0620/11 May/June 2025 Worked Answers (IGCSE Chemistry 0620 Core)

40 questions · 40 marks · 45 minutes

Question papers and mark schemes are copyright Cambridge International. We do not reproduce them: the worked answers here are written by The Practice Book. Have the paper open alongside. Get the official paper from Cambridge

1. Question 1

   1 marksChanges of state
   Step 1: Water vapour is a gas and rain is liquid water. Step 2: A gas turning into a liquid is called condensing. Step 3: So the process is condensing.
   Method:

   Identify that vapour is a gas and rain is a liquid, then name the gas to liquid change.
   Examiner tips

   • Name the starting state and the final state first, then choose the matching change.

2. Question 2

   1 marksChanges of state and particle energy
   Step 1: State 3 is a solid and state 1 is a liquid, so the change from state 3 to state 1 is melting. Step 2: When a solid melts, the particles gain energy and move more, so both the energy and the particle motion increase. Step 3: Only the change from state 3 to state 1 with both increasing is correct.
   Method:

   Identify each state as solid, liquid or gas, then track whether the change adds or removes energy.
   Examiner tips

   • Heating a substance always increases particle energy and movement; cooling does the reverse.

3. Question 3

   1 marksAtomic structure and proton number
   Step 1: The element is identified by its number of protons (the proton number). Step 2: An atom with 19 protons has proton number 19, which is potassium. Step 3: So Q is potassium.
   Method:

   Read the proton number and find the element with that number in the Periodic Table.
   Examiner tips

   • Match the proton number to the Periodic Table, ignore the neutron number when naming the element.

4. Question 4

   1 marksSubatomic particles
   Step 1: A neutron has a relative mass of 1 and no charge (relative charge 0). Step 2: A proton also has mass 1 but a charge of +1, and an electron has almost no mass. Step 3: So the particle is the neutron.
   Method:

   Recall the relative mass and charge of each subatomic particle and match the description.
   Examiner tips

   • Remember protons are positive, electrons negative, and neutrons neutral.

5. Question 5

   1 marksIsotopes
   Step 1: Any isotope of boron must have 5 protons, and a neutral atom has equal protons and electrons, so 5 electrons. Step 2: Boron-11 already has 6 neutrons, so a different isotope must have a different neutron number, such as 5 neutrons (boron-10). Step 3: The row with 5 electrons, 5 neutrons and 5 protons fits.
   Method:

   Keep 5 protons and 5 electrons for neutral boron, then pick a neutron number that differs from 6.
   Examiner tips

   • Fix the proton number to keep the element, then change only the neutron number for an isotope.

6. Question 6

   1 marksIonic bonding and properties
   Step 1: Potassium iodide is ionic, made of potassium cations (positive) and iodide anions (negative), so statement 1 reverses the charges and is wrong. Step 2: Ionic compounds conduct when molten or dissolved because the ions are free to move, so statement 2 is correct. Step 3: Ions are formed by transferring electrons, not sharing, so statement 3 is wrong, leaving only statement 2.
   Method:

   Recognise potassium iodide as ionic, then test each statement against ionic bonding rules.
   Examiner tips

   • Metal plus non-metal usually means ionic bonding with electron transfer.

7. Question 7

   1 marksCovalent bonding
   Step 1: Shared pairs of electrons are covalent bonds, found between non-metal atoms in molecules. Step 2: Methane and chlorine are molecules with covalent bonds, so they contain shared pairs. Step 3: Argon is a single unbonded atom and iron(III) oxide is ionic, so the answer is 2 and 4.
   Method:

   Classify each substance as atom, covalent molecule or ionic compound, then keep the covalent ones.
   Examiner tips

   • Look for molecules of non-metals; noble gases are single atoms and metal compounds are usually ionic.

