IGCSE Chemistry 0620/43 May/June 2025 Past Paper with Answers

Question 1

A list of substances includes aluminium oxide, calcium oxide, graphite, propene and silicon(IV) oxide. Which of these is a compound with a giant covalent structure?

1. silicon(IV) oxide, in which silicon and oxygen atoms are joined by a network of covalent bonds
2. graphite, which is a giant covalent structure but is an element, not a compound
3. calcium oxide, which is an ionic compound, not a covalent one
4. propene, which is a small simple covalent molecule

Question 2

A list of substances includes ethanol, graphite, propane, propene and silicon(IV) oxide. Which of these is an unsaturated hydrocarbon?

1. propene, an alkene containing a carbon-carbon double bond
2. propane, an alkane with only single carbon-carbon bonds
3. ethanol, which contains oxygen as well as carbon and hydrogen
4. graphite, which is a form of carbon only

Question 3

A list of substances includes aluminium oxide, calcium oxide, oxygen, propene and silicon(IV) oxide. Which of these is an amphoteric oxide?

1. aluminium oxide, which reacts with both acids and bases
2. calcium oxide, which is a basic oxide
3. silicon(IV) oxide, which is an acidic oxide
4. oxygen, which is an element, not an oxide

Question 4 · Answer A

Step 1: Conducting electricity when solid needs charged particles that are free to move, such as delocalised electrons. Step 2: Graphite has one delocalised electron per carbon atom that can move between its layers, so solid graphite conducts electricity. Step 3: Silicon(IV) oxide has no free electrons, and ionic calcium oxide only conducts when molten or dissolved, so the answer is graphite.

Question 5 · Answer A

Step 1: Count the atoms in each molecule from its formula. Step 2: Propene is $\text{C}_3\text{H}_6$, which has 3 carbon atoms plus 6 hydrogen atoms = 9 atoms (ethanol, $\text{C}_2\text{H}_6\text{O}$, also has 9 atoms). Step 3: Chlorine, nitrogen and oxygen are diatomic (2 atoms each), so the molecule with 9 atoms is propene.

Question 6 · Answer A

Step 1: In the blast furnace, limestone decomposes to calcium oxide, a basic oxide. Step 2: Calcium oxide reacts with the acidic sandy impurity silicon(IV) oxide to form molten slag (calcium silicate), which is run off. Step 3: So the two substances that react to form slag are silicon(IV) oxide and calcium oxide.

Question 7 · Answer A

Step 1: The general formula $\text{C}_n\text{H}_{2n+2}$ is the formula of the alkanes. Step 2: Propane, $\text{C}_3\text{H}_8$, is an alkane, so it fits $\text{C}_n\text{H}_{2n+2}$. Step 3: Propene is an alkene ($\text{C}_n\text{H}_{2n}$), ethanol is an alcohol and graphite is an element, so the answer is propane.

Question 8 · Answer A

Step 1: Ethene reacts with steam in the presence of a catalyst (an addition reaction) to form ethanol. Step 2: This catalytic hydration of ethene is one of the two industrial methods of making ethanol. Step 3: Propane and propene come from cracking and calcium oxide from heating limestone, so the answer is ethanol.

Question 9 · Answer A

Step 1: Clean dry air is about 78% nitrogen and 21% oxygen, with small amounts of other gases. Step 2: The gas at roughly 78% is therefore nitrogen. Step 3: Oxygen is about 21%, and chlorine and propane are not normal components of clean dry air, so the answer is nitrogen.

Question 10 · Answer A

Step 1: The nucleus is the dense central part of the atom. Step 2: Protons and neutrons are found together in the nucleus, while electrons orbit it in shells. Step 3: So the particles in the nucleus are protons and neutrons.

Question 11 · Answer A

Step 1: Protons = proton number = 16 for both species; neutrons = mass number − protons. Step 2: For $^{34}\text{S}$: neutrons = 34 − 16 = 18, and as a neutral atom electrons = protons = 16. Step 3: For $^{32}\text{S}^{2-}$: neutrons = 32 − 16 = 16, and the 2- charge means 2 extra electrons, so electrons = 16 + 2 = 18.

Question 12 · Answer A

Step 1: Relative atomic mass = [(mass × % abundance) summed] ÷ 100. Step 2: $(32 \times 95) + (34 \times 5) = 3040 + 170 = 3210$. Step 3: $3210 \div 100 = 32.1$, so the relative atomic mass is 32.1.

