0620/13 Oct/Nov 2025 Worked Answers (IGCSE Chemistry 0620 Core)
40 questions · 40 marks · 45 minutes
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Worked answers for 40 questions
- Step 1: In a solid and in a liquid the particles are packed closely together and touching, so they are a similar distance apart. Step 2: In a gas the particles are spread far apart with large spaces between them. Step 3: So the particles are a similar distance apart in the solid and liquid only.Method:Compare the spacing of particles in each state and group the two that are similar.Examiner tips
- Solids and liquids: particles close together; gases: particles far apart.
- Step 1: The container is rigid and sealed, so the volume of the gas cannot change, which means statement 1 is wrong. Step 2: Heating gives the gas molecules more energy so they move faster, making statement 2 correct. Step 3: Faster molecules hit the walls harder and more often, so the pressure rises, making statement 3 correct, giving 2 and 3.Method:Decide what stays fixed (volume) and what heating does (speed and pressure), then pick the correct statements.Examiner tips
- Fixed volume + heating → faster molecules and higher pressure.
- Step 1: The solid sodium chloride breaks up and goes into solution in the water, which is dissolving. Step 2: The dissolved particles then spread out from where they are concentrated to fill the whole beaker, which is diffusion. Step 3: So both dissolving and diffusion occur.Method:Identify the two stages: the solid dissolving and then the particles diffusing through the water.Examiner tips
- Dissolving puts the solid into solution; diffusion spreads the particles out evenly.
- Step 1: P is a metal and Q is a non-metal, so the compound is held together by ionic bonding (electrons transferred from metal to non-metal). Step 2: P has 2 outer electrons so it loses both to form P²⁺, and Q has 7 outer electrons so each Q gains 1 to form Q⁻. Step 3: Balancing the charges needs one P²⁺ to two Q⁻, giving the formula PQ₂ with ionic bonding.Method:Decide the bonding from metal/non-metal, work out the ion charges, then balance them for the formula.Examiner tips
- Metal + non-metal → ionic; use the ion charges to balance the formula.
- Step 1: A neutral sulfur atom has 16 electrons; gaining 2 to form S²⁻ gives 18 electrons, arranged 2,8,8. Step 2: A neutral calcium atom has 20 electrons; losing 2 to form Ca²⁺ gives 18 electrons, also arranged 2,8,8. Step 3: Both ions have 18 electrons in the configuration 2,8,8, so the ion could be either S²⁻ or Ca²⁺.Method:Find the number of electrons in each ion and check which give the configuration 2,8,8.Examiner tips
- Count electrons: add for negative ions, subtract for positive ions, then match the configuration.
- Step 1: Work out the proton numbers: W = 35−18 = 17, X = 37−20 = 17, Y = 39−20 = 19, Z = 40−22 = 18. Step 2: W and X both have proton number 17, so they are atoms of the same element (isotopes), making the first statement correct. Step 3: Y (19) and Z (18) are different elements in different groups and periods, and Z (18) has a lower proton number than Y (19), so the other statements are wrong.Method:Calculate the proton number of each atom, then test the statements against those values.Examiner tips
- Proton number = nucleon number − neutrons; isotopes share the same proton number.
- Step 1: An ionic bond forms between oppositely charged ions, that is positive cations and negative anions. Step 2: The attraction between these oppositely charged ions is strong, not weak. Step 3: So an ionic bond is a strong electrostatic attraction between cations and anions.Method:Pick the option with the correct particles (ions) and the correct strength (strong).Examiner tips
- Ionic bond = strong electrostatic attraction between oppositely charged ions.
- Step 1: Simple molecular compounds have weak forces between their molecules, so only a little energy is needed to separate them, giving a low melting point. Step 2: They have no free-moving charged particles, so they do not conduct electricity even when molten. Step 3: So a simple molecular compound has a low melting point and poor electrical conductivity when molten.Method:Recall the weak forces and lack of free ions in simple molecular compounds to set both properties.Examiner tips
- Simple molecular: low melting point, no charged particles so a poor conductor.
