May/June 2025 Paper 12 Worked Answers (IGCSE Chemistry 0620 Core)

40 questions · 40 marks · 45 minutes

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1. Question 1

   1 marksMelting and boiling points
   Step 1: L is a liquid at room temperature, so its boiling point must be above about 20 °C. Step 2: A pure substance has a single, sharp boiling point, so it cannot be a range of temperatures. Step 3: Only 59 °C is both above room temperature and a single value, so it is the boiling point.
   Method:

   Rule out any value below room temperature and any temperature range, then take the single value left.
   Examiner tips

   • Pure substances have sharp, single melting and boiling points; ranges suggest impurity.

2. Question 2

   1 marksEffect of temperature and pressure on gas volume
   Step 1: Heating a gas at constant pressure makes the particles move faster and spread out, so the volume increases. Step 2: Increasing the pressure at constant temperature squeezes the particles closer together, so the volume decreases. Step 3: So the volume increases in the first change and decreases in the second.
   Method:

   Treat each change separately and recall how particle spacing responds to heat and to pressure.
   Examiner tips

   • Temperature up means volume up; pressure up means volume down (for a fixed mass of gas).

3. Question 3

   1 marksDiffusion of gases
   Step 1: Gas particles are always moving and spread out to fill the available space by diffusion. Step 2: Over 24 hours the nitrogen dioxide and air particles mix completely, even though nitrogen dioxide is denser. Step 3: So both jars end up with the same even, pale red-brown colour.
   Method:

   Apply the idea that gas particles move and spread until the colour is uniform everywhere.
   Examiner tips

   • Diffusion continues until the gas is spread evenly throughout the whole space.

4. Question 4

   1 marksElements, compounds and mixtures
   Step 1: Oxygen ($\text{O}_2$) is an element and distilled water ($\text{H}_2\text{O}$) is a compound, mixed together. Step 2: Glucose dissolved in water is two compounds (glucose and water), and clean dry air is a mixture of several gases. Step 3: Stainless steel is a mixture of metal elements, so only oxygen in water is one element plus one compound.
   Method:

   Break each option into its components and count how many are elements and how many are compounds.
   Examiner tips

   • Classify each part of the mixture as an element or a compound before deciding.

5. Question 5

   1 marksAtomic number (proton number)
   Step 1: The atomic number, or proton number, counts the protons in the nucleus of an atom. Step 2: The total number of particles in the nucleus is the nucleon number, not the atomic number. Step 3: So the atomic number is the number of protons in the nucleus.
   Method:

   Recall that atomic number means proton number and pick the matching definition.
   Examiner tips

   • Atomic number = proton number; nucleon number = protons + neutrons.

6. Question 6

   1 marksIsotopes
   Step 1: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Step 2: A different number of neutrons changes the nucleon number (protons + neutrons). Step 3: So isotopes differ in nucleon number.
   Method:

   Recall that isotopes vary only in neutron number and select the property that this changes.
   Examiner tips

   • Same protons keep it the same element; different neutrons change the nucleon number.

7. Question 7

   1 marksIonic bonding by electron transfer
   Step 1: A chlorine atom has 7 outer electrons and needs one more to fill its outer shell. Step 2: It gains one electron from sodium to become a chloride ion with a 1- charge. Step 3: So each chlorine atom gains one electron.
   Method:

   Count chlorine outer electrons, decide it needs one more, and recall that only electrons transfer.
   Examiner tips

   • Non-metals near the right of the table usually gain electrons to fill their outer shell.

8. Question 8

   1 marksCovalent bonding and outer electrons
   Step 1: Carbon in methane forms four bonds, using all four of its outer electrons. Step 2: Hydrogen has only one outer electron and uses it in its single bond. Step 3: Chlorine in HCl and oxygen in water keep non-bonding pairs, so only carbon and hydrogen use all their outer electrons.
   Method:

   For each atom, count its outer electrons and check whether any are left as non-bonding pairs.
   Examiner tips

   • Carbon shares all four outer electrons; hydrogen shares its single electron.

