0620_m23_qp_22__Shreyashkar1301

Question 1

Substance M is a solid at 30 °C. The substance is heated to 80 °C and its temperature measured as it cools down to room temperature. The cooling curve is shown.

Between which times is substance M freezing?

A. P to Q
B. Q to R
C. R to S
D. S to T

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 1.1: Solids, liquids and gases — Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing (Core); Explain changes of state in terms of kinetic particle theory, including the interpretation of heating and cooling curves (Supplement)

▶️ Answer/Explanation

On a cooling curve, freezing is represented by the horizontal (flat) section where temperature remains constant as the liquid loses energy and converts to solid. The substance M starts as a liquid at 80 °C and cools; the flat plateau — where the temperature stays constant while the state change occurs — is the freezing point. This corresponds to the region Q to R, where thermal energy is released at constant temperature as particles become arranged into a fixed lattice. Sections P–Q and R–S show temperature falling, and S–T shows the solid continuing to cool.
✅ Answer: (B)

Question 2

Which gas has the fastest rate of diffusion?

A. Ar
B. C₂H₆
C. HCl
D. H₂S

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 1.2: Diffusion — Describe and explain the effect of relative molecular mass on the rate of diffusion of gases (Supplement)

▶️ Answer/Explanation

The rate of diffusion of a gas is inversely related to its relative molecular mass (Mr) — lighter gases diffuse faster. The Mr values are: Ar = 40, C₂H₆ = 30, HCl = 36.5, H₂S = 34. C₂H₆ (ethane) has the lowest Mr of 30, so it diffuses the fastest among the four options. Heavier molecules move more slowly at the same temperature, giving them slower diffusion rates.
✅ Answer: (B)

Question 3

There are two stable isotopes of bromine. The mass number of isotope 1 is 79. The mass number of isotope 2 is 81. Which statement is correct?

A. The isotopes have the same number of neutrons.
B. The isotopes have different chemical properties.
C. The isotopes have different numbers of protons.
D. The isotopes have the same number of outer electrons.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.3: Isotopes — Define isotopes as different atoms of the same element with the same number of protons but different numbers of neutrons (Core); State that isotopes have the same chemical properties because they have the same electronic configuration (Supplement)

▶️ Answer/Explanation

Isotopes of the same element have the same number of protons (same atomic number = 35 for bromine) but different numbers of neutrons — so A and C are incorrect. Because they have the same number of protons, they have the same number of electrons and therefore identical electronic configurations, giving them the same chemical properties — so B is incorrect. Since both isotopes are bromine (Group VII), they both have 7 outer-shell electrons, making D correct.
✅ Answer: (D)

Question 4

Which statement about ions and ionic bonds is correct?

A. Bromine atoms form negatively charged bromide ions.
B. Ionic bonds form between elements in Group VII of the Periodic Table.
C. Positive ions are formed when atoms lose protons.
D. Potassium iodide contains negatively charged potassium ions.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.4: Ions and ionic bonds — Describe the formation of positive ions (cations) and negative ions (anions); State that an ionic bond is a strong electrostatic attraction between oppositely charged ions (Core)

▶️ Answer/Explanation

Bromine is a Group VII non-metal and gains one electron to form a bromide ion (Br⁻), which is negatively charged — so A is correct. B is wrong because ionic bonds form between metals and non-metals (e.g. Group I and Group VII), not purely within Group VII. C is incorrect because positive ions form when atoms lose electrons, not protons. D is incorrect because potassium forms a positive ion (K⁺) in potassium iodide, while iodide (I⁻) is the negative ion.
✅ Answer: (A)

Question 5

Part of the Periodic Table is shown (elements F and G are in different groups).

Which type of chemical bonding is present in the oxide of F and in the oxide of G?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.4: Ions and ionic bonds — properties of ionic compounds (Core/Supplement)
• Topic 2.5: Simple molecules and covalent bonds — Describe formation of covalent bonds in simple molecules (Core)
• Topic 8.1: Arrangement of elements — describe metallic to non-metallic character across a period (Core)

▶️ Answer/Explanation

From the Periodic Table diagram, F is a metal (e.g. a Group I or II element) and G is a non-metal. The oxide of a metal (F) contains ionic bonding because the metal transfers electrons to oxygen, forming a giant ionic lattice. The oxide of a non-metal (G) contains covalent bonding as the atoms share electrons. The answer matching “ionic” for F’s oxide and “covalent” for G’s oxide is option C.
✅ Answer: (C)

