Question 1

Step 1: Cooling removes energy, so the particles slow down and move less. Step 2: In a solid the particles are held in fixed positions and can only vibrate. Step 3: So as the liquid freezes the positions of the particles become fixed.

Method:

Recall how particle arrangement and movement change when a liquid turns into a solid.

Examiner tips

$Cooling a liquid to a solid: particles slow down, get closer, and become fixed in place.$

Question 2

1 marksElements, compounds and mixtures from particle diagrams

Step 1: An element is made of only one kind of atom, shown as identical circles. Step 2: A compound is made of different atoms chemically joined, shown as different circles bonded together. Step 3: A mixture of an element and a compound contains both kinds of particle, not joined to each other, so the correct diagram has element molecules and compound molecules jumbled together.

Method:

Identify which picture contains both an element particle and a compound particle, mixed but not joined.

Examiner tips

$Element = one kind of atom; compound = different atoms joined; mixture = different particles not joined.$

Question 3

1 marksPeriods, groups and electron shells

Step 1: The period number equals the number of occupied electron shells and the group number (for the main groups) equals the number of outer shell electrons. Step 2: Sulfur is in Period 3 and Group VI, so it has three occupied shells and six outer electrons. Step 3: So the sulfur statement is correct, while the others give the wrong period or electron count.

Method:

Check each element against its period (shells) and group (outer electrons).

Examiner tips

$Use the position in the table: period gives the shells, group gives the outer electrons.$

Question 4

1 marksProtons, neutrons and electrons from nuclide notation

167 - 68 = 99

Method:

Read the proton number for protons and electrons, then subtract it from the nucleon number for neutrons.

Examiner tips

$Bottom number = protons (= electrons in a neutral atom); top minus bottom = neutrons.$

Question 5

1 marksIon formation in Group I metals

\text{Rb}^+

Method:

Decide whether the metal loses or gains electrons, then work out the resulting ion charge.

Examiner tips

$Metals lose electrons to form positive ions; the charge equals the group number for Group I.$

Question 6

1 marksDot-and-cross diagram of ammonia

Step 1: A nitrogen atom has five outer electrons and each hydrogen has one. Step 2: Nitrogen forms one shared pair (single covalent bond) with each of the three hydrogen atoms, using three of its outer electrons. Step 3: The remaining two nitrogen electrons stay as one non-bonding lone pair, so the correct diagram has three bonds and one lone pair.

Method:

Count nitrogen's five outer electrons, use three in bonds to hydrogen, and leave two as a lone pair.

Examiner tips

$In ammonia: three single N-H bonds plus one lone pair on nitrogen.$

Question 7

1 marksProperties of graphite

Step 1: In graphite each carbon is bonded to only three other carbons, so statement 1 is wrong. Step 2: Graphite conducts because it has free (delocalised) electrons, not free ions, so statement 2 is wrong. Step 3: Graphite has layers that slide over each other, which is why it is used as a lubricant, so only statement 3 is correct.

Method:

Test each statement against graphite's bonding, conduction mechanism and layered structure.

Examiner tips

$Graphite: 3 bonds per carbon, conducts via delocalised electrons, soft because layers slide.$

Question 8

1 marksBalanced equations with state symbols

\text{MgCl}_2

Method:

Get the correct salt formula, set the right state symbols, then check the equation balances.

Examiner tips

$Work out the salt formula from the ion charges, then balance and add state symbols.$

Question 9

1 marksMass of an element from formula mass

\text{CaCO}_3

Method:

Find the fraction of the formula mass due to carbon, then apply it to the given mass.

Examiner tips

$Mass of an element = (Ar of element / Mr of compound) x mass of compound.$

Question 10

1 marksDefinition of relative atomic mass

Step 1: Relative atomic mass takes into account that an element is a mixture of isotopes, so it is an average. Step 2: This average is compared with 1/12 of the mass of a carbon-12 atom, the standard reference. Step 3: So relative atomic mass is the average mass of the isotopes compared with 1/12 of a carbon-12 atom.

Method:

Pick the definition that uses an average over isotopes against the carbon-12 standard.

Examiner tips

$Key words: average, isotopes, and compared with 1/12 of a carbon-12 atom.$

Question 11

1 marksElectrolysis of a molten salt

\text{K}^+

Method:

List the ions in the molten salt, then send cations to the cathode and anions to the anode.

Examiner tips

$Molten binary salt: metal at the cathode, non-metal at the anode, no hydrogen or oxygen.$

Question 12

1 marksProduct of a hydrogen-oxygen fuel cell

Step 1: A hydrogen-oxygen fuel cell reacts hydrogen with oxygen, the reverse of electrolysis of water. Step 2: The only product of this reaction is water. Step 3: So water is produced in the fuel cell.