Question 13 · Answer A

Step 1: Relative atomic mass is defined on a scale where one atom of a chosen isotope is the standard. Step 2: The agreed standard is the carbon-12 isotope, $^{12}\text{C}$. Step 3: Hydrogen-1 and oxygen-16 were older standards, and carbon-14 is a different isotope, so the standard is carbon-12.

Question 14 · Answer A

Step 1: An oxygen atom is 2,6; gaining 2 electrons to form $\text{O}^{2-}$ gives 2,8 (10 electrons). Step 2: The atom with 10 electrons (2,8) is neon. A positive ion with 2,8 is $\text{Na}^+$ (sodium loses one electron), or $\text{Mg}^{2+}$ or $\text{Al}^{3+}$. Step 3: So the atom is neon and a suitable positive ion is $\text{Na}^+$.

Question 15 · Answer A

Step 1: Zinc is a metal, and metals are held together by metallic bonding. Step 2: Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons. Step 3: So the bonding in zinc is metallic bonding.

Question 16 · Answer A

Step 1: In a metal, the atoms lose their outer electrons, which become delocalised. Step 2: This leaves a lattice of positive metal ions surrounded by a sea of delocalised electrons. Step 3: Metallic bonding is the attraction between these positive ions and delocalised electrons, so the two particles are positive ions and (delocalised) electrons.

Question 17 · Answer A

Step 1: Electrical conduction needs charged particles that are free to move. Step 2: In a metal the delocalised electrons can move through the lattice when a voltage is applied. Step 3: The positive ions stay fixed in the lattice, so it is the movement of delocalised electrons that allows zinc to conduct electricity.

Question 18 · Answer A

Step 1: An alloy is made by mixing a metal with other elements (usually other metals). Step 2: The atoms are mixed but not chemically bonded into a compound, so an alloy is a mixture. Step 3: So an alloy is a mixture of a metal with one or more other elements.

Question 19 · Answer A

Step 1: Brass is a common alloy of two metals. Step 2: Brass is made from copper and zinc. Step 3: Since zinc is already given, the other metal in brass is copper.

Question 20 · Answer A

Step 1: Zinc carbonate reacts with the acid, fizzing as carbon dioxide is released, until the acid is used up. Step 2: When the acid has all reacted, any further zinc carbonate cannot react, so solid remains undissolved and the fizzing stops. Step 3: So the two observations are that solid stops dissolving (remains) and no more bubbles are produced.

Question 21 · Answer A

Step 1: The reaction makes zinc sulfate, which is soluble and stays in solution. Step 2: Filtration removes the unreacted solid zinc carbonate, leaving the dissolved product to pass through as the filtrate. Step 3: So the filtrate is aqueous zinc sulfate.

Question 22 · Answer A

Step 1: Zinc sulfate is the salt of zinc with sulfuric acid, so the added compound must be a base or carbonate of zinc. Step 2: Zinc oxide (or zinc hydroxide) is a base that reacts with dilute sulfuric acid to form zinc sulfate and water. Step 3: So a suitable compound, other than zinc carbonate, is zinc oxide.

Question 23 · Answer A

Step 1: A saturated solution has dissolved as much solute as it can at a given temperature. Step 2: Any extra solute added will not dissolve and stays as solid. Step 3: So a saturated solution contains the maximum concentration of dissolved solute at that specified temperature.

Question 24 · Answer A

Step 1: Reaction rate depends on how often reacting particles collide. Step 2: Large pieces have a smaller surface area than the same mass of powder, so fewer acid particles can collide with the solid at any moment. Step 3: The frequency of collisions decreases, so the rate of reaction decreases.

Question 25 · Answer A

Step 1: Hydrated refers to a substance that contains water of crystallisation. Step 2: This water is chemically combined within the crystal structure in a fixed proportion. Step 3: So hydrated means the crystals contain water that is chemically combined in their structure.

Question 26 · Answer A

Step 1: Balance iron: 4 Fe on the left needs $c = 2$ ($2\text{Fe}_2\text{O}_3$ gives 4 Fe). Step 2: Balance sulfur: $4\text{FeS}_2$ has 8 S, so $b = 8$ ($8\text{SO}_2$). Step 3: Balance oxygen: products have $(8 \times 2) + (2 \times 3) = 22$ O atoms, so $a = 22 \div 2 = 11$. So a = 11, b = 8, c = 2.