9. Question 9

   1 marksGiant covalent structures
   Step 1: A giant covalent structure is a large network of atoms joined by covalent bonds. Step 2: Graphite is a giant covalent structure made of carbon atoms. Step 3: Sodium chloride and magnesium chloride are giant ionic, and ammonia is a small molecule, so graphite is correct.
   Method:

   Separate giant covalent networks from ionic solids and simple molecules, then choose the network.
   Examiner tips

   • Diamond, graphite and silicon(IV) oxide are the common giant covalent examples.

10. Question 10

    1 marksDeducing molecular formulae from structures
    Step 1: Counting atoms, molecule 1 is $\text{C}_4\text{H}_8$, while molecules 2, 3 and 4 are each $\text{C}_3\text{H}_6\text{O}$. Step 2: So molecules 2, 3 and 4 share the same molecular formula, $\text{C}_3\text{H}_6\text{O}$, and molecule 1 is different. Step 3: That means exactly three of the molecules have the same molecular formula.
    Method:

    Work out each molecular formula by counting atoms, then compare how many formulae match.
    Examiner tips

    • Two molecules with different structures can still share the same molecular formula (isomers).

11. Question 11

    1 marksReacting masses
    Step 1: Moles of magnesium $= 6 \div 24 = 0.25$ mol. Step 2: The equation shows 2 mol of magnesium reacts with 1 mol of $\text{O}_2$, so 0.25 mol of Mg reacts with 0.125 mol of $\text{O}_2$. Step 3: Mass of oxygen $= 0.125 \times 32 = 4$ g.
    Method:

    Convert magnesium mass to moles, apply the 2:1 ratio, then turn moles of O2 back into mass.
    Examiner tips

    • Use the balancing numbers in the equation to set the mole ratio.

12. Question 12

    1 marksElectrolytic cell and electrodes
    Step 1: The electrode connected to the positive terminal is the anode, so X is the anode and Y (negative) is the cathode. Step 2: A salt only conducts electricity when it is molten or dissolved (aqueous), because the ions are then free to move. Step 3: So Z is an aqueous salt, giving X anode, Y cathode, Z aqueous salt.
    Method:

    Label the electrodes from their terminals, then choose the electrolyte state that can conduct.
    Examiner tips

    • Anode is positive, cathode is negative; electrolytes conduct only when molten or aqueous.

13. Question 13

    1 marksHydrogen-oxygen fuel cell
    Step 1: A hydrogen-oxygen fuel cell reacts hydrogen with oxygen to make water and release energy. Step 2: The balanced equation is $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Step 3: The other equations make hydrogen peroxide or split water, so they are wrong.
    Method:

    Recall the product is water and pick the correctly balanced hydrogen plus oxygen equation.
    Examiner tips

    • Hydrogen plus oxygen gives water; balance to 2H2 + O2 to 2H2O.

14. Question 14

    1 marksExothermic reactions and energy profiles
    Step 1: A rise in the temperature of the surroundings means the reaction is exothermic and gives out energy. Step 2: In an energy level diagram an exothermic reaction has products below reactants, and the larger the drop the more energy is released. Step 3: So the diagram with the largest downward drop from reactants to products gives the largest temperature increase.
    Method:

    Match the largest temperature rise to the most exothermic profile, the one with the biggest drop.
    Examiner tips

    • Exothermic = products below reactants; the bigger the drop, the more heat released.

15. Question 15

    1 marksPhysical and chemical changes
    Step 1: A chemical change makes one or more new substances. Step 2: Neutralising copper(II) oxide with an acid forms a new salt and water, so it is a chemical change. Step 3: Dissolving, distilling and freezing only change state or separate substances, so they are physical changes.
    Method:

    Decide for each process whether a new substance is formed; only neutralisation does.
    Examiner tips

    • Ask whether a new substance is made; if not, the change is physical.

16. Question 16

    1 marksRate of reaction from graphs
    Step 1: On a volume against time graph, the rate of reaction is shown by the steepness (gradient) of the curve. Step 2: The curve is steepest at the very start, when the reactants are most concentrated. Step 3: So the point at the start where the curve is steepest shows the fastest rate.
    Method:

    Find where the curve is steepest, which is at the very start, and choose that point.
    Examiner tips

    • Read rate from the gradient: steepest gradient means fastest rate.