Method:

Recall the fuel cell reaction makes water and rule out the reactants and carbon products.

Examiner tips

$A clean fuel cell using hydrogen and oxygen makes only water.$

Question 13

1 marksEndothermic dissolving

Step 1: The ammonium nitrate can be recovered unchanged, so it has only dissolved, not reacted to form a new substance. Step 2: The temperature decreases, which means energy is taken in from the surroundings, so the process is endothermic. Step 3: So the ammonium nitrate dissolves and the process is endothermic.

Method:

Decide dissolving versus reacting from recoverability, and endo versus exo from the temperature change.

Examiner tips

$Temperature down = endothermic; recoverable unchanged = a physical change (dissolving).$

Question 14

1 marksFastest rate from a results table

Step 1: The rate is fastest when the volume of gas increases most quickly, that is the largest change in volume over a time period. Step 2: From 0 to 30 s the volume rises by 32 cm³, from 30 to 60 s by 27 cm³, from 60 to 90 s by 9 cm³, and from 90 to 120 s by 6 cm³. Step 3: The biggest increase is in the first 0 to 30 s period, so the rate is fastest then.

Method:

Find the change in volume for each 30 s interval and pick the interval with the largest change.

Examiner tips

$Compare the increase in volume for each interval; the largest increase is the fastest rate.$

Question 15

1 marksColoured compounds of transition elements

Step 1: Transition elements form coloured compounds, and copper(II) compounds are typically blue. Step 2: Copper(II) sulfate dissolves in water to give a blue solution. Step 3: So copper(II) sulfate is the substance that forms a blue solution.

Method:

Recall transition metal compound colours and pick the one that is blue in water.

Examiner tips

$Copper(II) salts are blue; many transition metal compounds are coloured.$

Question 16

1 marksIdentifying the reduced substance

\text{H}_2\text{O}

Method:

Compare each reactant with its product and find which one loses oxygen.

Examiner tips

$Track the oxygen: the reactant that loses oxygen is reduced.$

Question 17

1 marksTest for the ammonium ion

Step 1: Warming an ammonium compound with aqueous sodium hydroxide releases ammonia gas, which is the test for the ammonium ion. Step 2: Ammonium chloride and ammonium nitrate both contain the ammonium ion, so both produce ammonia. Step 3: Aluminium nitrate has no ammonium ion, so only compounds 2 and 3 produce ammonia.

Method:

Find which compounds contain the ammonium ion, since only those release ammonia.

Examiner tips

$Test for ammonium ion: warm with sodium hydroxide and check for ammonia (turns damp red litmus blue).$

Question 18

1 marksNeutralisation

Step 1: Hydrogen ions come from an acid and hydroxide ions come from an alkali, and they combine to form water. Step 2: An acid reacting with an alkali to form water is the definition of neutralisation. Step 3: So the reaction is a neutralisation reaction.

Method:

Recognise H⁺ and OH⁻ as acid and alkali ions combining to water, which defines neutralisation.

Examiner tips

$H⁺ + OH⁻ \to H₂O is the neutralisation reaction; the pH moves towards 7.$

Question 19

1 marksAcidic oxides

Step 1: Acidic oxides are oxides of non-metals. Step 2: Carbon is a non-metal, so carbon dioxide is an acidic oxide. Step 3: Barium, copper and magnesium are metals, so their oxides are basic, leaving carbon dioxide as the acidic oxide.

Method:

Identify which element is a non-metal, since only non-metal oxides are acidic.

Examiner tips

$Acidic oxides come from non-metals (CO₂, SO₂); basic oxides come from metals.$

Question 20

1 marksHydrated salts and water of crystallisation

Step 1: A hydrated salt contains water of crystallisation, which is water chemically combined within the solid crystals. Step 2: This is different from a salt dissolved in water or a dry anhydrous salt. Step 3: So a hydrated salt is a solid salt that is chemically combined with water.

Method:

Recall that hydrated means water of crystallisation is built into the solid crystal.

Examiner tips

$Hydrated = water of crystallisation in the solid; anhydrous = no water.$

Question 21

1 marksMetallic character across a period

Step 1: Metals are found on the left of the Periodic Table and non-metals on the right. Step 2: So moving left to right across a period the elements change from metals to non-metals. Step 3: This means the metallic character decreases from left to right across a period.

Method:

Recall that metals sit on the left, so metallic character drops moving right across a period.