Question 6

Elements X and Y react to form a compound. Element X loses two electrons and element Y gains one electron. What is the charge on the ions of elements X and Y and what is the formula of the compound?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.4: Ions and ionic bonds — Describe formation of positive and negative ions; Deduce the formula of an ionic compound from the charges on the ions (Supplement)
• Topic 3.1: Formulae — Deduce the formula of an ionic compound from the relative numbers of ions (Supplement)

▶️ Answer/Explanation

Element X loses 2 electrons, forming a 2+ ion (X²⁺). Element Y gains 1 electron, forming a 1– ion (Y⁻). To balance the charges in the compound, 2 Y⁻ ions are needed for every 1 X²⁺ ion (2× (−1) = −2 cancels +2). Therefore the formula of the compound is XY₂. This matches option B, which shows X²⁺, Y⁻, and formula XY₂.
✅ Answer: (B)

Question 7

Which statement about graphite explains why it is used as an electrode?

A. It contains ions.
B. It has a giant covalent structure.
C. It is a metal.
D. It has mobile electrons.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.6: Giant covalent structures — Describe the giant covalent structure of graphite; Relate the structure and bonding of graphite to its use as an electrode (Core)

▶️ Answer/Explanation

Graphite conducts electricity because each carbon atom forms only 3 covalent bonds within layers, leaving one delocalised (mobile) electron per atom that is free to move along the layers and carry charge. This is why graphite can be used as an electrode — it allows electrons to flow through it. It is not a metal (C is wrong), and while it has a giant covalent structure (B is true but not the specific reason for electrical conductivity), the actual explanation is the presence of mobile electrons (D is the most precise and correct answer).
✅ Answer: (D)

Question 8

Methane, CH₄, burns in air to form carbon dioxide and water. What is the balanced equation for this reaction?

A. CH₄(g) + O₂(g) → CO₂(g) + 2H₂O(g)
B. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
C. CH₄(g) + 2O₂(g) → CO₂(g) + H₂O(g)
D. CH₄(g) + 3O₂(g) → CO₂(g) + 2H₂O(g)

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 3.1: Formulae — Construct word equations and symbol equations to show how reactants form products, including state symbols (Core)
• Topic 11.3: Fuels — Name methane as main constituent of natural gas; combustion of hydrocarbons (Core)

▶️ Answer/Explanation

Balancing CH₄ + O₂ → CO₂ + H₂O: carbon balance gives 1 CO₂; hydrogen balance gives 2 H₂O (since CH₄ has 4 H atoms, producing 2 H₂O). The total oxygen needed on the right is 2 (from CO₂) + 2 (from 2 H₂O) = 4 oxygen atoms, which requires 2 O₂ molecules on the left. The fully balanced equation is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g), which is option B.
✅ Answer: (B)

Question 9

The equation for the thermal decomposition of sodium hydrogencarbonate is shown.

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

The M_r of sodium hydrogencarbonate, NaHCO₃, is 84. The M_r of sodium carbonate, Na₂CO₃, is 106. In an experiment, 2.1 g of sodium hydrogencarbonate is heated but not all of it decomposes. All of the carbon dioxide is collected and measured at room temperature and pressure. The total volume of carbon dioxide produced is 0.21 dm³. The volume of 1 mole of a gas at r.t.p. is 24 dm³. Which statement is correct?

A. The mass of sodium carbonate produced is 0.93 g.
B. The mass of sodium carbonate produced is 1.33 g.
C. The percentage yield of carbon dioxide is 10%.
D. The percentage yield of carbon dioxide is 35%.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 3.3: The mole and the Avogadro constant — Calculate stoichiometric reacting masses; Calculate percentage yield given appropriate data (Supplement)

▶️ Answer/Explanation

Moles of CO₂ produced = 0.21 ÷ 24 = 0.00875 mol. From the equation, 2 mol NaHCO₃ produces 1 mol CO₂ and 1 mol Na₂CO₃, so 0.00875 mol CO₂ is produced from 0.00875 mol Na₂CO₃. Mass of Na₂CO₃ = 0.00875 × 106 = 0.9275 ≈ 0.93 g. To find % yield: moles of NaHCO₃ taken = 2.1 ÷ 84 = 0.025 mol; theoretical moles of CO₂ = 0.025 ÷ 2 = 0.0125 mol; % yield = (0.00875 ÷ 0.0125) × 100 = 70%, confirming A is correct for the mass of Na₂CO₃.
✅ Answer: (A)

Question 10

An electrolysis experiment is done using carbon electrodes. Hydrogen and oxygen are formed at the electrodes. What is the electrolyte?

