IGCSE Chemistry 0620/33 October/November 2025 Past Paper with Answers

Question 1

Which element is used to make food and drink cans because of its resistance to corrosion?

1. aluminium, Al, which forms a protective oxide layer that resists corrosion
2. iron, Fe, which rusts readily when exposed to air and water
3. sodium, Na, which reacts vigorously with water
4. calcium, Ca, which reacts steadily with water and air

Question 2

Which metal does not react with water, dilute acids or oxygen?

1. gold, Au, which is very unreactive and found native in nature
2. calcium, Ca, which reacts steadily with cold water
3. iron, Fe, which reacts with dilute acid and rusts in air
4. sodium, Na, which reacts violently with water

Question 3

In a hydrogen-oxygen fuel cell, hydrogen reacts with one other element to produce water and electrical energy. Which element reacts with hydrogen in this fuel cell?

1. oxygen, O, which combines with hydrogen to form water
2. nitrogen, N, which is unreactive in the fuel cell
3. carbon, C, which is not a reactant in a hydrogen fuel cell
4. helium, He, a noble gas that does not react

Question 4 · Answer A

Step 1: Methane has the formula $\text{CH}_4$, so it contains carbon and hydrogen only. Step 2: In methane one carbon atom is bonded to four hydrogen atoms. Step 3: Oxygen, nitrogen and sulfur are not present in methane, so the element bonded to hydrogen is carbon.

Question 5 · Answer A

Step 1: The number of nucleons is the nucleon (mass) number, which is 131. Step 2: The number of neutrons = nucleon number - proton number = 131 - 54 = 77. Step 3: So the atom has 77 neutrons and 131 nucleons.

Question 6 · Answer A

Step 1: When a gas is cooled, the particles move more slowly and take up less space, so decreasing the temperature decreases the volume. Step 2: When the pressure on a gas is reduced, the particles push the plunger out, so decreasing the pressure increases the volume. Step 3: So lowering the temperature decreases the volume while lowering the pressure increases the volume.

Question 7 · Answer A

Step 1: In a solid the particles are packed closely together in a regular arrangement (rows or a lattice). Step 2: They have only enough energy to vibrate about fixed positions, so they cannot move from place to place. Step 3: So solid xenon has a regular arrangement of particles that vibrate about fixed positions.

Question 8 · Answer A

Step 1: The concentration of each positive ion is given by its mass in the same 600 cm³ sample. Step 2: Calcium has the largest mass at 25.0 mg, compared with 6.2 mg, 2.7 mg and 0.1 mg for the others. Step 3: So the positive ion with the highest concentration is calcium.

Question 9 · Answer A

The ion $\text{PO}_4^{3-}$, made of phosphorus and oxygen with a 3- charge, is named phosphate (e.g. in calcium phosphate and many fertilisers).

Question 10 · Answer A

Step 1: To test for nitrate ions, add aqueous sodium hydroxide and a small piece of aluminium (or Devarda's alloy) and warm. Step 2: The nitrate ions are reduced to ammonia gas, which turns damp red litmus paper blue. Step 3: The silver nitrate test is for halides, limewater tests for carbon dioxide and the acid test detects carbonates, so the correct nitrate test produces ammonia.

Question 11 · Answer A

Step 1: The mass of ions is proportional to the volume of water. Step 2: Mass in 200 cm³ = 2.7 × (200 ÷ 600) = 2.7 × (1/3). Step 3: This gives 0.9 mg of sodium ions in 200 cm³.

Question 12 · Answer A

Step 1: Nitrate ions are a major component of fertilisers used on farmland. Step 2: When rain washes fertiliser off the land, the nitrate ions run into rivers, raising their concentration. Step 3: Dissolved gases and river-bed minerals do not add nitrate, so the source is fertilisers.

