IGCSE Chemistry 0620/31 October/November 2025 Past Paper with Answers

Question 1

Choosing only from the elements H, He, Li, C, N, Ne, Mg, Al, Si, S, Cl, K, Ti, Fe, Cu, Zn and I, which element forms an ion that gives a red colour in a flame test?

1. Li, because lithium ions give a red flame colour
2. K, because potassium ions give a lilac flame colour
3. Cu, because copper ions give a blue-green flame colour
4. Na, because sodium ions give a yellow flame colour

Question 2

Choosing only from the elements H, He, Li, C, N, Ne, Mg, Al, Si, S, Cl, K, Ti, Fe, Cu, Zn and I, which element is present in diamond?

1. C, because diamond is a giant covalent form of carbon
2. Si, because silicon forms a giant structure similar to diamond
3. Al, because aluminium is a hard, shiny metal
4. S, because sulfur is a hard, brittle non-metal

Question 3

Choosing only from the elements H, He, Li, C, N, Ne, Mg, Al, Si, S, Cl, K, Ti, Fe, Cu, Zn and I, which element forms an ion with a charge of 2-?

1. S, which gains two electrons to form a sulfide ion with a 2- charge
2. Cl, which gains one electron to form an ion with a 1- charge
3. N, which gains three electrons to form an ion with a 3- charge
4. Mg, which loses two electrons to form an ion with a 2+ charge

Question 4 · Answer A

Step 1: Bauxite is the main ore of aluminium. Step 2: Aluminium is extracted from bauxite by electrolysis, so the symbol is Al. Step 3: Iron comes from haematite and copper from copper ores, so the element extracted from bauxite is Al.

Question 5 · Answer A

Step 1: Electrical wiring needs a metal that conducts electricity well and can be drawn into wires. Step 2: Copper is an excellent electrical conductor and is ductile, so the symbol is Cu. Step 3: Sulfur is a non-metal, potassium is too reactive and titanium is used for strength, so the answer is Cu.

Question 6 · Answer A

Step 1: The proton number (18) equals the number of protons, so there are 18 protons. Step 2: The number of neutrons = mass number - proton number = 40 - 18 = 22. Step 3: So one atom has 18 protons and 22 neutrons.

Question 7 · Answer A

Step 1: The prefix mon- means one, so monatomic means one atom. Step 2: A monatomic element exists as single, separate atoms that are not joined to others. Step 3: Diatomic means two atoms joined, so monatomic means existing as single atoms.

Question 8 · Answer A

Step 1: Atoms react to gain a full (stable) outer shell of electrons. Step 2: Argon already has a full outer shell, so it has no tendency to gain, lose or share electrons. Step 3: Having a full outer shell makes argon stable and therefore unreactive.

Question 9 · Answer A

Step 1: Heating a gas gives its particles more energy so they move faster and spread out, pushing the plunger out, so the volume increases. Step 2: Increasing the pressure pushes the particles closer together, so the volume decreases. Step 3: So volume increases on heating and decreases when pressure is raised.

Question 10 · Answer A

Step 1: Compare the masses of the positive ions only. Step 2: Sodium has the largest mass, 8.3 mg per 1000 cm³, which is more than lithium (4.3), calcium (1.4) or ammonium (0.5). Step 3: So the positive ion with the highest concentration is sodium.

Question 11 · Answer A

The formula for the sulfate ion is $\text{SO}_4^{2-}$.

Question 12 · Answer A

Step 1: Halide ions are tested by acidifying with dilute nitric acid and adding aqueous silver nitrate. Step 2: With iodide ions a yellow precipitate of silver iodide forms. Step 3: A white precipitate shows chloride, the hydroxide test detects ammonium and limewater tests for carbon dioxide, so iodide gives a yellow precipitate.

Question 13 · Answer A

Step 1: The mass of nitrate is proportional to the volume of water. Step 2: Mass in 200 cm³ = 9.5 × (200 ÷ 1000) = 9.5 × 0.20. Step 3: This gives 1.9 mg of nitrate ions in 200 cm³.

Question 14 · Answer A

Step 1: NPK stands for nitrogen, phosphorus and potassium. Step 2: With potassium already provided, nitrogen comes from the nitrate ion $\text{NO}_3^{-}$ and phosphorus comes from the phosphate ion $\text{PO}_4^{3-}$. Step 3: So the pair that supplies N and P is nitrate and phosphate.