A. Aqueous copper(II) sulfate
B. Concentrated hydrochloric acid
C. Dilute aqueous sodium chloride
D. Molten potassium oxide

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 4.1: Electrolysis — Identify the products formed at the electrodes during the electrolysis of dilute sulfuric acid and dilute aqueous sodium chloride using inert electrodes (Core)

▶️ Answer/Explanation

When dilute aqueous sodium chloride is electrolysed with inert electrodes, H⁺ ions are preferentially discharged at the cathode (producing H₂) and OH⁻ ions at the anode (producing O₂), because the sodium and chloride ions are in low concentration. In concentrated NaCl, Cl₂ would form at the anode instead. Aqueous CuSO₄ gives Cu at the cathode and O₂ at the anode, and concentrated HCl gives Cl₂ at the anode — so only dilute aqueous NaCl gives both H₂ and O₂ under these conditions.
✅ Answer: (C)

Question 11

Concentrated aqueous copper(II) sulfate is electrolysed using copper electrodes. Which ionic half-equation describes the reaction taking place at the cathode?

A. 2H⁺ + 2e⁻ → H₂
B. 4OH⁻ → O₂ + 2H₂O + 4e⁻
C. Cu → Cu²⁺ + 2e⁻
D. Cu²⁺ + 2e⁻ → Cu

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 4.1: Electrolysis — Identify the products during the electrolysis of aqueous copper(II) sulfate using copper electrodes; Construct ionic half-equations for reactions at the cathode (to show reduction) (Supplement)

▶️ Answer/Explanation

The cathode is the negative electrode, where reduction (gain of electrons) occurs. In the electrolysis of CuSO₄ with copper electrodes, Cu²⁺ ions from solution are attracted to the cathode and gain 2 electrons to form copper metal: Cu²⁺ + 2e⁻ → Cu. At the anode (positive electrode), copper metal is oxidised (Cu → Cu²⁺ + 2e⁻), so option C describes the anode reaction. Options A and B are incorrect for this particular system with copper electrodes.
✅ Answer: (D)

Question 12

When powdered sodium carbonate and aqueous ethanoic acid are mixed, the temperature of the mixture falls. Which statement about this reaction is correct?

A. The reaction is endothermic and ΔH is negative.
B. The reaction is endothermic and ΔH is positive.
C. The reaction is exothermic and ΔH is negative.
D. The reaction is exothermic and ΔH is positive.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 5.1: Exothermic and endothermic reactions — State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in temperature (Core); State that ΔH is positive for endothermic reactions (Supplement)

▶️ Answer/Explanation

A fall in temperature of the surroundings (the mixture) indicates that the reaction is absorbing heat from the surroundings — this is the definition of an endothermic reaction. For endothermic reactions, the enthalpy change (ΔH) is positive because energy is taken in by the reaction system. A and D incorrectly pair the reaction type with the wrong sign of ΔH; option B correctly states endothermic with ΔH positive.
✅ Answer: (B)

Question 13

Magnesium powder reacts with an excess of dilute hydrochloric acid to produce hydrogen gas. Which statements about this reaction are correct?

The smaller the particles of magnesium powder, the more slowly the hydrogen is produced.
The higher the temperature, the faster the magnesium powder disappears.
The lower the concentration of dilute hydrochloric acid, the faster the rate of reaction.
The faster the magnesium powder disappears, the faster the rate of reaction.

A. 1 and 2
B. 2 and 3
C. 2 and 4
D. 3 and 4

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.2: Rate of reaction — Describe the effect of changing surface area, temperature, and concentration on the rate of reaction; Interpret data from rate of reaction experiments (Core)

▶️ Answer/Explanation

Statement 1 is incorrect — smaller particles have greater surface area, so they react faster, not more slowly. Statement 2 is correct — higher temperature increases particle kinetic energy and collision frequency, so Mg disappears faster. Statement 3 is incorrect — lower concentration decreases the rate. Statement 4 is correct — the rate of disappearance of magnesium is directly a measure of the rate of the reaction. Therefore, statements 2 and 4 are both correct.
✅ Answer: (C)

Question 14

The reaction between two aqueous compounds, X and Y, is slow and exothermic. The graph shows how the rate of this reaction changes with time.

A student suggests that the rate of reaction decreases with time because:

the activation energy decreases
the speed of the molecules of X and Y decreases
the concentration of both X and Y decreases with time.