17. Question 17

    1 marksReversible reactions
    Step 1: Two half-arrows pointing in opposite directions are the symbol for a reversible reaction. Step 2: They show the reaction can go both forwards and backwards. Step 3: So the symbol means the reaction is reversible.
    Method:

    Recall what the double arrow symbol stands for and choose the matching reaction type.
    Examiner tips

    • The reversible-reaction symbol is two half-arrows pointing opposite ways.

18. Question 18

    1 marksIdentifying redox reactions
    Step 1: A redox reaction involves both oxidation and reduction (a transfer of oxygen or electrons). Step 2: Incomplete combustion of propane and rusting of iron both involve oxygen being gained, so they are redox. Step 3: The silver nitrate test is a precipitation reaction and the decomposition of calcium carbonate is not redox, so the answer is 1 and 2.
    Method:

    Check each reaction for oxidation and reduction, then pick the two that show both.
    Examiner tips

    • Combustion and rusting are common redox reactions; precipitation and many decompositions are not.

19. Question 19

    1 marksThe pH scale
    Step 1: A pH of 14 is strongly alkaline, and alkaline solutions contain hydroxide ions. Step 2: A pH of 1 is strongly acidic (not alkaline), pH 5 is acidic (not neutral), and pH 9 is alkaline so it turns universal indicator blue or purple, not yellow. Step 3: So only the pH 14 solution containing hydroxide ions is correct.
    Method:

    Place each pH on the scale as acidic, neutral or alkaline and check the description.
    Examiner tips

    • Universal indicator: red for strong acid, green for neutral, purple for strong alkali.

20. Question 20

    1 marksBasic oxides and the Periodic Table
    Step 1: Basic oxides are formed by metals, so we need a part of the table that contains only metals. Step 2: Group I contains only reactive metals, which all form basic oxides. Step 3: Period 3 also contains non-metals (acidic oxides) and Group VIII are unreactive noble gases that do not form these oxides, so Group I only is correct.
    Method:

    Identify which region is made up only of metals, since only metals give basic oxides.
    Examiner tips

    • Metals make basic oxides; non-metals make acidic oxides.

21. Question 21

    1 marksArrangement and trends in the Periodic Table
    Step 1: Group VII atoms have 7 outer electrons and gain one electron to form ions, so their charge is $7 - 8 = -1$, matching the rule. Step 2: Elements in a group share the same number of outer electrons but not the same full configuration. Step 3: Metallic character decreases across a period and the table is ordered by proton number, so only the Group VII statement is correct.
    Method:

    Test each statement against the rules for groups, periods, metallic character and ordering.
    Examiner tips

    • Groups share outer electrons; the table is ordered by proton number; metals are on the left.

22. Question 22

    1 marksGroup I trends
    Step 1: Going down Group I the melting point decreases. Step 2: The density generally increases down the group. Step 3: The reactivity increases down the group, so the trends are melting point decreasing, density increasing, reactivity increasing.
    Method:

    Recall the three Group I trends going down the group and match them to the row.
    Examiner tips

    • Down Group I: melting point down, density up, reactivity up.

23. Question 23

    1 marksProperties of the halogens
    Step 1: The halogens are all diatomic molecules, so statement 1 is correct. Step 2: They are not all gases (bromine is a liquid and iodine is a solid at room temperature), so statement 2 is wrong. Step 3: Density increases down the group (statement 3 correct) but reactivity decreases down the group (statement 4 wrong), so the answer is 1 and 3.
    Method:

    Check each statement against the physical states and trends of the halogens.
    Examiner tips

    • Down Group VII: density increases but reactivity decreases.

24. Question 24

    1 marksProperties of transition elements
    Step 1: Transition elements are metals with high densities and high melting points. Step 2: They conduct heat and electricity and often form coloured compounds. Step 3: So a correct property of zirconium is its high density.
    Method:

    Recall the typical properties of transition elements and pick the matching one.
    Examiner tips

    • Transition metals: dense, high melting points, good conductors, coloured compounds.