Question 15 · Answer A

Step 1: Nitrate and phosphate ions are plant nutrients found in fertilisers. Step 2: When they wash into rivers they cause rapid growth of algae; the algae later decompose, using up dissolved oxygen (deoxygenation), which harms aquatic life. Step 3: So the ions responsible are nitrate and phosphate.

Question 16 · Answer A

Step 1: The process boils the water to form vapour and then cools the vapour back to liquid. Step 2: Boiling a liquid and condensing the vapour to collect the pure liquid is called distillation. Step 3: Filtration removes insoluble solids and chromatography separates dissolved substances, so this is distillation.

Question 17 · Answer A

Step 1: Count each type of atom in the displayed formula. Step 2: There are 10 carbon atoms, 16 hydrogen atoms and 2 oxygen atoms. Step 3: Writing these together gives the molecular formula $\text{C}_{10}\text{H}_{16}\text{O}_2$.

Question 18 · Answer A

Step 1: Unsaturated means the molecule contains a carbon-carbon double bond. Step 2: The feature that shows unsaturation is therefore the C=C double bond. Step 3: A C=O, O-H or C-H bond does not make a molecule unsaturated, so the answer is the C=C double bond.

Question 19 · Answer A

Step 1: Carbon: 11 × 12 = 132. Step 2: Hydrogen: 14 × 1 = 14, and oxygen: 3 × 16 = 48. Step 3: Adding gives 132 + 14 + 48 = 194, so the relative molecular mass is 194.

Question 20 · Answer A

Step 1: The word hydrocarbon combines hydrogen and carbon. Step 2: A hydrocarbon is a compound that contains only the elements hydrogen and carbon. Step 3: If oxygen or other elements are present it is not a hydrocarbon, so the correct meaning is a compound of hydrogen and carbon only.

Question 21 · Answer A

Step 1: Smaller molecules have lower boiling points and are collected near the top of the fractionating column. Step 2: Of the listed fractions, gasoline (petrol) contains the smallest molecules, so it has the lowest boiling point. Step 3: Lubricating oil, diesel oil and fuel oil have larger molecules and higher boiling points, so the answer is gasoline.

Question 22 · Answer A

Step 1: Viscosity (thickness) increases as the hydrocarbon molecules get larger. Step 2: Lubricating oil is the heaviest of the listed fractions, with the largest molecules, so it has the highest viscosity. Step 3: Gasoline, naphtha and kerosene are lighter fractions and flow more easily, so the answer is lubricating oil.

Question 23 · Answer A

Step 1: Each fraction has typical uses based on its molecule size. Step 2: Kerosene (paraffin) is the fraction used as jet fuel. Step 3: Gasoline fuels cars, diesel oil fuels lorries and lubricating oil makes lubricants, so jet fuel is kerosene.

Question 24 · Answer A

Step 1: Alkanes have the general formula $\text{C}_n\text{H}_{2n+2}$. Step 2: With n = 7, the number of hydrogen atoms is (2 × 7) + 2 = 16. Step 3: So heptane is $\text{C}_7\text{H}_{16}$.

Question 25 · Answer A

Step 1: Octane has 8 carbon atoms, between heptane (7) and nonane (9). Step 2: Its melting point should lie between heptane (-90 °C) and nonane (-54 °C). Step 3: A value such as -57 °C fits this range, so the best prediction is about -57 °C.

Question 26 · Answer A

Step 1: Heating a large hydrocarbon with a catalyst to break it into smaller molecules is called cracking. Step 2: Cracking is endothermic and produces smaller alkanes and alkenes. Step 3: Fractional distillation only separates molecules, polymerisation joins them and combustion burns them, so the reaction is cracking.

Question 27 · Answer A

Step 1: Ethene is $\text{C}_2\text{H}_4$, an alkene, so it has a carbon-carbon double bond. Step 2: Each carbon forms four bonds: two go into the C=C double bond region and two are C-H bonds, so each carbon bonds to two hydrogen atoms. Step 3: So the displayed formula shows a C=C double bond with two hydrogen atoms on each carbon.

Question 28 · Answer A

Step 1: Ethene molecules contain a C=C double bond and add together without losing any atoms. Step 2: Joining many unsaturated monomers into a polymer with no small molecule lost is addition polymerisation. Step 3: Condensation polymerisation loses a small molecule (such as water), so forming poly(ethene) is addition polymerisation.

Question 29 · Answer A

Step 1: Most plastics are non-biodegradable, so they build up in landfill sites and accumulate in the oceans. Step 2: If they are burned, they can release toxic or acidic gases. Step 3: So two genuine challenges are filling landfill and producing toxic gases on burning.

