 7 outer electrons, an incomplete shell, so the noble gases are 1 and 3.
    Method:

    Check each configuration for a complete outer shell and select those that are full.
    Examiner tips

    • A full outer shell means 2 electrons for the first shell or 8 for later shells.

22. Question 22

    1 marksComposition of alloys
    Step 1: An alloy is a mixture of a metal with one or more other elements, usually metals. Step 2: Brass is made from copper and zinc. Step 3: So brass is a mixture of the metals copper and zinc.
    Method:

    Recall the two metals in brass and that an alloy is a mixture.
    Examiner tips

    • Brass = copper + zinc, and alloys are mixtures of elements.

23. Question 23

    1 marksReactivity series from observations
    Step 1: A metal that reacts with cold water is more reactive than one that reacts only with acid, which is more reactive than one that does not react at all. Step 2: W reacts strongly with both (most reactive); Z reacts with cold water and acid (next); Y reacts only with acid; X does not react (least reactive). Step 3: From least to most reactive the order is X, Y, Z, W.
    Method:

    Order the metals by how vigorously they react, using the cold-water and acid observations.
    Examiner tips

    • Use cold-water reactions to separate the most reactive metals from the rest.

24. Question 24

    1 marksBarrier methods of rust prevention
    Step 1: A barrier method coats the iron to keep out air and water. Step 2: Greasing (2) and painting (3) both form a coating that acts as a barrier. Step 3: Alloying with chromium (1) changes the metal itself rather than coating it, so the barrier methods are 2 and 3.
    Method:

    Identify which methods form a coating over the iron and exclude alloying.
    Examiner tips

    • Greasing, painting and coating with another metal are barrier methods; alloying is not.

25. Question 25

    1 marksExtraction of aluminium
    Step 1: Bauxite is the ore of aluminium. Step 2: Aluminium is too reactive to be extracted by reduction with carbon, so it is extracted by electrolysis. Step 3: So the metal extracted from bauxite by electrolysis is aluminium.
    Method:

    Match the ore bauxite to aluminium and recall it is extracted by electrolysis.
    Examiner tips

    • Aluminium from bauxite is the standard example of metal extraction by electrolysis.

26. Question 26

    1 marksProperties and treatment of water
    Step 1: Dissolving a solute such as sodium chloride raises the boiling point of water above 100 °C. Step 2: Adding water to anhydrous cobalt(II) chloride turns it from blue to pink, not blue. Step 3: Not all dissolved metal ions are beneficial, and sedimentation removes insoluble (not soluble) impurities, so only the first statement is correct.
    Method:

    Test each statement against boiling-point, the cobalt chloride water test, and water treatment stages.
    Examiner tips

    • Dissolved salt raises the boiling point; cobalt chloride goes blue → pink with water.

27. Question 27

    1 marksStructure of ethanol
    Step 1: Ethanol is $\text{C}_2\text{H}_5\text{OH}$, an alcohol with two carbon atoms and one –OH group. Step 2: The two carbons are joined by a single bond, with the –OH on the end carbon, giving $\text{CH}_3\text{CH}_2\text{OH}$. Step 3: The structure with $\text{CH}_3$ and $\text{CH}_2\text{OH}$ on a single C–C bond is ethanol.
    Method:

    Build ethanol from two carbons, a single bond and an –OH group, then match the structure.
    Examiner tips

    • Alcohols have an –OH group; ethanol has two carbons joined by a single bond.

28. Question 28

    1 marksProperties and reactions of alkanes
    Step 1: Alkanes are saturated hydrocarbons that contain only carbon and hydrogen joined by single covalent bonds. Step 2: In ultraviolet light they react with chlorine by substitution, where a chlorine atom replaces a hydrogen atom. Step 3: So the correct statement is that they react with chlorine in a substitution reaction.
    Method:

    Recall the make-up and reactions of alkanes and eliminate the false statements.
    Examiner tips

    • Alkanes: only C and H, single covalent bonds, substitution with chlorine in UV light.

29. Question 29

    1 marksAir pollutants and their effects
    Step 1: Methane is a greenhouse gas that contributes to global warming. Step 2: Sulfur dioxide causes acid rain (not cancer), particulates cause breathing problems and smog (not acid rain), and carbon monoxide is toxic (it does not cause photochemical smog). Step 3: So the correct pairing is methane causing global warming.
    Method:

    Pair each pollutant with its correct effect and keep the true row.
    Examiner tips

    • Learn each pollutant with its main adverse effect; methane → global warming.

30. Question 30

    1 marksGeneral formula of a homologous series
    Step 1: A general formula gives the same hydrogen-to-carbon relationship for every member. Step 2: $\text{C}_2\text{H}_4$, $\text{C}_3\text{H}_6$ and $\text{C}_4\text{H}_8$ all fit $\text{C}_n\text{H}_{2n}$ (the alkenes). Step 3: The other groups mix different general formulae, so only this group shares one general formula.
    Method:

    Test each compound against CnH2n and CnH2n+2 and pick the group that all match one formula.
    Examiner tips

    • Alkanes are CnH2n+2 and alkenes are CnH2n; sort each compound by its pattern.