Which suggestions are correct?

A. 1 and 2
B. 1 and 3
C. 2 only
D. 3 only

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.2: Rate of reaction — Describe collision theory; Describe and explain the effect of changing concentration on rate using collision theory (Supplement)

▶️ Answer/Explanation

Activation energy is a fixed property of a reaction and does not change with time, so statement 1 is incorrect. Although the reaction is exothermic and could slightly increase the temperature (increasing molecular speed), this does not account for the decreasing rate — so statement 2 is not the cause. Statement 3 is correct: as X and Y are consumed, their concentrations decrease, reducing the frequency of successful collisions and therefore the rate. The decrease in rate over time is solely explained by decreasing concentrations of reactants.
✅ Answer: (D)

Question 15

Hydrogen reacts with iodine to form hydrogen iodide.

H₂(g) + I₂(g) → 2HI(g)

Which statements explain why the reaction is faster when the pressure is increased, at constant temperature?

At higher pressure, the molecules are moving faster.
At higher pressure, more of the molecules have the required activation energy.
At higher pressure, the molecules are closer together.
At higher pressure, the molecules collide more frequently.

A. 1 and 2
B. 1 and 3
C. 2 and 4
D. 3 and 4

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.2: Rate of reaction — Describe collision theory; Describe and explain the effect of changing pressure of gases on rate using collision theory (Supplement)

▶️ Answer/Explanation

At constant temperature, increasing pressure compresses the gas so molecules are closer together (statement 3 is correct), which increases the frequency of collisions between them (statement 4 is correct). Statement 1 is incorrect — at constant temperature, the average molecular speed does not change with pressure. Statement 2 is also incorrect — the fraction of molecules with energy ≥ Ea is fixed at constant temperature, regardless of pressure. Therefore, only statements 3 and 4 correctly explain the faster rate at higher pressure.
✅ Answer: (D)

Question 16

Ammonium sulfate is used as a fertiliser. It is made from ammonia and sulfuric acid.
The ……1…… is made by the ……2…… process in which ……3…… is used as a catalyst.
Which words complete gaps 1, 2 and 3?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.3: Reversible reactions and equilibrium — State the symbol equation for production of ammonia in the Haber process; State the typical conditions including the iron catalyst (Supplement)
• Topic 10.2: Fertilisers — State that ammonium salts and nitrates are used as fertilisers (Core)

▶️ Answer/Explanation

Ammonium sulfate is made by reacting ammonia with sulfuric acid. The ammonia (gap 1) is produced by the Haber process (gap 2), which involves reacting nitrogen and hydrogen at 450 °C and 200 atm using an iron catalyst (gap 3). The Haber process equation is N₂(g) + 3H₂(g) ⇌ 2NH₃(g). This matches option C: ammonia / Haber / iron.
✅ Answer: (C)

Question 17

The reversible reaction shown takes place in a closed system at constant temperature.

P(g) + Q(g) + R(g) ⇌ S(g) + T(g)

When the reaction has reached equilibrium, more T is added. After the addition of T, which other substances increase in concentration?

A. P, Q, R and S
B. P and Q only
C. P, Q and R only
D. S only

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.3: Reversible reactions and equilibrium — Predict and explain how the position of equilibrium is affected by changing concentration (Supplement)

▶️ Answer/Explanation

Adding more T increases the concentration of a product, so by Le Chatelier’s principle, the equilibrium shifts in the reverse direction (to the left) to reduce the excess T. The reverse reaction consumes T and S and produces P, Q, and R. Therefore, P, Q, and R all increase in concentration as the equilibrium shifts left. S does not increase — it is consumed by the reverse reaction along with T.
✅ Answer: (C)

Question 18

In which equation is the underlined substance acting as a reducing agent?

A. 3CO + Fe₂O₃ → 2Fe + 3CO₂
B. CO₂ + C → 2CO
C. CuO + H₂ → Cu + H₂O
D. CaO + H₂O → Ca(OH)₂

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 6.4: Redox — Define a reducing agent as a substance that reduces another substance and is itself oxidised; Identify oxidising agents and reducing agents in redox reactions (Supplement)

▶️ Answer/Explanation

A reducing agent is a substance that causes reduction of another substance and is itself oxidised (loses oxygen or gains oxygen number increases). In option A, CO is oxidised to CO₂ (gains oxygen) while Fe₂O₃ is reduced to Fe (loses oxygen) — so CO is the reducing agent. In B, CO₂ is reduced (gains carbon/loses oxygen notionally), not a reducing agent. In C, CuO is reduced (loses oxygen) — it is the oxidising agent, not a reducing agent. Option D is not a redox reaction at all.
✅ Answer: (A)

