IGCSE Chemistry 0620/33 May/June 2025 Past Paper with Answers

Question 1

A list of substances includes calcium, carbon, chlorine, gold, nitrogen and oxygen. Which of these is an element that forms an ion with a 3- charge?

1. nitrogen, which gains three electrons to form a 3- ion
2. oxygen, which gains two electrons to form a 2- ion
3. chlorine, which gains one electron to form a 1- ion
4. calcium, which loses two electrons to form a 2+ ion

Question 2

A list of substances includes calcium, carbon, copper, nitrogen, oxygen and potassium. Which of these is a transition element?

1. copper, found in the central block of the Periodic Table
2. calcium, found in Group II of the Periodic Table
3. potassium, found in Group I of the Periodic Table
4. nitrogen, found in Group V of the Periodic Table

Question 3

A list of substances includes carbon dioxide, chlorine, neon, nitrogen, oxygen and sulfur dioxide. Which of these is a gas that is identified using limewater?

1. carbon dioxide, which turns limewater milky
2. oxygen, which relights a glowing splint
3. chlorine, which bleaches damp litmus paper
4. nitrogen, which puts out a lighted splint

Question 4 · Answer A

Step 1: Clean dry air is about 78% nitrogen and 21% oxygen, with small amounts of other gases. Step 2: The gas making up roughly 78% is therefore nitrogen. Step 3: Oxygen is about 21%, while carbon dioxide and neon are only present in small or trace amounts, so the answer is nitrogen.

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 it is widely used in electrical wiring. Step 3: Calcium and potassium are too reactive, and carbon is a non-metal, so copper is the correct choice.

Question 6 · Answer A

Step 1: In the reactivity series, gold is one of the least reactive metals and is found uncombined (native) in nature. Step 2: Potassium and calcium are very reactive (they react with water), and copper is more reactive than gold. Step 3: So the least reactive metal in the list is gold.

Question 7 · Answer A

Step 1: A diatomic molecule contains two atoms joined together, and the question also needs a substance that is solid at room temperature. Step 2: Iodine is a Group VII element that exists as $\text{I}_2$ molecules and is a grey solid at room temperature and pressure. Step 3: Chlorine and nitrogen are diatomic but are gases, and neon exists as single atoms, so the answer is iodine.

Question 8 · Answer A

Step 1: In the blast furnace, limestone decomposes to calcium oxide, which is a basic oxide. Step 2: Calcium oxide reacts with the acidic sandy impurities (silicon dioxide) to form molten slag (calcium silicate), which is removed. Step 3: Carbon is the reducing agent and carbon dioxide is a waste gas, so the compound that forms slag is calcium oxide.

Question 9 · Answer A

Step 1: The relative atomic mass of an element can be read from the Periodic Table. Step 2: Carbon has a relative atomic mass of 12. Step 3: Oxygen is 16, nitrogen is 14 and calcium is 40, so the element with a relative atomic mass of 12 is carbon.

Question 10 · Answer A

Step 1: The formula $\text{MgSO}_4$ contains a magnesium ion and a sulfate ion, $\text{SO}_4^{2-}$. Step 2: A compound of magnesium with the sulfate ion is named magnesium sulfate. Step 3: Magnesium sulfide would be MgS and magnesium sulfite would be $\text{MgSO}_3$, so $\text{MgSO}_4$ is magnesium sulfate.

Question 11 · Answer A

Step 1: The mass of compounds is proportional to the volume of sea water. Step 2: The mass in 800 cm³ = 38.0 × (800 ÷ 2000) = 38.0 × 0.40. Step 3: This gives 15.2 g of compounds in 800 cm³.

Question 12 · Answer A

Step 1: Using the solubility rules, all common sodium, potassium and ammonium compounds are soluble, and so are all hydroxides of Group I metals. Step 2: Sodium hydroxide is therefore soluble in water. Step 3: Lead(II) sulfate, most carbonates (magnesium carbonate) and silver chloride are insoluble, so the soluble compound is sodium hydroxide.

Question 13 · Answer A

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

Question 14 · Answer A

Step 1: A potassium atom has 19 electrons arranged 2,8,8,1. Step 2: Losing the single outer electron leaves 18 electrons arranged 2,8,8. Step 3: Losing one negative electron leaves the ion with a charge of 1+, so the ion is 2,8,8 with a 1+ charge.