- Step 1: A structure like diamond is a giant covalent lattice with many strong covalent bonds holding every atom in place. Step 2: These strong bonds throughout the structure make it very hard. Step 3: Like diamond it has no free electrons or ions (so it does not conduct), is insoluble in water, and is not soft like graphite, so the likely property is that it is very hard.Method:Use the properties of diamond (giant covalent) to predict the property of boron nitride.Examiner tips
- Diamond-like (giant covalent) = very hard, non-conducting, insoluble.
- Step 1: A molecular formula tells you which elements are present and how many atoms of each there are, for example H₂O has two hydrogen atoms and one oxygen atom. Step 2: So it shows both the number and the type of the different atoms in a molecule. Step 3: It does not just give a total count, a count of elements, or the spatial arrangement, so the correct definition is the number and type of different atoms.Method:Choose the definition that includes both how many atoms and which kinds of atom are present.Examiner tips
- Molecular formula = number and type of each atom (e.g. CO₂ = 1 C and 2 O).
- Step 1: The formula has one calcium, two oxygen and two hydrogen atoms. Step 2: Add the masses: calcium 40, oxygen 2 × 16 = 32, hydrogen 2 × 1 = 2. Step 3: The total is 40 + 32 + 2 = 74, so the relative formula mass is 74.Method:Count each atom from the formula, multiply by its relative atomic mass, then add them up.Examiner tips
- The subscript 2 outside the bracket multiplies everything inside (both O and H).
- Step 1: Electroplating coats an object with an attractive metal layer, so it improves the appearance (statement 1 correct). Step 2: The coating can protect the metal underneath from reacting, making it more resistant to corrosion (statement 3 correct). Step 3: Electroplating does not insulate the object or make it softer, so statements 2 and 4 are wrong, giving 1 and 3.Method:Recall the purposes of electroplating and reject statements about insulating or softening.Examiner tips
- Electroplating: better appearance and corrosion resistance.
- Step 1: A hydrogen-oxygen fuel cell reacts hydrogen with oxygen to form water, so the equation is correct (statement 1). Step 2: The hydrogen is supplied as a fuel, not taken from the air, so statement 2 is wrong. Step 3: The only product is water, which is harmless and does not cause respiratory problems, so statement 3 is wrong, leaving statement 1 only.Method:Check each statement against how a fuel cell works and what it produces.Examiner tips
- Fuel cell: H₂ + O₂ → H₂O; the water product is clean and harmless.
- Step 1: In an endothermic reaction energy is taken in from the surroundings, so the products store more energy than the reactants. Step 2: On a reaction pathway diagram this is shown by the products line being higher than the reactants line. Step 3: So the endothermic diagram is the one where the products are at a higher energy than the reactants.Method:Recall that endothermic means energy in, so the products line is above the reactants line.Examiner tips
- Endothermic: products higher than reactants; exothermic: products lower.
- Step 1: A physical change makes no new substance and can usually be reversed; condensing steam to water just changes state, and dissolving sugar can be reversed by evaporation, so both are physical. Step 2: Rusting forms a new substance (hydrated iron oxide) and cracking breaks molecules into new ones, so both are chemical changes. Step 3: So the physical changes are 2 and 3.Method:Decide for each change whether a new substance is formed; if not, it is physical.Examiner tips
- No new substance = physical (state change, dissolving); new substance = chemical.
- Step 1: Heating blue hydrated copper(II) sulfate drives off its water of crystallisation, leaving white anhydrous copper(II) sulfate. Step 2: This is a reversible reaction: the white anhydrous solid takes water back from the moist air. Step 3: Absorbing water reverses the change and reforms the blue hydrated copper(II) sulfate, so water is absorbed from the air and the reaction reverses.Method:Recognise the reversible loss and regain of water of crystallisation in copper(II) sulfate.Examiner tips
- Hydrated (blue) ⇌ anhydrous (white) + water is a reversible reaction.