Question 13 · Answer A

Step 1: Insoluble substances are solids that do not dissolve in the water. Step 2: Filtration traps these insoluble solids while the water passes through; sedimentation lets them settle out. Step 3: Chlorination kills microbes and distillation removes dissolved substances, so insoluble solids are removed by filtration (or sedimentation).

Question 14 · Answer A

Step 1: Harmful microbes such as bacteria come from waste containing them. Step 2: Sewage (human and animal waste) entering a river is a common source of these microbes. Step 3: Dissolved gases and sand do not contain harmful microbes, so the source is sewage.

Question 15 · Answer A

Step 1: Microbes in drinking water are killed by adding chlorine to the water. Step 2: This process, chlorination, makes the water safe to drink. Step 3: Filtration and sedimentation only remove solids, and adding fertilisers would pollute the water, so chlorination is the correct treatment.

Question 16 · Answer A

Step 1: Bad tastes and odours come from dissolved substances in the water. Step 2: Carbon (activated carbon) adsorbs these substances, removing the taste and odour. Step 3: Chlorine kills microbes, sand filters solids and sodium chloride is a salt, so carbon is the substance that removes taste and odour.

Question 17 · Answer A

Step 1: A molecular formula lists the total number of atoms of each element in the molecule. Step 2: Compound B has 10 carbon, 12 hydrogen and 1 oxygen atom. Step 3: Writing these together gives the molecular formula $\text{C}_{10}\text{H}_{12}\text{O}$.

Question 18 · Answer A

Step 1: An unsaturated molecule contains at least one carbon-carbon double bond. Step 2: The C=C double bond is the feature that shows unsaturation, because it can take part in addition reactions. Step 3: Single bonds (C-H, C-C, O-H) are found in saturated molecules too, so only the C=C double bond shows unsaturation.

Question 19 · Answer A

Step 1: Multiply each element's relative atomic mass by the number of atoms: carbon 9 × 12 = 108, hydrogen 10 × 1 = 10, oxygen 3 × 16 = 48. Step 2: Add these together: 108 + 10 + 48. Step 3: This gives a relative molecular mass of 166.

Question 20 · Answer A

Step 1: Fractions with larger molecules have higher boiling points and condense lower in the fractionating column. Step 2: Lubricating oil is collected near the bottom, so it has the largest molecules and the highest boiling point. Step 3: Gasoline, naphtha and kerosene are collected higher up and have lower boiling points, so the highest boiling point fraction is lubricating oil.

Question 21 · Answer A

Step 1: Shorter hydrocarbon chains have smaller molecules, lower boiling points and are collected near the top of the column. Step 2: Gasoline/petrol is collected at the top, so it has the shortest chains. Step 3: Fuel oil, lubricating oil and diesel oil are heavier fractions with longer chains, so the shortest chain length is in gasoline/petrol.

Question 22 · Answer A

Step 1: Each petroleum fraction has typical uses based on its properties. Step 2: Lubricating oil is the fraction used for lubricants and for making waxes and polishes. Step 3: Gasoline is a fuel for cars, naphtha is a chemical feedstock and kerosene is jet fuel, so waxes and polishes come from lubricating oil.

Question 23 · Answer A

Step 1: A homologous series is a family of organic compounds. Step 2: Its members share the same functional group and so have similar chemical properties, with each member differing by $\text{CH}_2$. Step 3: They do not all share one molecular formula, so the correct meaning is a family of compounds with the same functional group and similar chemical properties.

Question 24 · Answer A

Step 1: Each member of this series increases by one carbon and two hydrogens: $\text{C}_n\text{H}_{2n+1}\text{OH}$. Step 2: For octanol, n = 8, so the hydrogen count is 2(8) + 1 = 17. Step 3: So octanol is $\text{C}_8\text{H}_{17}\text{OH}$, fitting between heptanol ($\text{C}_7\text{H}_{15}\text{OH}$) and nonanol ($\text{C}_9\text{H}_{19}\text{OH}$).