Question 30 · Answer A

Step 1: Down Group VII the colour darkens and the state changes from gas to solid. Step 2: Chlorine, near the top of the group, is a pale yellow-green gas at room temperature and pressure. Step 3: Bromine is a red-brown liquid and iodine a grey-black solid, so chlorine is a pale yellow-green gas.

Question 31 · Answer A

Step 1: Drinking water must be safe from disease-causing microbes. Step 2: Chlorine is added to kill bacteria and other microbes, making the water safe to drink. Step 3: Colour and taste are dealt with by other steps, so the role of chlorine is to kill microbes.

Question 32 · Answer A

Step 1: A substance is a gas at temperatures above its boiling point. Step 2: The boiling point of iodine is 184 °C, and 195 °C is higher than this. Step 3: So at 195 °C iodine is a gas, because the temperature is above its boiling point.

Question 33 · Answer A

Step 1: Group I metals are unusual metals because they are soft. Step 2: Potassium is soft enough to be cut with a knife, and like other metals it is shiny when freshly cut and conducts electricity. Step 3: It is not hard, it does conduct electricity and it is a solid, so the correct extra property is that it is soft.

Question 34 · Answer A

Step 1: Potassium is very reactive and less dense than water, so it floats and moves about on the surface. Step 2: The reaction releases heat, melting the potassium into a ball, and produces hydrogen gas, causing fizzing. Step 3: The heat ignites the hydrogen, giving a lilac flame, so all these observations are seen.

Question 35 · Answer A

Step 1: Hydrogen is a flammable gas. Step 2: When a lighted splint is held to hydrogen it burns rapidly with a characteristic squeaky pop. Step 3: A glowing splint relighting tests for oxygen and limewater tests for carbon dioxide, so the squeaky pop confirms hydrogen.

Question 36 · Answer A

Step 1: The product is $2\text{K}_2\text{O}$, which contains 4 potassium atoms and 2 oxygen atoms. Step 2: To balance, the left side needs 4 K atoms (so 4K) and 2 O atoms (so one $\text{O}_2$ molecule). Step 3: This gives $4\text{K} + \text{O}_2 \rightarrow 2\text{K}_2\text{O}$, balanced on both sides.

Question 37 · Answer A

Step 1: Potassium oxide is a metal oxide that dissolves to form an alkali. Step 2: Alkaline solutions have a pH greater than 7, usually around pH 11 to 14. Step 3: Of the choices, pH 12 is alkaline, while pH 7 is neutral and pH 5 and pH 1 are acidic, so the answer is pH 12.

Question 38 · Answer A

Step 1: Potassium (2,8,8,1) loses its single outer electron to become 2,8,8 with a 1+ charge. Step 2: Chlorine (2,8,7) gains that electron to become 2,8,8 with a 1- charge. Step 3: So both ions have the configuration 2,8,8, with potassium 1+ and chloride 1-.

Question 39 · Answer A

Step 1: Carbon (coke) burns in the hot air blast, releasing heat to keep the furnace very hot. Step 2: Burning carbon also produces carbon dioxide, which then reacts with more carbon to make carbon monoxide, the reducing agent. Step 3: So carbon is used both as a fuel to heat the furnace and to produce the gas that leads to reduction of the iron ore.

Question 40 · Answer A

Step 1: In the furnace, calcium carbonate (limestone) thermally decomposes to calcium oxide and carbon dioxide. Step 2: The calcium oxide is a base and reacts with the acidic silicon(IV) oxide (sand) impurity. Step 3: This reaction forms molten calcium silicate, which is the slag, so slag comes from calcium oxide reacting with silicon(IV) oxide.

Question 41 · Answer A

Step 1: Oxidation in terms of oxygen means gaining oxygen. Step 2: In the equation, CO (one oxygen) becomes $\text{CO}_2$ (two oxygens), so the carbon monoxide has gained oxygen. Step 3: Gaining oxygen means it is oxidised, while the iron(III) oxide loses oxygen and is reduced.

Question 42 · Answer A

Step 1: Stainless steel is iron mixed with other metals to stop it rusting. Step 2: The two metals most commonly added are chromium and nickel. Step 3: Copper/tin make bronze and zinc is used in brass or galvanising, so the answer is chromium and nickel.

Question 43 · Answer A

Step 1: Stainless steel is strong and resists rusting, so it is used where contact with water or food occurs. Step 2: Cutlery is a common use because it stays shiny and does not rust. Step 3: Wiring uses copper, windows use glass and balloons use rubber, so a correct use is making cutlery.