Question 19

An aqueous solution reacts with a solid. The products are an alkaline gas, a salt and water. What are the aqueous solution and the solid?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 7.1: Characteristic properties of acids and bases — Describe the characteristic properties of bases in terms of their reactions with ammonium salts (Core)

▶️ Answer/Explanation

The alkaline gas produced is ammonia (NH₃). Ammonia is released when an alkali (base) reacts with an ammonium salt — for example, aqueous sodium hydroxide reacting with solid ammonium chloride: NaOH(aq) + NH₄Cl(s) → NaCl(aq) + NH₃(g) + H₂O(l). The products match: alkaline gas (NH₃), salt (NaCl), and water. This corresponds to option D: aqueous sodium hydroxide and solid ammonium chloride.
✅ Answer: (D)

Question 20

Butanoic acid partially dissociates in aqueous solution. Which row about butanoic acid is correct?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 7.1: Characteristic properties of acids and bases — Define a strong acid as completely dissociated and a weak acid as partially dissociated in aqueous solution (Supplement)
• Topic 11.7: Carboxylic acids — Describe reactions of carboxylic acids with metals, bases and carbonates (Core)

▶️ Answer/Explanation

Butanoic acid is a carboxylic acid that partially dissociates in aqueous solution, making it a weak acid (not a strong acid). Like all acids, it reacts with carbonates to produce a salt, water, and carbon dioxide. A weak acid that reacts with carbonates — this combination matches option B in the table. It is not completely dissociated (ruling it out as a strong acid), but it does still react chemically with metals, bases, and carbonates.
✅ Answer: (B)

Question 21

Copper(II) sulfate is prepared by adding excess copper(II) carbonate to sulfuric acid. Why is an excess of copper(II) carbonate added?

A. To ensure all the copper(II) carbonate has reacted.
B. To ensure all the sulfuric acid has reacted.
C. To increase the rate of reaction.
D. To increase the amount of copper(II) sulfate produced.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 7.3: Preparation of salts — Describe the preparation of soluble salts by reaction of an acid with excess insoluble carbonate (Core)

▶️ Answer/Explanation

When preparing a soluble salt from an acid and an insoluble carbonate, adding an excess of the carbonate ensures that all of the acid is consumed — leaving none of the acid in the final solution. This is important because any remaining sulfuric acid would contaminate the copper(II) sulfate product. The excess insoluble carbonate is then easily removed by filtration, giving a pure salt solution that can be crystallised. This is why option B is correct.
✅ Answer: (B)

Question 22

Part of the Periodic Table is shown. Which element has two electrons in its outer shell and three electron shells?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.2: Atomic structure and the Periodic Table — Determine the electronic configuration of elements; State that the number of outer shell electrons equals the group number (Groups I–VII) and the number of occupied shells equals the period number (Core)

▶️ Answer/Explanation

An element with 2 outer-shell electrons is in Group II of the Periodic Table. An element with 3 electron shells (occupied) is in Period 3. The element in Group II, Period 3 is magnesium (Mg), with electronic configuration 2,8,2 — two electrons in its outer (third) shell across three shells in total. This matches the description in the question and is option C.
✅ Answer: (C)

Question 23

Elements in Group I and Group II show the same trends in their reactions with water and in their density.
Which row shows how the properties of barium compare with calcium?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 8.1: Arrangement of elements — Identify trends in groups given information about the elements (Supplement)
• Topic 9.4: Reactivity series — Describe reactions of metals with water; Deduce order of reactivity from experimental results (Core)

▶️ Answer/Explanation

In Group II, going down the group, reactivity increases and density increases. Barium (Ba) is below calcium (Ca) in Group II, so barium is more reactive with water than calcium and has a higher density. The rate of reaction of barium with cold water is more vigorous than calcium’s. This matches option A, which states barium has a greater density and reacts more vigorously (higher reactivity) with water than calcium.
✅ Answer: (A)

Question 24

Which pair of compounds shows a transition element in two different oxidation states?