Question 27 · Answer A

Step 1: Each oxygen atom has 6 outer electrons and needs 2 more to fill its shell. Step 2: The two atoms share two pairs of electrons, forming an O=O double bond (two dots and two crosses between them). Step 3: This leaves each oxygen with two non-bonding (lone) pairs, so the diagram shows a double bond plus two lone pairs on each oxygen.

Question 28 · Answer A

Step 1: The Contact process needs a temperature high enough for a good rate but not so high that the yield falls; about 450 °C is used. Step 2: A pressure of about 2 atm is enough, and a vanadium(V) oxide catalyst speeds up the reaction. Step 3: So the three conditions are about 450 °C, about 2 atm, and a vanadium(V) oxide catalyst.

Question 29 · Answer A

Step 1: Sulfur dioxide ($\text{SO}_2$) reacts with oxygen ($\text{O}_2$) to form sulfur trioxide ($\text{SO}_3$). Step 2: Balancing gives 2 $\text{SO}_2$ + 1 $\text{O}_2$ → 2 $\text{SO}_3$ (4 S? no, 2 S each side; O: left 4+2=6, right 6). Step 3: So the balanced equation is $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$.

Question 30 · Answer A

Step 1: Oleum is formed when sulfur trioxide is absorbed into concentrated sulfuric acid. Step 2: One $\text{SO}_3$ combines with one $\text{H}_2\text{SO}_4$ to give $\text{H}_2\text{S}_2\text{O}_7$. Step 3: Checking atoms confirms $\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7$.

Question 31 · Answer A

Step 1: An element on its own always has an oxidation number of 0, so carbon as C is 0. Step 2: In $\text{CO}_2$ each oxygen is −2, giving −4 in total, so carbon must be +4 to make the molecule neutral. Step 3: So carbon is 0 in C and +4 in $\text{CO}_2$.

Question 32 · Answer A

Step 1: Moles of $\text{NaHCO}_3$ = mass ÷ $M_r$ = 4.20 ÷ 84 = 0.0500 mol. Step 2: The equation shows 2 $\text{NaHCO}_3$ give 2 $\text{CO}_2$ (a 1:1 ratio), so moles of $\text{CO}_2$ = 0.0500 mol. Step 3: Volume = moles × 24 000 = 0.0500 × 24 000 = 1200 cm³.

Question 33 · Answer A

Step 1: At equilibrium the forward and reverse reactions are still happening, but at the same rate. Step 2: Because the rates are equal, the concentrations of reactants and products no longer change. Step 3: So two characteristics are: rate of forward = rate of reverse, and concentrations stay constant.

Question 34 · Answer A

Step 1: A lower temperature slows particles down, so below 300 °C the rate of reaction is too low. Step 2: The forward reaction is exothermic, so a higher temperature shifts the equilibrium backwards, lowering the yield. Step 3: So below 300 °C the rate is too slow, and above 300 °C the yield of ethanoic acid decreases.

Question 35 · Answer A

Step 1: A catalyst speeds up both directions equally, so it does not change the position of equilibrium: no change to the concentration of $\text{CH}_3\text{COOH}$. Step 2: The forward reaction goes from 2 moles of gas to 1 mole of gas, so increasing the pressure shifts the equilibrium towards the product, increasing the concentration of $\text{CH}_3\text{COOH}$. Step 3: Higher pressure also pushes particles closer together, so the rate of the forward reaction increases.

Question 36 · Answer A

Step 1: Transition elements and their compounds are widely used as catalysts. Step 2: From the list, cobalt is the transition element, so it is the suitable catalyst. Step 3: Sodium and magnesium are reactive main-group metals and carbon is a non-metal, so the best answer is cobalt because it is a transition element.

Question 37 · Answer A

Step 1: Carboxylic acids are named by the number of carbon atoms: meth- (1), eth- (2), prop- (3), but- (4). Step 2: The acid with one carbon atom uses the prefix meth-, giving methanoic acid (HCOOH). Step 3: So the carboxylic acid with one carbon atom is methanoic acid.