Question 44 · Answer A

Step 1: The more vigorously a metal reacts with the acid, the more reactive it is. Step 2: Titanium does not react (least reactive), zinc reacts slowly, magnesium reacts quickly and calcium reacts most vigorously (most reactive). Step 3: So from least to most reactive the order is titanium, zinc, magnesium, calcium.

Question 45 · Answer A

Step 1: A reactive metal reacting with an acid produces a salt and hydrogen. Step 2: The salt of ethanoic acid is an ethanoate, so calcium gives calcium ethanoate, and hydrogen gas is released. Step 3: So the products are calcium ethanoate and hydrogen.

Question 46 · Answer A

Step 1: Electrolysis needs an ionic compound that is molten or dissolved in water so the ions can move. Step 2: An electric current is passed through this electrolyte. Step 3: The current breaks the compound down into simpler substances at the electrodes, so electrolysis is the decomposition of an ionic compound by electricity.

Question 47 · Answer A

Step 1: In electrolysis the cathode is the negative electrode. Step 2: It is connected to the negative terminal of the power supply, and positive ions (cations) move towards it. Step 3: The positive electrode is the anode, so the cathode is the one joined to the negative terminal.

Question 48 · Answer A

Step 1: During the electrolysis of dilute sulfuric acid, hydrogen forms at the cathode and oxygen forms at the anode. Step 2: At the anode, hydroxide ions from the water are discharged to release oxygen. Step 3: So the product at the anode is oxygen, not hydrogen, sulfur or chlorine.

Question 49 · Answer A

Step 1: An inert electrode is one that takes no part in the chemical reaction. Step 2: It conducts the current but does not react with the electrolyte or the products. Step 3: So inert means unreactive, not that it dissolves or acts as a catalyst.

Question 50 · Answer A

Step 1: In graphite the layers of carbon atoms can slide over each other. Step 2: This makes graphite slippery, so it is used as a lubricant (and in pencil leads). Step 3: Graphite conducts electricity (so it is not an insulator) and is soft (so it is not used for cutting), so a correct use is as a lubricant.

Question 51 · Answer A

Step 1: In a chemical change, new substances with different properties are made. Step 2: Here copper(II) sulfate, water and carbon dioxide are produced, which are new substances. Step 3: So this is a chemical change because new substances are formed.

Question 52 · Answer A

Step 1: The excess copper(II) carbonate is an insoluble solid in a solution. Step 2: Filtration separates an insoluble solid from a liquid: the solid stays on the filter paper and the solution passes through. Step 3: So filtration removes the excess copper(II) carbonate.

Question 53 · Answer A

Step 1: Copper(II) sulfate is made by reacting a copper compound with sulfuric acid. Step 2: Copper(II) oxide is a base that reacts with sulfuric acid to give copper(II) sulfate and water. Step 3: Sodium chloride, calcium carbonate and magnesium oxide would not give copper(II) sulfate, so the answer is copper(II) oxide.

Question 54 · Answer A

Step 1: Aqueous copper(II) sulfate is the compound dissolved in water. Step 2: Hydrated copper(II) sulfate is the solid that is chemically combined with water (water of crystallisation). Step 3: Anhydrous copper(II) sulfate is the solid containing no water, so the correct matching is aqueous = dissolved, hydrated = combined with water, anhydrous = no water.

Question 55 · Answer A

Step 1: A reaction has finished when no more gas is produced, so the curve becomes a horizontal (constant) line. Step 2: The line first becomes horizontal at about 119 s, where the volume stays at 250 cm³. Step 3: So the reaction finishes at 119 s.

Question 56 · Answer A

Step 1: Larger pieces of the same mass have a smaller total surface area. Step 2: A smaller surface area means fewer collisions per second between acid particles and the magnesium, so the reaction is slower. Step 3: A slower reaction takes longer to finish, so the time increases.

Question 57 · Answer A

Step 1: The total volume of hydrogen depends on the amount (mass) of magnesium that reacts, not on the size of the pieces. Step 2: The same total mass of magnesium is used, so the same number of moles of magnesium reacts. Step 3: Therefore the total volume of hydrogen produced is unchanged.

Question 58 · Answer A

Step 1: If the products are at a lower energy level than the reactants, energy has been released to the surroundings. Step 2: Releasing energy to the surroundings is an exothermic change. Step 3: So this is exothermic, because the products are lower in energy than the reactants.