- Step 1: Reduction is the loss of oxygen and oxidation is the gain of oxygen. Step 2: The silver oxide loses its oxygen to become silver, so it is reduced, while the carbon gains oxygen to become carbon dioxide, so it is oxidised. Step 3: So the substance that is reduced is the silver oxide.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.
- Step 1: A base dissolving in water makes an alkaline solution, which contains hydroxide ions, OH⁻. Step 2: Thymolphthalein is colourless in acid and neutral solutions but turns blue in alkali. Step 3: So in the alkaline solution the thymolphthalein turns blue, and the ion responsible is the hydroxide ion.Method:Identify the solution as alkaline (OH⁻), then recall the alkaline colour of thymolphthalein.Examiner tips
- Dissolved base = alkali (OH⁻); thymolphthalein is blue in alkali.
- Step 1: Ammonium chloride contains the ammonium ion, and calcium hydroxide is a base. Step 2: Warming an ammonium compound with a base releases ammonia gas, which is the test for the ammonium ion. Step 3: So the gas produced is ammonia.Method:Recognise the ammonium-salt-plus-base reaction that releases ammonia.Examiner tips
- Ammonium ion + base + heat → ammonia gas (turns damp red litmus blue).
- Step 1: Chemical properties are decided by the number of electrons in the outer shell. Step 2: Elements in the same group have the same number of outer shell electrons. Step 3: So L and M have similar chemical properties because they have the same number of electrons in the outer shell.Method:Link similar chemical properties to having the same number of outer shell electrons.Examiner tips
- Same group = same number of outer electrons = similar chemistry.
- Step 1: For metals, reactivity increases down a group, so the lower metal Y is more reactive than the higher metal X. Step 2: For non-metals, reactivity decreases down a group, so the higher non-metal W is more reactive than the lower non-metal Z. Step 3: So Y is more reactive than X, and W is more reactive than Z.Method:Apply the opposite reactivity trends for metals and non-metals to the given positions.Examiner tips
- Down a group: metal reactivity up, non-metal reactivity down.
- Step 1: Use the solubility rules: most sulfates are soluble except calcium, barium and lead; most hydroxides are insoluble except group I and ammonium; silver chloride is insoluble. Step 2: Calcium sulfate, magnesium hydroxide and silver chloride are all insoluble. Step 3: Each of the other sets contains at least one soluble compound (such as sodium sulfate, calcium chloride or calcium nitrate), so only the first set is all insoluble.Method:Apply the solubility rules to each compound and find the set where all three are insoluble.Examiner tips
- Remember nitrates and sodium/ammonium salts are always soluble; silver chloride is insoluble.
- Step 1: The two metals are mixed, not chemically combined, so 18 carat gold is a mixture (statement 1) and an alloy (statement 3), which is a mixture of a metal with other elements. Step 2: Because the metals are not chemically joined it is not a compound, so statement 2 is wrong. Step 3: Metals conduct electricity, so the alloy conducts (statement 4), giving 1, 3 and 4.Method:Decide mixture vs compound from how the metals are combined, then add metal properties.Examiner tips
- Alloy = mixture of metals: not a compound, still conducts electricity.
- Step 1: G reacts vigorously with cold water and its oxide is not reduced by carbon, so G is the most reactive. Step 2: E reacts with steam but not cold water, placing it below G; H does not react with water or steam but does react with dilute acid, placing it below E. Step 3: F does not react with water or acid and its oxide is reduced by carbon, so F is the least reactive, giving the order G, E, H, F.Method:Rank the metals using how vigorously each reacts with water, steam and acid, and whether carbon reduces its oxide.Examiner tips
- Cold water > steam > dilute acid only > oxide reduced by carbon, from most to least reactive.