8. Question 8

   1 marksGiant covalent structures
   Step 1: A giant covalent structure has many atoms joined by covalent bonds in a large network. Step 2: Diamond is a giant covalent structure of carbon atoms. Step 3: Ammonia, carbon dioxide and water are all small molecules, so the answer is diamond.
   Method:

   Separate giant covalent networks from simple molecules and choose the network.
   Examiner tips

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

9. Question 9

   1 marksChemical formulae
   Step 1: Sodium ions are $\text{Na}^+$ and carbonate ions are $\text{CO}_3^{2-}$, so two sodium ions balance one carbonate, giving $\text{Na}_2\text{CO}_3$. Step 2: Cobalt(II) chloride should be $\text{CoCl}_2$, xenon is a single atom Xe, and ammonium sulfate is $(\text{NH}_4)_2\text{SO}_4$. Step 3: Only sodium carbonate, $\text{Na}_2\text{CO}_3$, is written correctly.
   Method:

   Use the ion charges for each compound and check that the given formula balances.
   Examiner tips

   • Write the ion charges first, then cross them over to balance the formula.

10. Question 10

    1 marksReacting masses
    Step 1: Moles of $\text{MgCO}_3 = 21.0 \div 84 = 0.25$ mol. Step 2: The equation shows 1 mol of $\text{MgCO}_3$ gives 1 mol of MgO, so 0.25 mol of MgO forms. Step 3: Mass of $\text{MgO} = 0.25 \times 40 = 10.0$ g.
    Method:

    Convert 21.0 g to moles, apply the 1:1 ratio, then convert moles of MgO back to mass.
    Examiner tips

    • Always convert mass to moles before applying the equation ratio.

11. Question 11

    1 marksElectrolysis products
    Step 1: Negative ions move to the positive electrode (anode). Step 2: In concentrated sodium chloride solution, chloride ions are discharged in preference to hydroxide, giving chlorine gas. Step 3: So the main product at the anode is chlorine.
    Method:

    Identify the ions going to the anode and apply the discharge rule for concentrated halides.
    Examiner tips

    • At the anode, concentrated halide ions are preferred over hydroxide.

12. Question 12

    1 marksHydrogen-oxygen fuel cells and air composition
    Step 1: Air is about 21% oxygen, so the volume of air needed is the oxygen volume divided by 0.21. Step 2: $630 \div 0.21 = 3000$. Step 3: So at least 3000 $\text{dm}^3$ of clean, dry air is needed.
    Method:

    Divide the required oxygen volume by the oxygen fraction of air (0.21).
    Examiner tips

    • Remember the approximate composition of air: about 78% nitrogen and 21% oxygen.

13. Question 13

    1 marksExothermic and endothermic reactions
    Step 1: Endothermic reactions take in energy, so the surroundings cool down, making statement 1 wrong. Step 2: Exothermic reactions release energy to the surroundings, so statement 2 is wrong. Step 3: In an exothermic reaction the products are at a lower energy than the reactants, so only statement 3 is correct.
    Method:

    Define exothermic and endothermic clearly, then test each statement against the definitions.
    Examiner tips

    • On a pathway diagram, exothermic products sit below the reactants.

14. Question 14

    1 marksPhysical and chemical changes
    Step 1: A physical change does not make a new substance and can usually be reversed. Step 2: The metal melting (1) and water turning to steam (3) are changes of state, so they are physical. Step 3: Producing hydrogen (2) and raising the pH (4) involve new substances, so they are chemical, leaving 1 and 3.
    Method:

    Classify each observation as making a new substance (chemical) or only changing state (physical).
    Examiner tips

    • Ask whether a new substance is formed; if only the state changes, it is physical.

15. Question 15

    1 marksCatalysts
    Step 1: A catalyst speeds up a reaction without being used up. Step 2: Because it is not consumed, the mass of the catalyst is the same before and after the reaction. Step 3: It does not change how much oxygen forms or the overall energy released.
    Method:

    Recall the definition of a catalyst and rule out any statement implying it is consumed or changes the yield.
    Examiner tips

    • Catalysts change the rate only; amounts of product and energy change are unaffected.

16. Question 16

    1 marksOxidation and reduction
    Step 1: Reduction is loss of oxygen and oxidation is gain of oxygen. Step 2: Copper(II) oxide loses oxygen to become copper, so CuO is reduced. Step 3: Carbon gains oxygen to form carbon dioxide, so carbon is oxidised, not reduced.
    Method:

    Follow the oxygen through the equation to decide which species is reduced and which is oxidised.
    Examiner tips

    • Use OIL RIG with oxygen: loss of oxygen is reduction, gain of oxygen is oxidation.

