IGCSE Chemistry 0620/21 May/June 2025 Past Paper with Answers

Question 1

A gas is placed in a sealed container with a fixed volume and is then heated. Three statements are given. 1: the gas molecules move around faster. 2: the gas particles collide with the walls of the container more frequently. 3: the pressure in the container decreases. Which statements are correct?

1. 1 and 2
2. 1 only
3. 2 and 3
4. 2 only

Question 2

A sample of ethanol is heated at a constant rate and its heating curve is recorded, with points labelled 1 to 6 along the curve. Section 2 to 3 is a lower horizontal plateau (melting) and section 4 to 5 is a higher, longer horizontal plateau (boiling); the sloping sections 1 to 2, 3 to 4 and 5 to 6 are where the solid, liquid and gas warm up. Which statement about the heating curve is correct?

1. more energy is absorbed during the plateau between 4 and 5 than during the plateau between 2 and 3
2. the ethanol changes its physical state during the sloping section between 3 and 4
3. the ethanol condenses during the sloping section between 2 and 3
4. energy is absorbed during the section between 3 and 4 but no energy is absorbed between 4 and 5

Question 3

Which neutral atom has an equal number of protons, neutrons and electrons?

1. a nitrogen-14 atom, $^{14}_{\;7}\text{N}$
2. an argon-40 atom, $^{40}_{18}\text{Ar}$
3. a hydrogen-1 atom, $^{1}_{1}\text{H}$
4. a sodium-23 atom, $^{23}_{11}\text{Na}$

Question 4 · Answer A

Step 1: Let the fraction of europium-151 be $x$, so $151x + 153(1-x) = 152$. Step 2: Expanding gives $153 - 2x = 152$, so $2x = 1$ and $x = 0.500$. Step 3: So europium-151 makes up 50.0% of the sample.

Question 5 · Answer A

Step 1: Ionic compounds conduct electricity only when the ions are free to move, which happens when molten or dissolved. Step 2: Molten potassium chloride has mobile ions, so it conducts, making that statement correct. Step 3: Its high melting point is due to strong ionic (not covalent) bonds, it does not conduct as a solid, and it is soluble in water, so the other statements are wrong.

Question 6 · Answer A

Step 1: Each nitrogen atom has five outer electrons and needs to reach a full outer shell of eight. Step 2: The two atoms share three pairs of electrons, forming a triple bond, which gives each nitrogen a share of eight electrons. Step 3: That uses three of each atom's five outer electrons, leaving one lone pair on each nitrogen, so the correct diagram has a triple bond and one lone pair per atom.

Question 7 · Answer A

Step 1: Silicon(IV) oxide is a giant covalent structure in which silicon and oxygen atoms are joined by single covalent bonds. Step 2: Each covalent bond is one shared pair of electrons, so that statement is correct. Step 3: It contains atoms (not ions), it does not conduct electricity, and its tetrahedral arrangement is like diamond rather than graphite, so the other statements are wrong.

Question 8 · Answer A

Step 1: The empirical formula is the simplest whole-number ratio of atoms. Step 2: Lactic acid $\text{C}_3\text{H}_6\text{O}_3$ divides by 3 to give $\text{CH}_2\text{O}$, and glucose $\text{C}_6\text{H}_{12}\text{O}_6$ divides by 6 to give $\text{CH}_2\text{O}$, so they match. Step 3: Propanoic acid $\text{C}_3\text{H}_6\text{O}_2$ does not simplify to $\text{CH}_2\text{O}$, so only lactic acid and glucose share an empirical formula.

Question 9 · Answer A

Step 1: Moles of calcium carbonate = 1.00 ÷ 100 = 0.0100 mol, and moles of HCl = 0.0500 × (50.0 ÷ 1000) = 0.00250 mol. Step 2: The acid is limiting because 0.00250 mol of HCl needs only 0.00125 mol of CaCO3, and 2 mol of HCl give 1 mol of CO2, so moles of CO2 = 0.00250 ÷ 2 = 0.00125 mol. Step 3: Volume of CO2 = 0.00125 × 24 000 = 30 cm³.

Question 10 · Answer A

Step 1: Copper is deposited on the steel cathode, so the steel turns reddish-brown, making statement 1 correct. Step 2: The copper anode dissolves to replace the copper ions removed at the cathode, so the solution stays the same blue colour, making statement 2 wrong. Step 3: Because the copper anode dissolves, it becomes thinner, so statement 3 is correct, giving 1 and 3 only.

Question 11 · Answer A

Step 1: At the anode negative ions are oxidised, which means they lose electrons. Step 2: With concentrated sodium chloride the chloride ions are discharged, so two chloride ions lose two electrons to form a chlorine molecule. Step 3: This gives $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$, with electrons on the right because they are lost.

Question 12 · Answer A

Step 1: A hydrogen-oxygen fuel cell combines hydrogen and oxygen, the reverse of the electrolysis of water. Step 2: The only product is water, and the balanced equation is $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Step 3: So the overall reaction makes water from hydrogen and oxygen.