Question 15 · Answer A

Step 1: In a solid the particles are packed very close together, usually touching, in a regular arrangement. Step 2: They have only enough energy to vibrate about fixed positions, so they cannot move from place to place. Step 3: So the particles in a solid are close together and vibrate about fixed positions.

Question 16 · Answer A

Step 1: Phosphate ions are found in fertilisers (and in some detergents). Step 2: When fertilisers are washed off farmland by rain, the phosphate ions are carried into rivers and the sea. Step 3: Dissolved gases and insoluble rock do not provide phosphate ions, so fertilisers are a correct source.

Question 17 · Answer A

Step 1: Water is covalent, so the atoms share electrons. Each hydrogen shares one pair of electrons with the oxygen, forming a single O-H bond. Step 2: Oxygen has 6 outer electrons; two are used in the two bonds, leaving 4 non-bonding electrons (two lone pairs). Step 3: So each hydrogen has one shared pair and the oxygen carries two non-bonding pairs.

Question 18 · Answer A

Step 1: Fractional distillation separates liquids that have different boiling points. Step 2: The hydrocarbons in petroleum have different boiling points because they have different sized molecules, so they condense at different heights in the column. Step 3: So the property used is their different boiling points.

Question 19 · Answer A

Step 1: The petroleum is heated so it vaporises and the vapours rise up the fractionating column. Step 2: The column has a temperature gradient: hot at the bottom and cooler towards the top. Step 3: As the vapours rise and cool, each fraction condenses when it reaches its own boiling point, so different fractions are collected at different heights.

Question 20 · Answer A

Step 1: Cracking uses heat and a catalyst to break large hydrocarbon molecules into smaller ones. Step 2: This produces smaller alkanes and useful alkenes. Step 3: Fractional distillation only separates molecules, polymerisation joins small molecules together and neutralisation is an acid-base reaction, so the process is cracking.

Question 21 · Answer A

Step 1: Bitumen is a thick tarry fraction used for surfacing and making roads. Step 2: Lubricating oil provides waxes and polishes, and naphtha is used as a chemical feedstock for making other chemicals. Step 3: So the correct matching is bitumen → roads, lubricating oil → waxes and polishes, naphtha → chemical feedstock.

Question 22 · Answer A

Step 1: Ethanoic acid is $\text{CH}_3\text{COOH}$, so it has a CH3 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 with a C=O and an O-H, with all bonds drawn, including the O-H bond.

Question 23 · Answer A

Step 1: A carboxylic acid reacting with a carbonate makes a salt, water and carbon dioxide. Step 2: The salt is named from the metal (sodium) and the acid (ethanoic acid), giving sodium ethanoate. Step 3: So the salt produced is sodium ethanoate.

Question 24 · Answer A

Step 1: Complete combustion of a hydrocarbon in excess oxygen gives carbon dioxide and water. Step 2: The carbon in ethane is oxidised to carbon dioxide and the hydrogen is oxidised to water. Step 3: So the two products are carbon dioxide and water.

Question 25 · Answer A

Step 1: One way to make ethanol is fermentation, in which yeast converts glucose into ethanol and carbon dioxide. Step 2: The other way is the catalytic addition of steam to ethene (hydration of ethene). Step 3: So the two industrial methods are fermentation and the catalytic addition of steam to ethene.

Question 26 · Answer A

Step 1: The molecular formula is found by counting every atom of each element in the displayed formula. Step 2: Counting gives 7 carbon atoms, 12 hydrogen atoms and 4 oxygen atoms. Step 3: So the molecular formula is $\text{C}_7\text{H}_{12}\text{O}_4$.

Question 27 · Answer A

Step 1: Unsaturated molecules contain at least one carbon-carbon double bond (C=C). Step 2: A molecule with only single C-C bonds is saturated, so the presence of a C=C is what makes a molecule unsaturated. Step 3: An O-H group or being a hydrocarbon does not decide saturation, so the key feature is a carbon-carbon double bond.

Question 28 · Answer A

Step 1: The group number of a main-group element equals the number of electrons in its outer shell. Step 2: Nitrogen has the electronic configuration 2,5, so it has 5 outer electrons. Step 3: Having 5 outer electrons places nitrogen in Group V.