- Step 1: Some metal compounds provide essential minerals, and dissolved oxygen is needed by aquatic animals to breathe, so 1 and 3 are beneficial. Step 2: Plastics are pollutants that harm aquatic life, so statement 2 is not beneficial. Step 3: Nitrates and phosphates cause too much plant growth and remove oxygen, harming aquatic life, so statement 4 is not beneficial, giving 1 and 3.Method:Sort each substance into helpful (oxygen, trace minerals) or harmful (plastics, excess nutrients).Examiner tips
- Dissolved oxygen and trace minerals help aquatic life; plastics and excess fertilisers harm it.
- Step 1: In water treatment, activated carbon absorbs substances that cause unpleasant tastes and odours. Step 2: Chlorine is added separately to kill bacteria, not to remove tastes and odours. Step 3: So the element used to remove tastes and odours is carbon.Method:Recall the role of carbon (absorbing tastes/odours) versus chlorine (sterilising) in water treatment.Examiner tips
- Carbon (activated) removes tastes and odours; chlorine kills microbes.
- Step 1: NPK fertilisers must supply nitrogen, phosphorus or potassium. Step 2: Sodium nitrate contains the nitrate ion, which provides nitrogen for plant growth. Step 3: The other salts (aluminium sulfate, magnesium chloride and calcium ethanoate) supply none of N, P or K, so the correct choice is sodium nitrate.Method:Find the salt that contains one of the NPK nutrients (here, nitrogen from the nitrate).Examiner tips
- NPK = nitrogen, phosphorus, potassium; a nitrate provides nitrogen.
- Step 1: Photosynthesis uses carbon dioxide and water as raw materials and needs light energy, so 1, 3 and 4 are essential. Step 2: Oxygen is a product of photosynthesis, not a requirement, so statement 2 is not needed. Step 3: So the essential things are carbon dioxide, light and water (1, 3 and 4).Method:Separate the reactants and energy needed (CO₂, water, light) from the product (oxygen).Examiner tips
- Reactants of photosynthesis: carbon dioxide + water + light energy → glucose + oxygen.
- Step 1: In reaction M, calcium carbonate is broken down by heat alone into simpler substances, which is thermal decomposition. Step 2: In reaction N, methane reacts with oxygen and burns, which is combustion. Step 3: So M is thermal decomposition and N is combustion.Method:Identify each reaction by its driver: heat breaking a compound apart, or burning in oxygen.Examiner tips
- Thermal decomposition = broken down by heat; combustion = burning in oxygen.
- Step 1: Test each formula against . For , 2n+2 = 2(3)+2 = 8, which matches, so it is an alkane. Step 2: needs 6 hydrogens to be an alkane, needs 10, and needs 12, so none of these match. Step 3: Only one compound () fits the alkane formula.Method:Calculate 2n+2 for each formula and count how many have that number of hydrogen atoms.Examiner tips
- Alkane check: hydrogens should equal 2 × (carbons) + 2.
- Step 1: Pentane is a saturated alkane with only single bonds. Step 2: Alkanes react with chlorine (in the presence of light) by swapping a hydrogen atom for a chlorine atom, which is a substitution reaction. Step 3: So the reaction of pentane with chlorine is substitution.Method:Use the fact that alkanes are saturated to identify the reaction with chlorine as substitution.Examiner tips
- Alkane + chlorine (with light) = substitution; addition needs a C=C double bond.
- Step 1: Natural gas is mostly methane, typically around 90% by composition. Step 2: The other alkanes and the non-hydrocarbons together make up only a small part. Step 3: So the correct bar chart has a very tall methane bar and only small bars for the other alkanes and the non-hydrocarbons.Method:Pick the chart where the methane bar is by far the tallest, matching natural gas composition.Examiner tips
- Natural gas ≈ 90% methane, so its bar is much taller than the rest.