Question 25 · Answer A

Step 1: Melting points increase steadily up the homologous series. Step 2: Nonanol lies between octanol (-16 °C) and decanol (7 °C), so its melting point should be between these two values. Step 3: A value such as -4 °C lies between -16 °C and 7 °C, so it is a reasonable prediction; values below -16 °C or above 7 °C do not fit the trend.

Question 26 · Answer A

Step 1: Complete combustion of a fuel containing carbon, hydrogen and oxygen happens in excess oxygen. Step 2: The carbon is fully oxidised to carbon dioxide and the hydrogen forms water. Step 3: Carbon monoxide and soot form only in incomplete combustion, so the other product here is carbon dioxide.

Question 27 · Answer A

Step 1: Anhydrous copper(II) sulfate is white and is used to test for water. Step 2: When water is added it forms hydrated copper(II) sulfate, which is blue. Step 3: So the colour change observed is from white to blue.

Question 28 · Answer A

Step 1: Fermentation makes ethanol from a sugar. The reactant is aqueous glucose. Step 2: Yeast is added to provide enzymes, and the mixture is kept warm at about 25-35 °C in the absence of oxygen. Step 3: Ethene + steam is the catalytic (hydration) route, not fermentation, and the other options use wrong reactants or conditions, so the correct set is aqueous glucose with yeast at 25-35 °C.

Question 29 · Answer A

Step 1: Ethanoic acid is $\text{CH}_3\text{COOH}$: a methyl group joined to a carboxylic acid group. Step 2: The carboxylic acid carbon has one double bond to oxygen (C=O) and a single bond to an O-H group. Step 3: So the displayed formula shows CH3 joined to a carbon bearing a C=O and an O-H, with all bonds drawn.

Question 30 · Answer A

Step 1: The group number of a main-group element equals the number of electrons in its outer shell. Step 2: Silicon is in Group IV because it has 4 electrons in its outer electron shell. Step 3: The number of shells gives the period, not the group, so silicon is in Group IV because of its 4 outer electrons.

Question 31 · Answer A

Step 1: A substance is solid below its melting point, liquid between its melting and boiling points, and gas above its boiling point. Step 2: At 175 °C the temperature is below tin's melting point of 232 °C. Step 3: So tin is a solid at 175 °C because it has not yet reached its melting point.

Question 32 · Answer A

Step 1: Down Group VII the elements get darker and change state: chlorine is a gas, bromine a liquid and iodine a solid. Step 2: Iodine is a grey-black solid at room temperature and pressure. Step 3: The gaseous and liquid options describe chlorine and bromine, so iodine is a grey-black solid.

Question 33 · Answer A

Step 1: Calcium reacts with cold water to form calcium hydroxide and hydrogen gas. Step 2: Bubbles of hydrogen gas are seen (effervescence) and the calcium gradually gets smaller as it reacts; the mixture also warms up and a white solid forms. Step 3: So a correct pair of observations is that bubbles are given off and the calcium gets smaller.

Question 34 · Answer A

Step 1: A reactive metal reacting with water forms a metal hydroxide and hydrogen gas. Step 2: Calcium reacts with water to form calcium hydroxide and hydrogen. Step 3: Calcium oxide forms with oxygen not water, and oxygen is not a product, so the equation is calcium + water -> calcium hydroxide + hydrogen.

Question 35 · Answer A

Step 1: Acids have a pH below 7; the stronger or more concentrated the acid, the lower the pH. Step 2: Dilute nitric acid is a strong acid, so it has a low pH such as pH 1. Step 3: pH 7 is neutral and pH 9 and pH 13 are alkaline, so the pH of dilute nitric acid is pH 1.

Question 36 · Answer A

Step 1: Neutralisation is the reaction of hydrogen ions with hydroxide ions to form water. Step 2: Combining $\text{H}^+$ with $\text{OH}^-$ gives $\text{H}_2\text{O}$, which balances for hydrogen and oxygen. Step 3: The other ions do not balance the equation, so the missing ion is $\text{OH}^-$.