17. Question 17

    1 marksIndicators in acids and alkalis
    Step 1: Aqueous sodium hydroxide is an alkali. Step 2: In an alkali, thymolphthalein is blue and methyl orange is yellow. Step 3: So the correct row is aqueous sodium hydroxide giving thymolphthalein blue and methyl orange yellow.
    Method:

    Classify the solution as acid or alkali, then match both indicator colours.
    Examiner tips

    • Learn the acid and alkali colours for each common indicator before the exam.

18. Question 18

    1 marksTest for ammonia and ammonium ions
    Step 1: Damp red litmus turning blue shows an alkaline gas, which is ammonia. Step 2: Ammonia is released when an ammonium salt is warmed with a base, so one substance must be a base and the other an ammonium compound. Step 3: A basic oxide with ammonium chloride fits, so that is the correct pair.
    Method:

    Identify the alkaline gas as ammonia, then choose the base and ammonium compound that release it.
    Examiner tips

    • Ammonia is the only common gas that turns damp red litmus paper blue.

19. Question 19

    1 marksPreparation of salts
    Step 1: Copper is too unreactive to react with dilute acid, so a copper compound must be used. Step 2: Copper(II) carbonate reacts with dilute sulfuric acid to give copper(II) sulfate, water and carbon dioxide. Step 3: So the reactants are dilute acid plus a metal carbonate.
    Method:

    Recognise copper is unreactive, then pick the copper compound that reacts with acid to give the salt.
    Examiner tips

    • For unreactive metals, react their oxide or carbonate with the acid.

20. Question 20

    1 marksArrangement of the Periodic Table
    Step 1: Group number gives the number of outer electrons, and Group II atoms lose 2 electrons to form 2+ ions. Step 2: The table is ordered by proton number, not relative atomic mass, and elements in the same group (not period) have similar properties. Step 3: Group VII reactivity decreases down the group, so only the Group II statement is correct.
    Method:

    Check each statement against the rules for groups, periods and ordering of the table.
    Examiner tips

    • Groups share properties; the table is ordered by proton number.

21. Question 21

    1 marksMetallic character trends
    Step 1: Metallic character increases towards the left-hand side of the Periodic Table. Step 2: The element on the far left is furthest into the metal region. Step 3: So the far left element shows the most metallic character.
    Method:

    Use the trend that metallic character increases to the left and select the leftmost element.
    Examiner tips

    • Metallic character rises going left and down the Periodic Table.

22. Question 22

    1 marksGroup I properties and trends
    Step 1: Going down Group I, melting point falls and density generally rises. Step 2: A higher melting point than caesium and a lower density than sodium both point to an element near the top of the group. Step 3: Lithium is at the top of Group I, so E is lithium.
    Method:

    Apply the Group I trends in melting point and density to locate the element near the top.
    Examiner tips

    • Down Group I: melting point decreases and density tends to increase.

23. Question 23

    1 marksGroup VII displacement reactions
    Step 1: The halogen $\text{X}_2$ displaces Y from its halide, so $\text{X}_2$ must be the more reactive halogen. Step 2: Chlorine is more reactive than iodine, so chlorine ($\text{X}_2$) displaces iodine from iodide ($\text{Y}^-$). Step 3: So $\text{X}_2$ is chlorine, $\text{Y}^-$ is iodide, and chlorine is more reactive than iodine.
    Method:

    Identify the displacing halogen as the more reactive one and match the explanation.
    Examiner tips

    • Reactivity decreases down Group VII, so chlorine displaces bromine and iodine.

24. Question 24

    1 marksProperties of transition elements
    Step 1: High melting and boiling points, a coloured oxide and use as a catalyst are all typical of the transition elements. Step 2: Group I metals have low melting points, Group VII are non-metals, and Group VIII are unreactive gases. Step 3: So the element is one of the transition elements.
    Method:

    Match the listed properties to the typical features of transition elements.
    Examiner tips

    • Transition metals: high melting points, coloured compounds and catalytic activity.