Examiner tips

$Across a period: metallic character decreases; down a group: it increases.$

Question 22

1 marksProperties of Group I elements

Step 1: Group I metals react with water to give hydrogen and a hydroxide, so statement 1 is correct. Step 2: Going down Group I the melting point decreases (so statement 2 is wrong) and the density generally increases (so statement 3 is wrong). Step 3: The reactivity increases down the group, so statement 4 is correct, giving 1 and 4.

Method:

Check each statement against the known reactions and trends of the alkali metals.

Examiner tips

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

Question 23

1 marksReactivity and properties of the halogens

Step 1: No reaction when X meets Y's ions means X is less reactive than Y, and no reaction when Y meets Z's ions means Y is less reactive than Z. Step 2: So the reactivity order is Z (most reactive) then Y then X (least reactive), which makes Z chlorine, Y bromine and X iodine. Step 3: Bromine (Y) is a red-brown liquid at room temperature and pressure, so that statement is correct.

Method:

Use the displacement results to order reactivity, identify each halogen, then check the statements.

Examiner tips

$A more reactive halogen displaces a less reactive one; bromine is the red-brown liquid.$

Question 24

1 marksProperties of a transition element

Step 1: Transition elements have high melting points and high densities, so a very low melting point or a low density rules an element out. Step 2: They also form coloured compounds, so a coloured oxide is expected rather than a white one. Step 3: Only the row with a high melting point of 1085 °C, a high density and a coloured (red solid) oxide fits a transition element.

Method:

Match all three properties to a transition element: high melting point, high density, coloured oxide.

Examiner tips

$Look for high melting point, high density and a coloured compound together.$

Question 25

1 marksProperties of Group VIII (noble gases)

Step 1: The noble gases are monatomic gases, so statement 3 is correct. Step 2: Helium has only two outer electrons, so not all of them have eight, making statement 1 wrong. Step 3: The noble gases are unreactive and do not react with sodium, so statement 2 is wrong, leaving only statement 3.

Method:

Check each statement against the noble gases being unreactive monatomic gases, noting helium.

Examiner tips

$Noble gases are monatomic and unreactive; helium is the exception with two outer electrons.$

Question 26

1 marksComparing metal and non-metal properties

Step 1: Metals conduct heat well, so potassium has the higher thermal conductivity. Step 2: Metals are malleable while non-metals are brittle, so sulfur has the lower malleability. Step 3: So potassium has the higher thermal conductivity and sulfur has the lower malleability.

Method:

Use the metal/non-metal properties: the metal conducts heat best, the non-metal is least malleable.

Examiner tips

$Metal = good heat conductor and malleable; non-metal = poor conductor and brittle.$

Question 27

1 marksChoosing aluminium for power cables

Step 1: Overhead power cables need to be as light as possible, so the lower density of aluminium is a reason to choose it, making statement 1 a valid explanation. Step 2: Copper having the higher conductivity is a reason to choose copper, not aluminium, so statement 2 does not explain using aluminium. Step 3: Aluminium being more reactive is not a reason to use it for cables, so only statement 1 explains the choice.

Method:

Decide which property makes aluminium better than copper for light overhead cables.

Examiner tips

$For overhead cables, low density (light weight) is the key advantage of aluminium.$

Question 28

1 marksOrder of reactivity from reactions

Step 1: A metal that reacts with cold water is the most reactive, so L is the most reactive. Step 2: A metal that reacts with steam (but not cold water) is more reactive than one that only reacts with acid, so J comes above K. Step 3: A metal that does not react with acid is the least reactive, so M is last, giving the order L, J, K, M.

Method:

Rank the metals by the strongest reaction each one shows, from cold water down to no reaction.

Examiner tips

$Reactivity ranking by reaction: cold water > steam > acid only > no reaction.$

Question 29

1 marksChemical name for rust

The chemical name for rust, the substance formed when iron corrodes in air and water, is hydrated iron(III) oxide .

Method:

Recall that rusting needs water and oxygen and gives hydrated iron(III) oxide.

Examiner tips

$Rust = hydrated iron(III) oxide; both water and oxygen are needed to form it.$

Question 30

1 marksHarmful dissolved substances in water

Step 1: Dissolved oxygen is needed by aquatic animals to breathe, so it is helpful, not harmful. Step 2: Nitrates and phosphates from fertilisers can cause excess plant growth that removes oxygen from the water, harming aquatic life. Step 3: So only the nitrates and phosphates (2 and 3) are harmful.

Method:

Separate the helpful dissolved oxygen from the nutrient pollutants that harm aquatic life.