Question 29 · Answer A

Step 1: Both isotopes have proton number 7, so each atom has 7 protons and, being neutral, 7 electrons. Step 2: Neutrons = nucleon number − proton number, so nitrogen-14 has 14 − 7 = 7 neutrons and nitrogen-15 has 15 − 7 = 8 neutrons. Step 3: So nitrogen-14 is 7 protons, 7 neutrons, 7 electrons and nitrogen-15 is 7 protons, 8 neutrons, 7 electrons.

Question 30 · Answer A

Step 1: Oxides of nitrogen form when nitrogen and oxygen from the air react together at high temperatures. Step 2: Car engines reach high enough temperatures for this to happen, so they are a major source of oxides of nitrogen. Step 3: Photosynthesis, evaporation and burning hydrogen do not produce oxides of nitrogen, so car engines are a correct source.

Question 31 · Answer A

Step 1: Count the hydrogen atoms on the left: $4\text{NH}_3$ gives 4 × 3 = 12 hydrogen atoms. Step 2: Each water molecule, $\text{H}_2\text{O}$, contains 2 hydrogen atoms, so 12 ÷ 2 = 6 water molecules are needed. Step 3: So $n = 6$, giving $6\text{H}_2\text{O}$ (and this also balances the oxygen atoms).

Question 32 · Answer A

Step 1: Nitrogen dioxide is made of two non-metals, nitrogen and oxygen. Step 2: Non-metal atoms bond by sharing electrons, which is covalent bonding. Step 3: Ionic bonding needs a metal and a non-metal, and metallic bonding needs metal atoms, so the bonding in $\text{NO}_2$ is covalent.

Question 33 · Answer A

Step 1: Oxides of non-metals are acidic, while oxides of metals are basic. Step 2: Nitrogen is a non-metal, so its oxide nitrogen dioxide is an acidic oxide. Step 3: So nitrogen dioxide is acidic because nitrogen is a non-metal.

Question 34 · Answer A

Step 1: Add the masses: aluminium = 1 × 27 = 27, nitrogen = 3 × 14 = 42, oxygen = 9 × 16 = 144. Step 2: Total relative formula mass = 27 + 42 + 144. Step 3: This gives 213.

Question 35 · Answer A

Step 1: An acid reacting with a metal carbonate always gives a salt, carbon dioxide and water. Step 2: Nitric acid gives a nitrate salt, so copper(II) carbonate gives copper(II) nitrate, plus carbon dioxide and water. Step 3: So the three products are copper(II) nitrate, carbon dioxide and water.

Question 36 · Answer A

Step 1: First remove the leftover (excess) magnesium oxide by filtering, leaving pure magnesium chloride solution. Step 2: Heat the solution to evaporate water to the point of crystallisation (until crystals just start to form), then allow it to cool so crystals grow. Step 3: Finally dry the crystals between sheets of filter paper, giving pure, dry crystals.

Question 37 · Answer A

Step 1: The anode is always the electrode connected to the positive terminal of the power supply. Step 2: The electrolyte is the substance that conducts electricity and is broken down, here the molten cobalt(II) bromide. Step 3: So the anode is the positive electrode and the electrolyte is the molten cobalt(II) bromide.

Question 38 · Answer A

Step 1: Molten cobalt(II) bromide contains $\text{Co}^{2+}$ ions and $\text{Br}^-$ ions, with no water present. Step 2: The positive bromide ions lose electrons at the anode to form bromine, while the cobalt ions gain electrons at the cathode to form cobalt metal. Step 3: So bromine forms at the positive electrode and cobalt forms at the negative electrode.

Question 39 · Answer A

Step 1: Simple molecular compounds have low melting points because only weak forces between molecules need to be overcome. Step 2: They also do not conduct electricity because they have no ions or free electrons. Step 3: Compounds C (−182 °C) and E (−101 °C) both have low melting points and do not conduct when molten, so C and E are the simple molecular compounds.

Question 40 · Answer A

Step 1: A chemical change forms one or more new substances and is usually difficult to reverse. Step 2: Decomposing vegetation breaks down into new substances, so it is a chemical change. Step 3: Boiling, dissolving and mixing are physical changes because no new substances are made, so the chemical change is decomposing vegetation.