25. Question 25

    1 marksReactivity series from observations
    Step 1: A metal that reacts with cold water is more reactive than one that only reacts with acid, which is more reactive than one that does not react at all. Step 2: T reacts vigorously with water (most reactive), Q reacts slowly with water, R reacts only with acid, and S does not react (least reactive). Step 3: From least to most reactive the order is S, R, Q, T.
    Method:

    Order the metals by how vigorously they react, using water reactions to separate the top metals.
    Examiner tips

    • Rank metals by the strongest reaction they show: cold water beats acid, acid beats no reaction.

26. Question 26

    1 marksReactions of metals with acid and steam
    Step 1: Magnesium reacts with dilute sulfuric acid to give magnesium sulfate and hydrogen. Step 2: Magnesium also reacts with steam to give magnesium oxide and hydrogen. Step 3: So hydrogen is formed in both reactions.
    Method:

    Recall the products of magnesium with acid and with steam and check for hydrogen in each.
    Examiner tips

    • Metals above hydrogen release hydrogen with acids; reactive metals also give hydrogen with steam.

27. Question 27

    1 marksAluminium ore and extraction
    Step 1: The main ore of aluminium is bauxite, so statement 2 is correct. Step 2: Aluminium is less reactive than calcium, and it is extracted by electrolysis (not carbon) because it is above carbon in reactivity. Step 3: Brass is an alloy of copper and zinc, not aluminium, so only statement 2 is correct.
    Method:

    Test each statement against aluminium reactivity, extraction method and the composition of brass.
    Examiner tips

    • Metals above carbon in reactivity are extracted by electrolysis.

28. Question 28

    1 marksAlloys (types of steel)
    Step 1: Cutlery must be hard and must resist corrosion from food and washing. Step 2: Stainless steel (type 3) is hard and resistant to corrosion, so it suits cutlery. Step 3: Types 1 and 2 both corrode, so only type 3 is suitable.
    Method:

    Decide the properties cutlery needs, then choose the steel that has them.
    Examiner tips

    • Match the property needed (here, corrosion resistance) to the right alloy.

29. Question 29

    1 marksRusting of iron
    Step 1: Grease stops the iron in tube P from rusting, so no oxygen is used and the water level does not change. Step 2: The untreated iron in tube Q rusts, using up the oxygen in the trapped air. Step 3: As oxygen is used up in tube Q the trapped gas volume falls and water rises, so P shows no change and Q rises.
    Method:

    Decide which iron rusts, then link oxygen use to the rising water level.
    Examiner tips

    • Oxygen used in rusting reduces the trapped gas, so water rises to fill the space.

30. Question 30

    1 marksFertilisers (NPK)
    Step 1: The three elements needed are nitrogen (N), phosphorus (P) and potassium (K). Step 2: Potassium nitrate supplies potassium and nitrogen, and calcium phosphate supplies phosphorus. Step 3: Together they provide all three elements, so this is the correct pair.
    Method:

    List the elements in each compound and find the pair that covers N, P and K.
    Examiner tips

    • Tick off N, P and K across both compounds to make sure none is missing.

31. Question 31

    1 marksChemical test for water
    Step 1: The chemical tests for water use anhydrous cobalt(II) chloride (blue to pink) and anhydrous copper(II) sulfate (white to blue). Step 2: Litmus and methyl orange are indicators that test for acids and alkalis, not for water. Step 3: So substances 1 and 2 detect water.
    Method:

    Recall the chemical tests for water and rule out the acid-base indicators.
    Examiner tips

    • Learn the two colour-change tests for water with the anhydrous salts.

32. Question 32

    1 marksIncomplete combustion of fuels
    Step 1: Incomplete combustion happens when there is not enough oxygen. Step 2: Burning methane with too little oxygen produces carbon monoxide (and sometimes carbon) along with water. Step 3: So carbon monoxide is produced.
    Method:

    Recall the products of incomplete combustion of a hydrocarbon and select carbon monoxide.
    Examiner tips

    • Incomplete combustion of hydrocarbons gives carbon monoxide and possibly soot.