A. Cr₂O₃ and Cr₂(SO₄)₃
B. Cu₂O and CuCO₃
C. ZnS and ZnSO₄
D. NiO and Ni(NO₃)₂

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 8.4: Transition elements — Describe transition elements as having ions with variable oxidation numbers, including iron(II) and iron(III) (Supplement)
• Topic 6.4: Redox — Use oxidation numbers to identify redox changes (Supplement)

▶️ Answer/Explanation

In Cu₂O, copper has oxidation state +1 (since 2×Cu + (−2) = 0, so Cu = +1). In CuCO₃, copper has oxidation state +2 (since Cu + (−2) from CO₃²⁻ = 0, so Cu = +2). These two compounds show copper in oxidation states +1 and +2, which are two different oxidation states for the same transition element. In options A and D, chromium is +3 in both and nickel is +2 in both respectively. Zinc (C) is not typically a transition element and shows only +2.
✅ Answer: (B)

Question 25

Which description of brass is correct?

A. A compound of copper and zinc
B. A compound of copper and tin
C. A mixture of copper and zinc
D. A mixture of copper and tin

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 9.3: Alloys and their properties — Describe an alloy as a mixture of a metal with other elements; Describe brass as a mixture of copper and zinc (Core)

▶️ Answer/Explanation

Brass is an alloy, which means it is a mixture (not a compound) of two or more metals. An alloy does not have a fixed composition and the metals retain their metallic bonding — these are key differences from a compound. Brass is specifically a mixture of copper and zinc, whereas bronze is an alloy of copper and tin. Options A and B are incorrect because brass is a mixture, not a compound, and B names the wrong metal.
✅ Answer: (C)

Question 26

What is the symbol of the metal used in the manufacture of aircraft because of its low density?

A. Al
B. Cu
C. Fe
D. Zn

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 9.2: Uses of metals — Describe the uses of aluminium in the manufacture of aircraft because of its low density (Core)

▶️ Answer/Explanation

Aluminium (symbol Al) has a notably low density of about 2.7 g/cm³, much lower than iron (7.9 g/cm³) or copper (8.9 g/cm³). This low density, combined with its good strength when alloyed and resistance to corrosion, makes aluminium and its alloys ideal for constructing aircraft bodies and frames. Copper is used for electrical wiring, iron/steel for structural construction, and zinc for galvanising — none primarily for aircraft manufacture due to their high density.
✅ Answer: (A)

Question 27

Which substances react to form hydrogen gas?

1. Calcium and water
2. Silver and dilute hydrochloric acid
3. Magnesium and steam
4. Zinc and dilute hydrochloric acid

A. 1, 3 and 4 B. 1 and 3 only C. 2 and 4 D. 4 only

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 9.4: Reactivity series — Describe the reactions of potassium, sodium, calcium with cold water; Describe reactions of magnesium, zinc, iron with dilute hydrochloric acid; State the order of the reactivity series (Core)

▶️ Answer/Explanation

Calcium reacts with cold water to produce H₂ (1 ✓). Silver is below hydrogen in the reactivity series and does not react with dilute hydrochloric acid (2 ✗). Magnesium reacts with steam to produce MgO and H₂ (3 ✓). Zinc reacts with dilute hydrochloric acid to produce ZnCl₂ and H₂ (4 ✓). Therefore, substances 1, 3 and 4 all produce hydrogen gas, making option A correct.
✅ Answer: (A)

Question 28

Coke (carbon) and limestone are two raw materials used in the extraction of iron from hematite.
Which type of reaction occurs when each substance is heated during the process?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 9.6: Extraction of metals — Describe the extraction of iron from hematite in the blast furnace, including combustion of carbon, reduction of CO₂ to CO, thermal decomposition of limestone (Core)

▶️ Answer/Explanation

In the blast furnace, coke (carbon) undergoes combustion: C + O₂ → CO₂ — this is an oxidation/combustion reaction. Limestone (calcium carbonate, CaCO₃) undergoes thermal decomposition when heated: CaCO₃ → CaO + CO₂. Combustion is an oxidation reaction, and CaCO₃ breaking down on heating is thermal decomposition. This combination matches option B: coke — combustion; limestone — thermal decomposition.
✅ Answer: (B)

Question 29

Some combustion reactions produce pollutant gases. Which reactions produce a pollutant gas that is not present in clean air?