25. Question 25

    1 marksNoble gases
    Step 1: Argon and krypton are noble gases that exist as single atoms, are unreactive and have eight outer electrons. Step 2: Helium is a noble gas but has only two outer electrons, not eight, so it does not fit. Step 3: Oxygen exists as $\text{O}_2$ molecules and is reactive, so the answer is 2 and 4 only.
    Method:

    Test each gas against single atoms, unreactive and eight outer electrons; exclude helium and oxygen.
    Examiner tips

    • Most noble gases have eight outer electrons, but helium has only two.

26. Question 26

    1 marksProperties and reactivity of metals
    Step 1: Calcium is above magnesium in the reactivity series, so it reacts faster with cold water (statement 1 correct). Step 2: All metals conduct electricity, so potassium and silver are both good conductors (statement 4 correct). Step 3: Aluminium is harder to extract than iron and copper does not react with dilute acid, so statements 2 and 3 are wrong, giving 1 and 4.
    Method:

    Check each statement against the reactivity series and the general properties of metals.
    Examiner tips

    • More reactive metals react faster but are harder to extract; all metals conduct.

27. Question 27

    1 marksReactivity series from observations
    Step 1: The more bubbles of hydrogen a metal produces with acid, the more reactive it is. Step 2: P gives the most bubbles, then Q (few), then S (very few), and R gives none. Step 3: So from most to least reactive the order is P, Q, S, R.
    Method:

    Order the metals by the amount of bubbling, from most (P) to none (R).
    Examiner tips

    • Rank the metals by how vigorously they bubble with the acid.

28. Question 28

    1 marksConditions needed for rusting
    Step 1: Iron needs both water and oxygen to rust. Step 2: Only test-tube 1 has both water and air present. Step 3: Test-tube 2 has no water and test-tube 3 has no oxygen (boiled water under oil), so only the nail in test-tube 1 rusts.
    Method:

    Check each test-tube for both water and oxygen; only the one with both shows rusting.
    Examiner tips

    • Both water and oxygen must be present for iron to rust.

29. Question 29

    1 marksProperties and uses of alloys
    Step 1: An alloy contains atoms of different sizes, which disrupt the regular layers of atoms. Step 2: This makes it harder for the layers to slide, so the alloy is harder and keeps its shape better than the pure metal. Step 3: So alloys are used because they are harder and stronger.
    Method:

    Recall how mixing different atoms changes the structure, making alloys harder and stronger.
    Examiner tips

    • Alloys are harder than pure metals because their different-sized atoms block the sliding layers.

30. Question 30

    1 marksPercentage of oxygen in air
    Step 1: Air is about 21% oxygen, so 100 $\text{cm}^3$ of air contains about 21 $\text{cm}^3$ of oxygen. Step 2: The heated copper removes the oxygen, leaving mainly nitrogen. Step 3: So the volume left is about $100 - 21 = 79$ $\text{cm}^3$.
    Method:

    Take 21% of 100 cm3 as the oxygen removed and subtract it from the starting volume.
    Examiner tips

    • Air is about 21% oxygen and 78% nitrogen; copper removes only the oxygen.

31. Question 31

    1 marksAcid rain and air pollutants
    Step 1: Acid rain is caused by acidic gases dissolving in rainwater. Step 2: Both sulfur dioxide and oxides of nitrogen form acids in rainwater, so both contribute to acid rain. Step 3: Carbon monoxide is a toxic gas but does not cause acid rain, so the answer is oxides of nitrogen and sulfur dioxide.
    Method:

    Pick the acidic pollutant gases and exclude carbon monoxide, which is not acidic.
    Examiner tips

    • Sulfur dioxide and oxides of nitrogen are the acid rain gases.

32. Question 32

    1 marksHomologous series
    Step 1: Compounds 1 ($\text{C}_2\text{H}_4$) and 2 ($\text{C}_4\text{H}_8$) both fit the general formula $\text{C}_n\text{H}_{2n}$, the alkenes. Step 2: Compound 3 ($\text{C}_4\text{H}_{10}$) fits $\text{C}_n\text{H}_{2n+2}$, the alkanes. Step 3: So only 1 and 2 are in the same homologous series.
    Method:

    Match each formula to a general formula and group those that share the same one.
    Examiner tips

    • Alkenes are CnH2n; alkanes are CnH2n+2.