Question 38 · Answer A

Step 1: Butanoic acid is $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$, which has 4 carbon atoms. Step 2: Counting atoms gives 4 C, 8 H and 2 O. Step 3: So the molecular formula is $\text{C}_4\text{H}_8\text{O}_2$.

Question 39 · Answer A

Step 1: An ester has the linkage -COO- joining an acid part to an alcohol part. Step 2: The smallest ester with two carbon atoms is methyl methanoate, from methanoic acid (HCOOH, 1 C) and methanol ($\text{CH}_3\text{OH}$, 1 C). Step 3: Its displayed formula shows H-C with a C=O double bond and a single bond to O, which joins to a $\text{CH}_3$ group (H-CO-O-$\text{CH}_3$).

Question 40 · Answer A

Step 1: An ester is named as [alcohol part][acid part]: the first word comes from the alcohol and the -oate from the acid. Step 2: "Propyl" comes from propanol (propan-1-ol), and "butanoate" comes from butanoic acid. Step 3: So propyl butanoate is made from butanoic acid and propan-1-ol.

Question 41 · Answer A

Step 1: Divide each percentage by its $A_r$: C 58.82 ÷ 12 = 4.90; H 9.80 ÷ 1 = 9.80; O 31.38 ÷ 16 = 1.96. Step 2: Divide by the smallest (1.96): C 4.90 ÷ 1.96 = 2.5; H 9.80 ÷ 1.96 = 5; O 1.96 ÷ 1.96 = 1, giving the ratio 2.5 : 5 : 1. Step 3: Multiply by 2 to get whole numbers: 5 : 10 : 2, so the empirical formula is $\text{C}_5\text{H}_{10}\text{O}_2$.

Question 42 · Answer A

Step 1: Group I elements (lithium, sodium, potassium and so on) react with water to form alkaline solutions. Step 2: Because of this they are called the alkali metals. Step 3: The halogens are Group VII, the noble gases are Group VIII/0 and the transition elements are the central block, so Group I are the alkali metals.

Question 43 · Answer A

Step 1: In Group I, reactivity increases going down the group. Step 2: Lithium is at the top of Group I, so it is the least reactive. Step 3: Potassium (lower down) is more reactive than sodium, which is more reactive than lithium, so lithium is the least reactive.

Question 44 · Answer A

Step 1: Lithium is less dense than water, so it floats and moves around on the surface. Step 2: The reaction produces hydrogen gas, so bubbles (fizzing) are seen and the lithium gradually gets smaller as it dissolves/disappears. Step 3: So two valid observations are that the lithium floats and moves on the surface and that bubbles (fizzing) are produced.

Question 45 · Answer A

Step 1: Lithium reacts with water to give lithium hydroxide (LiOH) and hydrogen ($\text{H}_2$). Step 2: Balancing: 2 Li and 2 $\text{H}_2\text{O}$ give 2 LiOH and 1 $\text{H}_2$. Step 3: Checking atoms confirms $2\text{Li} + 2\text{H}_2\text{O} \rightarrow 2\text{LiOH} + \text{H}_2$.

Question 46 · Answer A

Step 1: Group I metals are soft enough to cut with a knife, unlike the hard transition metals. Step 2: Group I metals also have low densities (some float on water), much lower than the dense transition metals. Step 3: So two differences are that Group I metals are softer and have lower densities than transition elements.

Question 47 · Answer A

Step 1: Down Group VII the state changes: chlorine gas, bromine liquid, iodine solid. Step 2: Chlorine at room temperature is a gas, and its colour is pale yellow-green. Step 3: So chlorine is a pale yellow-green gas.

Question 48 · Answer A

Step 1: The potassium ions ($\text{K}^+$) are spectator ions, so they are left out of the ionic equation. Step 2: Chlorine gains electrons and bromide ions lose electrons: $\text{Cl}_2 + 2\text{Br}^- \rightarrow 2\text{Cl}^- + \text{Br}_2$. Step 3: This is balanced for atoms and charge, so it is the correct ionic equation.

Question 49 · Answer A

Step 1: Energy to break bonds = I-I + Cl-Cl = 150 + 242 = 392 kJ. Step 2: Energy released forming bonds = 2 × I-Cl = 2 × 218 = 436 kJ. Step 3: $\Delta H$ = bonds broken − bonds formed = 392 − 436 = −44 kJ/mol (the negative sign shows it is exothermic).