Question 37 · Answer A

Step 1: Methyl orange is red in acidic solution and yellow in neutral or alkaline solution. Step 2: As lithium carbonate neutralises the nitric acid, the solution stops being acidic, so the methyl orange changes from red to yellow. Step 3: The reverse change and the other colours are wrong for methyl orange, so the change is from red to yellow.

Question 38 · Answer A

Step 1: Metal ions can be identified by the colour they give in a flame test. Step 2: Lithium ions give a red flame colour. Step 3: A lilac flame indicates potassium, silver nitrate tests for halides and limewater tests for carbon dioxide, so lithium ions give a red flame.

Question 39 · Answer A

Step 1: Lithium chloride is ionic, so electrons are transferred. Lithium (2,1) loses its single outer electron to become $\text{Li}^+$ with no outer electrons. Step 2: Chlorine (2,8,7) gains that electron to reach eight outer electrons, becoming $\text{Cl}^-$. Step 3: So the diagram shows $\text{Li}^+$ with an empty outer shell and $\text{Cl}^-$ with eight outer electrons (seven dots plus one cross), with 1+ and 1- charges.

Question 40 · Answer A

Step 1: In the reaction one compound breaks down into two simpler substances when heated. Step 2: Breaking a compound down using heat is called thermal decomposition. Step 3: Neutralisation, combustion and precipitation describe different reactions, so heating calcium carbonate is thermal decomposition.

Question 41 · Answer A

Step 1: Metal oxides are generally basic, while non-metal oxides are generally acidic. Step 2: Calcium is a metal, so calcium oxide is a metal oxide and is therefore basic. Step 3: It is not a non-metal oxide and does react with acids, so calcium oxide is a basic oxide.

Question 42 · Answer A

Step 1: The name desulfurisation tells us a sulfur-containing gas is being removed. Step 2: Calcium oxide (a base) reacts with acidic sulfur dioxide to remove it from the flue gases. Step 3: Nitrogen, oxygen and carbon monoxide are not the target, so the gas removed is sulfur dioxide.

Question 43 · Answer A

Step 1: Reduction is the loss of oxygen from a substance. Step 2: In the equation, zinc oxide (ZnO) loses its oxygen to become zinc (Zn), while the carbon gains the oxygen to form $\text{CO}_2$. Step 3: Because the zinc oxide loses oxygen, it is reduced.

Question 44 · Answer A

Step 1: Brass is an alloy made by mixing two metals. Step 2: Brass is a mixture of copper and zinc, so the other metal alongside zinc is copper. Step 3: Iron, tin and lead are not the main metals in brass, so the answer is copper.

Question 45 · Answer A

Step 1: In an alloy, atoms of different sizes disrupt the regular layers, making it harder and stronger than the pure metal. Step 2: Brass is therefore harder, stronger and more resistant to corrosion than pure zinc, which is useful for hard-wearing water taps. Step 3: The other options describe properties brass does not have, so brass is chosen because it is harder, stronger and corrosion-resistant.

Question 46 · Answer A

Step 1: Sulfur dioxide is an acidic gas that dissolves in rain water. Step 2: This forms acidic rain, which damages buildings, plants and aquatic life. Step 3: Sulfur dioxide does not deplete ozone, add oxygen or make rain alkaline, so its adverse effect is acid rain.

Question 47 · Answer A

Step 1: Plants take in carbon dioxide from the air during photosynthesis. Step 2: Planting more trees increases the amount of carbon dioxide removed from the atmosphere. Step 3: Burning fossil fuels, cutting down forests and using more cars all add carbon dioxide, so the helpful strategy is planting trees.

Question 48 · Answer A

Step 1: The faster a metal produces bubbles and heat with acid, the more reactive it is. Step 2: W shows no reaction (least reactive), Fe reacts very slowly, Mg reacts quickly, and Ba reacts most vigorously (most reactive). Step 3: So from least to most reactive the order is W, Fe, Mg, Ba.