Question 13 · Answer A

Step 1: The mixture gets hot, so energy is released to the surroundings, which means the reaction is exothermic. Step 2: In an exothermic reaction the products have less stored energy than the reactants, so the reactants are drawn higher than the products. Step 3: So the correct choice is exothermic with the reactants at a higher energy level than the products.

Question 14 · Answer A

Step 1: The equation uses the $\rightleftharpoons$ symbol, which shows the reaction can go both forwards and backwards. Step 2: A reaction that can proceed in both directions is described as reversible. Step 3: So this equation shows a reversible reaction.

Question 15 · Answer A

Step 1: A lower temperature gives the particles less energy, so their average kinetic energy is lower. Step 2: Slower particles collide less often, so the frequency of collisions is also lower. Step 3: So at 30 °C both the frequency of collisions and the average kinetic energy are lower than at 50 °C.

Question 16 · Answer A

Step 1: A lower temperature always slows a reaction, so statement 1 is correct, but because the forward reaction is exothermic a lower temperature actually raises the yield, so statement 2 is wrong. Step 2: Higher pressure favours the side with fewer gas molecules (the ammonia side), so pressures above 200 atm would raise, not lower, the yield, making statement 3 wrong. Step 3: Very high pressures are expensive and difficult to maintain safely, so statement 4 is correct, giving 1 and 4.

Question 17 · Answer A

Step 1: Acidified potassium manganate(VII) oxidises ethanol to ethanoic acid, so the ethanol undergoes oxidation. Step 2: As it acts as the oxidising agent the manganate(VII) is reduced and its purple colour fades. Step 3: So the reaction is an oxidation and the colour changes from purple to colourless.

Question 18 · Answer A

Step 1: A strong acid is fully dissociated, while a weak acid is only partly dissociated. Step 2: Acid R dissociates only 0.0048 of 0.10 mol and acid S only 0.0179 of 0.10 mol, so both are only partly dissociated. Step 3: Because neither acid is fully dissociated, both R and S are weak acids.

Question 19 · Answer A

Step 1: An insoluble salt is made by precipitation, then separated by filtration, washed, and dried. Step 2: Crystallisation is used to obtain a soluble salt from its solution by evaporation, so it is not needed for an insoluble salt. Step 3: So the process not used is crystallisation.

Question 20 · Answer A

Step 1: Hydrated copper(II) sulfate is the familiar blue crystal with five molecules of water of crystallisation, so $x = 5$. Step 2: Hydrated cobalt(II) chloride is the pink solid with six molecules of water of crystallisation, so $y = 6$. Step 3: So $x = 5$ and $y = 6$.

Question 21 · Answer A

Step 1: Metals are on the left of the Periodic Table and non-metals on the right. Step 2: So across Period 3 the elements change from metals (such as sodium) to non-metals (such as chlorine). Step 3: The number of shells stays the same across a period and conductivity falls rather than rises, so only the metal-to-non-metal change is correct.

Question 22 · Answer A

Step 1: Going down Group VII the halogens become less reactive, so reactivity decreases. Step 2: At the same time the atomic number, density and melting point all increase down the group. Step 3: So the property that decreases is reactivity.

Question 23 · Answer A

Step 1: Both iron and lithium are metals, so both conduct electricity well and are malleable, making statements 1 and 3 correct for both. Step 2: Lithium is a Group I metal with a low melting point, so statement 2 is not true for both, and only transition elements like iron form ions with variable oxidation numbers, so statement 4 is not true for both. Step 3: So the properties shared by both metals are 1 and 3.

Question 24 · Answer A

Step 1: Noble gases are very unreactive, so describing them as reactive makes statement 1 wrong, although they are indeed monatomic. Step 2: Noble gases do have a full outer shell of electrons, so statement 2 is correct. Step 3: So statement 2 is correct but statement 1 is not.

Question 25 · Answer A

Step 1: Copper is a good electrical conductor and is ductile, so it can be drawn into wires for electrical wiring, making this statement correct. Step 2: Brass is copper mixed with zinc (copper with tin is bronze), copper has a high density so it is not used for overhead cables, and takeaway containers use aluminium. Step 3: So the correct statement is that copper is used in electrical wiring and is ductile.

Question 26 · Answer A

Step 1: Reacting with dilute acid to give hydrogen, and only slowly with cold water but quickly with steam, fits a moderately reactive metal. Step 2: Magnesium reacts with acids, reacts very slowly with cold water and vigorously with steam, which matches Z. Step 3: Gold and silver are too unreactive to react with acid, and potassium reacts violently with cold water, so Z is magnesium.

Question 27 · Answer A

Step 1: A metal displaces another from solution when it is more reactive (more able to form positive ions). Step 2: J displaces K, L and M, so J is the most reactive; L displaces K and M; K displaces only M; and M displaces none, giving the order J > L > K > M. Step 3: Since J is above K in this order, J has a greater tendency to form positive ions than K, so that statement is correct.