1. 2CH₄ + 3O₂ → 2CO + 4H₂O
2. 2H₂ + O₂ → 2H₂O
3. C + O₂ → CO₂
4. N₂ + O₂ → 2NO

A. 1 and 3
B. 1 and 4
C. 2 and 3
D. 3 and 4

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 10.3: Air quality and climate — State the source and adverse effects of air pollutants including carbon monoxide (from incomplete combustion) and oxides of nitrogen (from car engines) (Core)

▶️ Answer/Explanation

Clean air contains N₂, O₂, CO₂ (in small amounts), noble gases, and water vapour. Reaction 1 produces CO (carbon monoxide), which is not present in clean air — it is a pollutant from incomplete combustion. Reaction 2 produces only H₂O — not a pollutant. Reaction 3 produces CO₂, which is present in clean air and so is not a pollutant in this context. Reaction 4 produces NO (nitrogen oxide), which is not present in clean air and is a pollutant. Therefore, reactions 1 and 4 both produce pollutant gases absent from clean air.
✅ Answer: (B)

Question 30

One mole of alkane Y produces 72 dm³ of carbon dioxide when burned in excess oxygen, measured at room temperature and pressure.
What is Y?

A. Butane
B. Ethane
C. Methane
D. Propane

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 3.3: The mole and Avogadro constant — Use the molar gas volume (24 dm³ at r.t.p.) in calculations (Supplement)
• Topic 11.4: Alkanes — State bonding in alkanes; combustion of alkanes (Core)

▶️ Answer/Explanation

Moles of CO₂ produced = 72 ÷ 24 = 3 mol. Since 1 mole of alkane Y produces 3 moles of CO₂, the alkane must contain 3 carbon atoms per molecule — because each carbon atom produces 1 mole of CO₂ on complete combustion. An alkane with 3 carbons is propane (C₃H₈). Combustion: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O confirms 3 moles of CO₂ per mole of propane.
✅ Answer: (D)

Question 31

The structure of organic compound X is shown.

What is X?

A. Ethyl ethanoate
B. Ethyl methanoate
C. Methyl ethanoate
D. Methyl methanoate

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.2: Naming organic compounds — Name and draw the displayed formulae of unbranched esters which can be made from unbranched alcohols and carboxylic acids, each containing up to four carbon atoms (Supplement)

▶️ Answer/Explanation

Esters are named from the alcohol part (first word, ending in -yl) and the acid part (second word, ending in -anoate). The structure shown has the formula CH₃COOCH₃: the acid part (CH₃COO–) comes from ethanoic acid (2 carbons), giving “ethanoate”; the alcohol part (–OCH₃) comes from methanol (1 carbon), giving “methyl”. Therefore, the compound is methyl ethanoate (CH₃COOCH₃), which is option C.
✅ Answer: (C)

Question 32

What is the structural formula of the compound formed in the addition reaction of propene with bromine?

A. CH₃CHBrCH₂Br
B. CH₂BrCH₂CH₂Br
C. CHBr₂CH₂CH₃
D. CH₃CBr₂CH₃

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.5: Alkenes — Describe the properties of alkenes in terms of addition reactions with bromine; Draw the structural or displayed formulae of the products (Supplement)

▶️ Answer/Explanation

Propene is CH₃CH=CH₂. In an addition reaction with bromine (Br₂), the double bond between the second and third carbons breaks and each carbon gains one bromine atom. The Br atoms add across the double bond: CH₃CHBrCH₂Br — bromine adds to C-2 and C-3 (counting from the methyl end). This is a 1,2-dibromoropropane structure, matching option A. Option B would result from bromine adding to C-1 and C-2, which does not occur in this way.
✅ Answer: (A)

Question 33

Ethanol is produced industrially by fermentation and also by a catalysed addition reaction involving steam. Which row describes one advantage of each process?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.6: Alcohols — Describe the manufacture of ethanol by fermentation and by catalytic addition of steam to ethene; Describe the advantages and disadvantages of each method (Supplement)

▶️ Answer/Explanation

Fermentation uses glucose from renewable plant material (sugar cane, etc.) as a feedstock — making it sustainable and usable in countries without oil — but it is slow, produces dilute ethanol, and requires separation. Steam hydration (addition of steam to ethene) is a continuous process giving a high rate of production and pure ethanol, but uses a non-renewable feedstock (ethene from crude oil). Option A correctly states: fermentation uses a renewable resource (advantage of fermentation); steam hydration gives a faster/continuous production rate (advantage of steam hydration).
✅ Answer: (A)

Question 34

Carboxylic acids react with alcohols when warmed with an acid catalyst.
Which type of substance is formed in this reaction?