Examiner tips

$Nitrates and phosphates cause eutrophication; dissolved oxygen supports aquatic life.$

Question 31

1 marksComposition of clean dry air

Step 1: Clean, dry air is about 78% nitrogen and about 21% oxygen, with small amounts of argon and carbon dioxide. Step 2: Only nitrogen, at about 78%, is over 30% of the air. Step 3: So the gas that is over 30% of clean, dry air is nitrogen.

Method:

Recall the proportions of air and pick the gas above 30%.

Examiner tips

$Air is about 78% nitrogen and 21% oxygen.$

Question 32

1 marksElements supplied by fertilisers

Step 1: Fertilisers supply the main nutrients that plants need and that crops remove from the soil. Step 2: These nutrients are nitrogen, phosphorus and potassium, often labelled NPK. Step 3: So the elements replaced by fertilisers are nitrogen, potassium and phosphorus.

Method:

Recall the NPK nutrients and choose the option listing nitrogen, potassium and phosphorus.

Examiner tips

$NPK fertilisers supply nitrogen, phosphorus and potassium.$

Question 33

1 marksPhotosynthesis and the carbon cycle

Step 1: The reaction uses carbon dioxide and water, with chlorophyll and sunlight, to make glucose and oxygen. Step 2: This is the reaction carried out by green plants, which removes carbon dioxide from the air. Step 3: So the process is photosynthesis.

Method:

Match the reactants, products and conditions to the named process, photosynthesis.

Examiner tips

$Photosynthesis: carbon dioxide + water \to glucose + oxygen, using light and chlorophyll.$

Question 34

1 marksFeatures of a homologous series

Step 1: A homologous series is a family of compounds with the same general formula and the same functional group. Step 2: They have similar chemical properties because of this shared functional group, but their physical properties such as boiling point change down the series. Step 3: So the members are compounds with the same functional group.

Method:

Recall the definition of a homologous series and choose the same functional group option.

Examiner tips

$Homologous series: same functional group and general formula, gradually changing physical properties.$

Question 35

1 marksIdentifying organic families from formulae

\text{C}_n\text{H}_{2n}\text{O}_2

Method:

Match each formula to a family using its atoms, then test the statements.

Examiner tips

$Use general formulae: alcohols CnH2n+1OH, carboxylic acids CnH2nO2, hydrocarbons have only C and H.$

Question 36

1 marksProperties of petroleum fractions

Step 1: Fractions collected lower down the column have longer molecules, higher boiling points and higher viscosity. Step 2: Fuel oil has longer molecules than diesel oil, so it is more viscous, making that statement correct. Step 3: The other statements reverse the trends for volatility, chain length or boiling point, so they are wrong.

Method:

Use the trends down the fractionating column to test each statement about the fractions.

Examiner tips

$Down the column: longer chains, higher boiling point, higher viscosity, lower volatility.$

Question 37

1 marksProperties of the alkanes

\text{C}_n\text{H}_{2n+2}

Method:

Use the saturated nature and general formula of alkanes to test each statement.

Examiner tips

$Alkanes: saturated, general formula CnH2n+2, unreactive except combustion and substitution.$

Question 38

1 marksProducts of cracking

\text{C}_4\text{H}_{10}

Method:

Balance the carbon and hydrogen atoms and pick the alkane-plus-alkene pair that matches C6H14.

Examiner tips

$Check the products add up to the original formula; cracking gives an alkane plus an alkene.$

Question 39

1 marksInterpreting a chromatogram

Step 1: In chromatography, a substance always travels the same distance up the paper in the same conditions, so spots at the same height could be the same substance. Step 2: P's single spot and one of Q's spots reach the same height, so P and Q could share a coloured substance. Step 3: The line where the spots start is the baseline (not the solvent front), several spots show a mixture (not a pure substance), and the solvent rises above the baseline, so only the first statement is correct.

Method:

Use the rule that matching spot heights suggest the same substance, and recall the chromatography terms.

Examiner tips

$Same height = could be the same substance; baseline is where spots start, solvent front is where solvent ends.$

Question 40

1 marksIdentifying a salt from flame and acid tests

Step 1: A yellow flame in a flame test shows that the sodium ion is present, so the metal is sodium. Step 2: Effervescence with dilute acid shows a carbonate, because carbonates react with acids to give carbon dioxide gas. Step 3: So T contains sodium ions and carbonate ions, making it sodium carbonate.

Method:

Use the flame colour to find the metal ion and the acid test to find the carbonate, then combine them.

Examiner tips

$Combine the cation test (flame colour) with the anion test (effervescence = carbonate).$

May/June 2025 Paper 13 Worked Answers (IGCSE Chemistry 0620 Core)

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