- Step 1: Alcohols burn in oxygen (combustion) to give carbon dioxide and water, so water is a product of their combustion. Step 2: Alcohols have the -OH functional group, which is different from the -COOH group of carboxylic acids, so that statement is wrong. Step 3: Fermentation happens at about 30 °C, not 300 °C, and ethanol is made from ethene with steam, not with hydrogen, so the only correct statement is that water is produced from the combustion of alcohols.Method:Test each statement against alcohol combustion, functional groups, and how ethanol is made.Examiner tips
- Combustion of alcohols → carbon dioxide + water; the alcohol group is -OH.
- Step 1: Hydrogen gas is produced when an acid reacts with a reactive metal, so P must be the metal sodium (sodium oxide would give water, not hydrogen). Step 2: An acid reacting with a metal gives a salt, and the salt of ethanoic acid is sodium ethanoate, so Q is sodium ethanoate. Step 3: So P is sodium and Q is sodium ethanoate.Method:Use the hydrogen product to identify P as a metal, then name the ethanoate salt as Q.Examiner tips
- Hydrogen produced means a metal reacted; the salt is sodium ethanoate.
- Step 1: The repeat unit -CH₂-CH₂- comes from a small molecule with a carbon-carbon double bond that opened up to join the chain. Step 2: That monomer is ethene, CH₂=CH₂, and the polymer made from it is named poly(ethene). Step 3: So the polymer is poly(ethene) and the monomer is ethene.Method:Recognise the repeat unit of poly(ethene) and name it after its monomer, ethene.Examiner tips
- Monomer = ethene (has C=C); polymer = poly(ethene).
- Step 1: A plastic is hard to break down in the environment if it is unreactive, so it stays for a long time. Step 2: Not reacting with water (statement 2) and being resistant to oxidation (statement 3) both mean it does not break down, so these explain why it is difficult to remove. Step 3: Being a poor conductor (1) and having a low density (4) do not affect how easily it breaks down, so the answer is 2 and 3.Method:Pick the properties that stop the plastic breaking down (chemical unreactivity), not its physical properties.Examiner tips
- Persistent plastics are unreactive: they resist water and oxidation.
- Step 1: The graph needs the volume of gas at different times, so an apparatus that measures gas volume is required. Step 2: A gas syringe collects the gas and reads its volume directly from a scale. Step 3: A beaker and test-tube do not measure gas volume accurately, and a pipette measures liquid volumes, so the correct apparatus is a gas syringe.Method:Match the measurement needed (volume of gas) to the apparatus that reads gas volume directly.Examiner tips
- Use a gas syringe to measure the volume of a gas over time.
- Step 1: Dye X did not move from the baseline, which means it is insoluble in the ethanol solvent, so statement 2 is correct. Step 2: One of S's spots is level with W, so S contains the dye W, making statement 4 correct. Step 3: S shows three spots (not two, so statement 1 is wrong) and none is level with Z (so statement 3 is wrong), leaving 2 and 4 only.Method:Read which spots moved and which line up, then test each statement against the chromatogram.Examiner tips
- Baseline spot = insoluble in the solvent; matching heights = the same substance.
- Step 1: A white precipitate with sodium hydroxide points to calcium, aluminium or zinc, because chromium gives a green precipitate. Step 2: The aluminium and zinc hydroxide precipitates dissolve in excess sodium hydroxide, but the calcium hydroxide precipitate does not. Step 3: Since the white precipitate is insoluble in excess sodium hydroxide, the cation is the calcium ion.Method:Use the white colour and the insolubility in excess sodium hydroxide to identify the calcium ion.Examiner tips
- Calcium hydroxide: white and insoluble in excess; aluminium and zinc hydroxides dissolve in excess.
- Step 1: A gas that bleaches damp litmus paper, turning it white, is chlorine. Step 2: Ammonia would turn damp red litmus blue, and carbon dioxide and oxygen do not bleach litmus. Step 3: So the gas that bleaches the litmus paper white is chlorine.Method:Match the bleaching (turning white) of damp litmus to the test for chlorine gas.Examiner tips
- Chlorine bleaches damp litmus white; ammonia turns red litmus blue.
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