Question 49 · Answer A

Step 1: In electrolysis the anode is always the electrode connected to the positive terminal of the power supply. Step 2: The nickel object being plated is the cathode (negative electrode), so the anode is the positive electrode (the copper). Step 3: The negative electrode is the cathode and the electrolyte is not an electrode, so the anode is the electrode connected to the positive terminal.

Question 50 · Answer A

Step 1: To plate copper onto an object, the electrolyte must contain copper ions. Step 2: A soluble copper salt such as copper(II) sulfate provides $\text{Cu}^{2+}$ ions, which are deposited onto the cathode as copper. Step 3: Sodium chloride, sodium hydroxide and plain sulfuric acid contain no copper ions, so the correct electrolyte is aqueous copper(II) sulfate.

Question 51 · Answer A

Step 1: During electrolysis of molten lead(II) bromide, the bromide ions are discharged at the anode. Step 2: This forms bromine, which is seen as a red-brown vapour at the anode. Step 3: Chlorine, hydrogen and sodium are not produced here, so the gas at the anode is red-brown bromine.

Question 52 · Answer A

Step 1: A chemical change makes new substances, while a physical change does not. Step 2: Electrolysis of molten lead(II) bromide produces lead and bromine, which are new substances different from the starting compound. Step 3: Because new substances are formed, it is a chemical change.

Question 53 · Answer A

Step 1: The salt formed is named after the acid: an ethanoate salt comes from ethanoic acid. Step 2: Since the product is cobalt(II) ethanoate, the dilute acid used must be ethanoic acid. Step 3: Hydrochloric, sulfuric and nitric acids give chloride, sulfate and nitrate salts, so the acid here is ethanoic acid.

Question 54 · Answer A

Step 1: Hydrogen is tested using a lighted (burning) splint held near the gas. Step 2: The hydrogen burns rapidly, giving a characteristic squeaky pop. Step 3: Relighting a glowing splint tests for oxygen, limewater tests for carbon dioxide and damp red litmus tests for ammonia, so the hydrogen test gives a squeaky pop.

Question 55 · Answer A

Step 1: Hydrated means the solid contains water of crystallisation, while anhydrous means the solid contains no water. Step 2: Aqueous means the substance is dissolved in water to form a solution. Step 3: So hydrated = solid containing water, anhydrous = solid with no water, and aqueous = dissolved in water.

Question 56 · Answer A

In a chemical equation the state symbol (l) means liquid (for example, $\text{H}_2\text{O(l)}$ is liquid water), while (s) is solid, (g) is gas and (aq) is aqueous.

Question 57 · Answer A

Step 1: To read the volume at 24 s, follow the time axis up to the curve and across to the volume axis. Step 2: At 24 s the curve is in its steep early region, just above the 50 cm³ gridline, giving about 55 cm³. Step 3: 110 cm³ is the final plateau value and the other readings do not match 24 s, so the volume at 24 s is 55 cm³.

Question 58 · Answer A

Step 1: A lower concentration means fewer acid particles in the same volume, so there are fewer collisions per second. Step 2: Fewer collisions per second means a slower rate of reaction. Step 3: A slower reaction takes more time to finish, so the time taken increases.

Question 59 · Answer A

Step 1: The total volume of carbon dioxide depends on the amount of the limiting reactant, the sodium hydrogencarbonate. Step 2: The acid is still in excess and the amount of sodium hydrogencarbonate is unchanged, so the same number of moles of carbon dioxide is made. Step 3: Therefore the total volume of carbon dioxide does not change; only the rate changes.

Question 60 · Answer A

Step 1: In a reaction pathway diagram, the relative heights show whether energy is taken in or given out. Step 2: Here the products are drawn higher in energy than the reactants, which means energy has been taken in from the surroundings. Step 3: A reaction that takes in energy is endothermic, so the diagram with products above reactants shows an endothermic reaction.