Question 28 · Answer A

Step 1: Iron is less reactive than carbon, so it can be extracted by reducing its oxide with carbon (as carbon monoxide) in the blast furnace. Step 2: Removing oxygen from iron(III) oxide is a reduction. Step 3: So the extraction of iron from hematite uses reduction.

Question 29 · Answer A

Step 1: Galvanising coats iron with zinc, not silver, so statement 1 is wrong. Step 2: The zinc layer acts as a barrier that keeps oxygen and water away from the iron, so statement 2 is correct. Step 3: Zinc is more reactive than iron, so it loses electrons more readily and protects the iron sacrificially, making statement 3 correct, giving 2 and 3 only.

Question 30 · Answer A

Step 1: Ammonium phosphate supplies both nitrogen (in the ammonium ion) and phosphorus (in the phosphate ion). Step 2: Potassium chloride supplies potassium, so together with ammonium phosphate all three elements are present. Step 3: The other mixtures miss either phosphorus or potassium, so the correct mixture is ammonium phosphate with potassium chloride.

Question 31 · Answer A

Step 1: Greenhouse gases trap thermal energy that the Earth radiates back outwards. Step 2: Statement 1 (carbon dioxide absorbs thermal energy from the Earth) and statement 4 (methane reflects thermal energy from the Earth back to the Earth) both describe trapping of the Earth's heat, so they explain the warming. Step 3: Statements 2 and 3 are about energy from the Sun and do not explain how the gases trap the Earth's heat, so the answer is 1 and 4.

Question 32 · Answer A

Step 1: Air is drawn into the engine, and at the high temperatures inside the engine the nitrogen and oxygen from the air react together. Step 2: This reaction forms oxides of nitrogen, which then leave in the exhaust. Step 3: So the oxides form from nitrogen and oxygen in the air reacting at the high temperatures in the engine.

Question 33 · Answer A

Step 1: Structural isomers have the same molecular formula but different structures, so they must match $\text{C}_3\text{H}_6\text{O}_2$. Step 2: Compound 1 ($\text{HCOOCH}_2\text{CH}_3$) and compound 2 ($\text{CH}_3\text{CH}_2\text{COOH}$) are both $\text{C}_3\text{H}_6\text{O}_2$, so they are isomers. Step 3: Compound 3 ($\text{HOCH}_2\text{CH}_2\text{CH}_2\text{OH}$) is $\text{C}_3\text{H}_8\text{O}_2$, which has two extra hydrogen atoms, so it is not an isomer, giving 1 and 2 only.

Question 34 · Answer A

Step 1: Propane has a chain of three carbon atoms, giving 2 carbon-carbon bonds. Step 2: There are 8 hydrogen atoms, each joined to a carbon by 1 bond, giving 8 carbon-hydrogen bonds. Step 3: The total number of covalent bonds is 2 + 8 = 10.

Question 35 · Answer A

Step 1: Fermentation relies on enzymes in yeast and is a slow batch process, while the catalytic reaction of ethene with steam is fast and continuous, so that comparison is correct. Step 2: Fermentation works at around 30 to 40 °C (not 100 °C), and the ethene-with-steam route does need a catalyst, so those statements are wrong. Step 3: The ethene route gives a high yield, so the yield comparison is also wrong, leaving the slow-versus-fast statement.

Question 36 · Answer A

Step 1: A solution with a pH of 3 that reacts with calcium carbonate to give carbon dioxide is acidic. Step 2: Among the organic families, only carboxylic acids are acidic and react with carbonates to release carbon dioxide. Step 3: So J is a carboxylic acid.

Question 37 · Answer A

Step 1: A carboxylic acid and an alcohol form an ester link, $\text{C(=O)}{-}\text{O}$, with the loss of water, and this is repeated for a condensation polymer. Step 2: Each -COOH of the diacid joins to an -OH of the diol, losing water and forming an ester linkage, so both carbonyl groups become ester links to the three-carbon diol chain. Step 3: So the correct repeat unit is the polyester linking the diacid and diol through ester groups, which is "a repeat unit joined by simple carbon-carbon bonds with no ester linkages, as if the monomers had added together without losing water".

Question 38 · Answer A

Step 1: Measuring 25.0 cm³ to one decimal place needs apparatus with a fine, accurate scale. Step 2: A burette has a precise graduated scale and a tap, so it can deliver an accurate volume such as 25.0 cm³. Step 3: A beaker and conical flask are only for holding liquids, and a measuring cylinder is far less precise, so the burette is the correct choice.

Question 39 · Answer A

Step 1: The copper(II) sulfate is dissolved in the solution, so it must be brought out of solution to form crystals. Step 2: Evaporating some of the water makes the solution saturated, and on cooling the dissolved salt forms crystals. Step 3: So the crystals are obtained by evaporation (to crystallisation), not by chromatography, condensation or filtration.

Question 40 · Answer A

Step 1: Each amino acid travels a characteristic distance set by its Rf value. Step 2: If two of the amino acids have the same Rf value, their spots travel the same distance and land in the same place, so they overlap into a single spot. Step 3: That makes four amino acids show as only three spots, so the correct reason is that two of them have the same Rf value.