Question 41 · Answer A

Step 1: The temperature change is the difference between final and initial temperatures, ignoring direction. Step 2: Experiment 1 changes by 6, Experiment 2 by 8, Experiment 3 by 6 and Experiment 4 by 5. Step 3: The largest change is 8 °C, in Experiment 2.

Question 42 · Answer A

Step 1: Endothermic reactions take in heat, so the temperature falls. The most endothermic is the one with the largest temperature drop. Step 2: Experiment 1 falls by 6 °C and Experiment 4 falls by 5 °C, while Experiments 2 and 3 rise (exothermic). Step 3: The largest fall is 6 °C in Experiment 1, so it is the most endothermic.

Question 43 · Answer A

Step 1: On a reaction pathway diagram the vertical axis is labelled energy. Step 2: For an exothermic reaction the products are at a lower energy than the reactants, because energy is released. Step 3: So the reactants are on the left at a higher level and the products on the right at a lower level, with the energy axis labelled.

Question 44 · Answer A

Step 1: Water treatment first removes solid particles by sedimentation and filtration, so stage 1 is sedimentation/filtration. Step 2: After the carbon stage (which removes tastes and odours), the final stage kills microbes by chlorination. Step 3: So stage 1 is sedimentation/filtration and stage 3 is chlorination.

Question 45 · Answer A

Step 1: Water is detected with anhydrous copper(II) sulfate (or anhydrous cobalt(II) chloride). Step 2: When water is added, white anhydrous copper(II) sulfate turns blue. Step 3: Limewater tests for carbon dioxide and litmus tests for chlorine, so the correct test is white-to-blue with anhydrous copper(II) sulfate.

Question 46 · Answer A

Step 1: Carbon dioxide is a greenhouse gas, so it causes global warming and climate change. Step 2: Oxides of nitrogen dissolve in rain to form acids, causing acid rain (and they can cause respiratory problems). Step 3: So carbon dioxide causes global warming and oxides of nitrogen cause acid rain.

Question 47 · Answer A

Step 1: The more vigorously a metal reacts with acid, the more reactive it is. Step 2: A reacts strongly at room temperature (most reactive), D reacts strongly only on warming, C reacts weakly on warming, and B does not react (least reactive). Step 3: So the order from most to least reactive is A, D, C, B.

Question 48 · Answer A

Step 1: A reaction speeds up when particles collide more often or more successfully. Step 2: Increasing the acid concentration gives more frequent collisions, and using smaller pieces of metal increases the surface area for collisions (adding a catalyst would also work). Step 3: So two ways are increasing concentration and increasing surface area.

Question 49 · Answer A

Step 1: Flame tests give characteristic colours for different metal ions. Step 2: Lithium ions give a red flame colour. Step 3: Lilac is potassium, yellow is sodium and blue-green is copper, so lithium gives a red flame.

Question 50 · Answer A

Step 1: An alkali is a soluble base that produces ions in water. Step 2: All aqueous alkalis contain hydroxide ions, $\text{OH}^-$, which make the solution alkaline. Step 3: Hydrogen ions make acids, while oxide and chloride ions are not what makes a solution alkaline, so the answer is the hydroxide ion.

Question 51 · Answer A

Step 1: Thymolphthalein is an indicator that is colourless in acids and neutral solutions. Step 2: In an alkali it turns blue. Step 3: So thymolphthalein is blue in an aqueous alkali.

Question 52 · Answer A

Step 1: An alloy contains atoms of different elements, so it has atoms of different sizes. Step 2: The different-sized atoms disrupt the regular layers found in a pure metal. Step 3: So the arrangement that best represents an alloy shows atoms of two different sizes mixed together, not identical atoms in a regular pattern.

Question 53 · Answer A

Step 1: In an alloy, atoms of different sizes disrupt the regular layers, making it harder for layers to slide. Step 2: This makes pewter harder and stronger than pure tin. Step 3: So the useful property is that pewter is harder/stronger than pure tin.

Question 54 · Answer A

Step 1: Typical physical properties of metals include being malleable, ductile, good conductors of electricity and having high melting points. Step 2: Two correct properties are that metals are malleable and good conductors of electricity. Step 3: Being brittle, non-conducting, low-melting or gaseous are not typical of metals, so the correct pair is malleable and good electrical conductor.