33. Question 33

    1 marksFuels and petroleum
    Step 1: Gasoline (petrol) is separated from petroleum, so the answer to the first question is yes. Step 2: Gasoline is used as a fuel for cars, so the answer to the second question is also yes. Step 3: Following yes then yes through the flowchart leads to outcome A.
    Method:

    Answer each flowchart question for gasoline and trace the path to the final outcome.
    Examiner tips

    • Work through a flowchart one decision at a time, following the correct yes or no branch.

34. Question 34

    1 marksTest for unsaturation with bromine
    Step 1: Aqueous bromine is only decolourised by an unsaturated hydrocarbon (one with a carbon-carbon double bond). Step 2: Methane and ethane have only single bonds (saturated), so the bromine stays orange. Step 3: Ethene has a double bond (unsaturated), so it changes the bromine from orange to colourless.
    Method:

    Decide which hydrocarbon is unsaturated, then assign the colour change only to that one.
    Examiner tips

    • Bromine water test: decolourised means unsaturated (a double bond is present).

35. Question 35

    1 marksManufacture of ethanol
    Step 1: Ethene reacts with steam in an addition reaction using a catalyst. Step 2: The water adds across the double bond to form ethanol. Step 3: So the product is ethanol.
    Method:

    Recall the catalytic addition of steam to ethene and name the alcohol product.
    Examiner tips

    • Ethene plus steam gives ethanol; this is how ethanol is manufactured industrially.

36. Question 36

    1 marksReactions of ethanoic acid
    Step 1: Ethanoic acid is an acid, so its aqueous solution has a pH below 7, not 10, making statement 1 wrong. Step 2: Like other acids, it reacts with metal carbonates to give carbon dioxide (statement 2 correct). Step 3: It also reacts with reactive metals such as magnesium to give hydrogen (statement 3 correct), so the answer is 2 and 3 only.
    Method:

    Use the typical reactions of acids to test each statement about ethanoic acid.
    Examiner tips

    • Ethanoic acid reacts like any acid: carbonates give CO2, reactive metals give hydrogen.

37. Question 37

    1 marksPolymers and the environment
    Step 1: Poly(ethene) is a very unreactive polymer that is not broken down by natural processes. Step 2: Because it does not decompose in water, it stays in the environment for a very long time. Step 3: So the bags build up in the oceans because poly(ethene) does not decompose in water.
    Method:

    Choose the statement about the polymer not breaking down, which causes it to accumulate.
    Examiner tips

    • Poly(ethene) is non-biodegradable, so it does not break down in the environment.

38. Question 38

    1 marksChromatography setup
    Step 1: The sample spot must stay out of the solvent at the start. Step 2: If the solvent is at or above the spot, the sample dissolves straight into the solvent and washes away instead of separating up the paper. Step 3: So the solvent level must start below the sample spot.
    Method:

    Decide where the solvent must be so the sample spot is not dissolved into it at the start.
    Examiner tips

    • Always start the solvent below the baseline spot so the sample is not washed away.

39. Question 39

    1 marksFractional distillation
    Step 1: Liquids with different boiling points can be separated by boiling them off one at a time. Step 2: Fractional distillation uses a fractionating column to separate liquids by their boiling points. Step 3: So fractional distillation is the correct process.
    Method:

    Match the need to separate liquids by boiling point to fractional distillation.
    Examiner tips

    • Use fractional distillation when separating liquids with different boiling points.

40. Question 40

    1 marksTests for the chloride ion
    Step 1: Sodium ions give no precipitate with aqueous sodium hydroxide, so there is no visible reaction. Step 2: Chloride ions give a white precipitate of silver chloride with acidified aqueous silver nitrate. Step 3: So sodium chloride gives no visible reaction with sodium hydroxide and a white precipitate with silver nitrate.
    Method:

    Apply the cation test (sodium hydroxide) and the anion test (silver nitrate) to sodium chloride.
    Examiner tips

    • Test for chloride: add acidified silver nitrate and look for a white precipitate.