33. Question 33

    1 marksHomologous series
    Step 1: Members of a homologous series have the same functional group. Step 2: Because they share the same functional group, they have similar chemical properties. Step 3: So the correct row is similar chemical properties and the same functional group.
    Method:

    Recall the definition of a homologous series and match the property and functional-group description.
    Examiner tips

    • Same functional group leads to similar chemical reactions across the series.

34. Question 34

    1 marksReactions of alkanes
    Step 1: $\text{C}_4\text{H}_{10}$ is an alkane, which is saturated with only single bonds. Step 2: Alkanes react with chlorine by substitution, in which a chlorine atom replaces a hydrogen atom. Step 3: Alkanes do not undergo addition because they have no double bond, so the answer is chlorine by substitution.
    Method:

    Identify Z as a saturated alkane, then choose the substitution reaction with chlorine.
    Examiner tips

    • Alkanes are saturated; their typical reaction with chlorine is substitution in light.

35. Question 35

    1 marksFractional distillation of petroleum
    Step 1: Boiling point increases as the fractions get heavier, with larger molecules boiling at higher temperatures. Step 2: Gasoline and naphtha are light fractions, then kerosene, then diesel oil is heavier. Step 3: So the order of increasing boiling point is gasoline, naphtha, kerosene, diesel oil.
    Method:

    Rank the named fractions from lightest to heaviest to give increasing boiling point.
    Examiner tips

    • Order fractions from light (low boiling point) to heavy (high boiling point).

36. Question 36

    1 marksCracking of alkanes
    Step 1: Cracking breaks large alkanes into smaller, more useful molecules including petrol, so statement 1 is correct. Step 2: Cracking produces both smaller alkanes and alkenes, so statement 2 (only alkenes) is wrong. Step 3: The alkenes made are used to make polymers, so statement 3 is correct, giving 1 and 3.
    Method:

    Check each statement against the products of cracking: smaller alkanes plus alkenes.
    Examiner tips

    • Cracking yields smaller alkanes (for fuels) and alkenes (for polymers).

37. Question 37

    1 marksProperties of ethene
    Step 1: Ethene contains only carbon and hydrogen, so it is a hydrocarbon (1). Step 2: Ethene is the monomer used to make poly(ethene), so it is a monomer (2). Step 3: Ethene has a double bond, so it is unsaturated and reactive, making statements 3 and 4 wrong, leaving 1 and 2.
    Method:

    Test each describing word against the structure and reactions of ethene.
    Examiner tips

    • Alkenes like ethene are unsaturated and reactive, and act as monomers.

38. Question 38

    1 marksMeasuring volume accurately
    Step 1: Measuring 25.00 $\text{cm}^3$ to two decimal places needs an accurate piece of apparatus. Step 2: A burette measures volumes accurately to the nearest 0.05 $\text{cm}^3$. Step 3: A beaker, measuring cylinder and dropping pipette are far less accurate, so a burette is most suitable.
    Method:

    Match the precision required (two decimal places) to the most accurate apparatus.
    Examiner tips

    • Choose apparatus that matches the precision shown by the number of decimal places.

39. Question 39

    1 marksSeparation techniques
    Step 1: After the reaction there is an insoluble solid (excess magnesium oxide) in a solution. Step 2: Filtration separates an insoluble solid from a liquid. Step 3: So filtration is used to remove the excess solid.
    Method:

    Recognise the insoluble excess in solution and choose filtration to remove it.
    Examiner tips

    • Filtration is the standard method to separate an insoluble solid from a liquid.

40. Question 40

    1 marksTests for cations
    Step 1: Copper(II) ions give a blue precipitate with aqueous sodium hydroxide. Step 2: Copper(II) sulfate is soluble, so there is no visible change with dilute sulfuric acid, and copper gives a blue-green flame. Step 3: All three results match copper(II) nitrate, so this row is correct.
    Method:

    Match each metal ion to its known hydroxide precipitate, sulfate behaviour and flame colour.
    Examiner tips

    • Use all three tests together; one wrong result rules out the whole row.