31. Question 31

    1 marksFractional distillation of petroleum
    Step 1: Gasoline is used as a fuel for cars, not for waxes and polishes, so row 1 is wrong. Step 2: Bitumen is collected at the bottom of the column, below gasoline, not above it, so row 2 is wrong. Step 3: Kerosene as jet fuel collected below gasoline (row 3) and refinery gas for heating and cooking at the top (row 4) are both correct, so only rows 3 and 4 are right.
    Method:

    Verify both the use and the column position for each row, then keep the fully correct rows.
    Examiner tips

    • Top of the column = light fractions (refinery gas, gasoline); bottom = bitumen.

32. Question 32

    1 marksProducts of cracking
    Step 1: Cracking breaks long alkanes into smaller alkanes, alkenes and sometimes hydrogen. Step 2: Alkenes (3) and hydrogen (1) are produced, but alcohols (2) and carboxylic acids (4) are not. Step 3: So the substances produced are 1 and 3.
    Method:

    Recall the products of cracking and select hydrogen and alkenes.
    Examiner tips

    • Cracking gives shorter alkanes plus alkenes (and hydrogen); no oxygen compounds form.

33. Question 33

    1 marksManufacture of ethanol
    Step 1: The catalytic addition of steam to ethene needs a high temperature and a high pressure. Step 2: The conditions used are about 300 °C and 6000 kPa, with a phosphoric acid catalyst. Step 3: So the correct row is 300 °C and 6000 kPa.
    Method:

    Recall the high temperature and high pressure used to hydrate ethene to ethanol.
    Examiner tips

    • Hydration of ethene: about 300 °C, 6000 kPa, phosphoric acid catalyst.

34. Question 34

    1 marksReactions of ethanoic acid
    Step 1: Ethanoic acid is a weak acid and shows typical acid reactions. Step 2: With reactive metals it forms a salt and hydrogen gas. Step 3: With carbonates it gives carbon dioxide (not hydrogen), with oxides it gives salt and water (not oxygen), and being acidic it does not turn red litmus blue, so the metal reaction is correct.
    Method:

    Apply the typical reactions of acids and select the correct products with a metal.
    Examiner tips

    • Acid + reactive metal → salt + hydrogen is a standard acid reaction.

35. Question 35

    1 marksAddition polymerisation
    Step 1: The monomer $\text{CH}_2{=}\text{CH}_2$ has a C=C double bond, so it is ethene, not ethane, making statement 1 wrong. Step 2: Joining unsaturated monomers into one long chain with no other product is addition polymerisation, so statement 2 is correct. Step 3: Poly(ethene) is unreactive and does not easily decompose, so statement 3 is correct, giving 2 and 3.
    Method:

    Check each statement: identify the monomer, the type of polymerisation, and the polymer property.
    Examiner tips

    • A monomer with a C=C bond undergoes addition polymerisation; the polymer is unreactive.

36. Question 36

    1 marksMelting and boiling points as purity tests
    Step 1: A pure substance melts or boils sharply at a fixed temperature. Step 2: The wax melts over a range (45–49 °C), which shows it is impure, while the liquid boils at a fixed 141 °C, which shows it is pure. Step 3: So the wax is impure and the liquid is pure.
    Method:

    Compare each result with the rule that pure substances change state at a fixed temperature.
    Examiner tips

    • Sharp fixed temperature = pure; a melting or boiling range = impure.

37. Question 37

    1 marksInterpreting a chromatogram
    Step 1: A single spot means a pure substance, while two or more spots mean a mixture. Step 2: Dyes P, Q and R each give one spot, so they are pure, but dye S gives two spots, so it is a mixture. Step 3: One of S's spots is level with Q, so S contains dye Q plus one other substance.
    Method:

    Count the spots for each dye and match spot heights to identify what S contains.
    Examiner tips

    • One spot = pure; matching heights show the same substance is present.

38. Question 38

    1 marksFiltration
    Step 1: Filtration separates an insoluble solid from a liquid. Step 2: Copper(II) hydroxide is an insoluble solid and aqueous calcium chloride is a solution, so the solid stays on the filter paper and the solution passes through. Step 3: The other mixtures are two solutions, two miscible liquids, or two solids, none of which can be separated by filtration.
    Method:

    Find the mixture that is one insoluble solid plus a liquid, which filtration can separate.
    Examiner tips

    • Filtration works only when one component is an insoluble solid and the other is a liquid.

39. Question 39

    1 marksTests for gases
    Step 1: A gas that turns damp red litmus paper blue is ammonia, so the gas from solid 1 is ammonia. Step 2: A gas that relights a glowing splint is oxygen, so the gas from solid 2 is oxygen. Step 3: So solid 1 gives ammonia and solid 2 gives oxygen.
    Method:

    Match each gas test to its gas: litmus test for ammonia, glowing splint for oxygen.
    Examiner tips

    • Ammonia: damp red litmus → blue. Oxygen: relights a glowing splint.

40. Question 40

    1 marksTests for cations
    Step 1: Adding aqueous sodium hydroxide to metal ions forms metal hydroxide precipitates of characteristic colours. Step 2: Copper(II) ions give a blue precipitate of copper(II) hydroxide. Step 3: Calcium gives white, chromium(III) gives green, and iron(II) gives a green precipitate, so the blue precipitate is copper(II).
    Method:

    Recall the hydroxide precipitate colours and pick the ion that gives blue.
    Examiner tips

    • Cu2+ → blue, Fe2+ → green, Fe3+ → red-brown, Ca2+ → white with sodium hydroxide.