A. An alkene
B. An ester
C. A salt
D. A polymer

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.7: Carboxylic acids — Describe the reaction of a carboxylic acid with an alcohol using an acid catalyst to form an ester (Supplement)

▶️ Answer/Explanation

When a carboxylic acid reacts with an alcohol in the presence of a concentrated acid catalyst (e.g. H₂SO₄) and heat, an esterification reaction occurs. The –OH of the carboxylic acid and the –H of the alcohol’s –OH group combine to form water, while the remaining parts join to form an ester linkage (–COO–). For example, ethanoic acid + ethanol → ethyl ethanoate + water. The product is always an ester, not a salt, alkene, or polymer.
✅ Answer: (B)

Question 35

Nylon is formed by condensation polymerisation. Which structure represents nylon?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.8: Polymers — Describe and draw the structure of nylon, a polyamide; Describe the differences between addition and condensation polymerisation (Supplement)

▶️ Answer/Explanation

Nylon is a polyamide formed by condensation polymerisation of a dicarboxylic acid and a diamine, eliminating water at each step. The repeat unit of nylon contains amide linkages (–CO–NH–). The key structural feature to identify is the presence of both –C=O and –NH– groups alternating along the chain, which is the amide bond (–CONH–). Option C shows the correct polyamide chain structure with the –NH– and –C=O linkages characteristic of nylon.
✅ Answer: (C)

Question 36

Which structure represents the repeat unit of the addition polymer formed from but-1-ene?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 11.8: Polymers — Deduce the structure or repeat unit of an addition polymer from a given alkene and vice versa (Supplement)

▶️ Answer/Explanation

But-1-ene has the formula CH₂=CHCH₂CH₃. In addition polymerisation, the double bond opens and the monomer units link together. The repeat unit retains all 4 carbons of the monomer but with a single C–C bond replacing the double bond, adding –CH₂– connections to the chain. The pendant group is –CH₂CH₃ (ethyl group) on the second carbon. The correct repeat unit is –[CH₂–CH(CH₂CH₃)]–, which matches option A.
✅ Answer: (A)

Question 37

2.00 g of powdered calcium carbonate is added to 50.0 cm³ of hydrochloric acid.
Which apparatus is used to measure these quantities of calcium carbonate and hydrochloric acid?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 12.1: Experimental design — Name appropriate apparatus for the measurement of mass and volume, including balances and measuring cylinders (Core)

▶️ Answer/Explanation

The mass of a solid (2.00 g of calcium carbonate) is measured using a balance, not a measuring cylinder. The volume of a liquid (50.0 cm³ of hydrochloric acid) is measured using a measuring cylinder, not a balance. A burette or pipette would be used only in titration contexts. The correct pairing is balance for the solid and measuring cylinder for the liquid, which corresponds to option A.
✅ Answer: (A)

Question 38

The diagram shows a chromatogram obtained from the colours of three different sweets, X, Y and Z.

How many different red dyes are present in the sweets?

A. 1
B. 2
C. 3
D. 4

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 12.3: Chromatography — Describe how paper chromatography is used to separate mixtures of soluble coloured substances; Interpret simple chromatograms to identify unknown substances (Core)

▶️ Answer/Explanation

In a chromatogram, each distinct spot at a unique R_f value represents a different dye. The question specifies counting only the red dyes. Spots that travel to the same height (same R_f value) in different samples are the same dye. By examining the chromatogram from the three sweets, the red spots appear at two distinct heights/positions — meaning there are 2 different red dyes present across all three sweets combined, even though some sweets may share the same red dye.
✅ Answer: (B)

Question 39

A mixture contains sand and an aqueous solution of sodium chloride.
Which processes are used to obtain a sample of solid sand and a sample of solid sodium chloride from the mixture?

A. Crystallisation followed by filtration
B. Evaporation followed by filtration
C. Filtration followed by crystallisation
D. Simple distillation followed by crystallisation

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 12.4: Separation and purification — Describe and explain methods of separation including filtration and crystallisation; Suggest suitable separation techniques given information about the substances (Core)

▶️ Answer/Explanation

Sand is an insoluble solid in water, while NaCl is dissolved. First, filtration separates the insoluble sand (residue) from the NaCl solution (filtrate). The sand remains on the filter paper. Then, the filtrate (sodium chloride solution) is heated/evaporated slowly — crystallisation — to obtain solid sodium chloride crystals. The correct sequence is filtration first, then crystallisation, which is option C.
✅ Answer: (C)

Question 40

A student tests an unknown compound M. The compound:

produces a lilac flame using a flame test
produces a gas which turns limewater cloudy when dilute hydrochloric acid is added.

What is M?

A. Sodium sulfate
B. Sodium carbonate
C. Potassium sulfate
D. Potassium carbonate

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 12.5: Identification of ions and gases — Describe the use of a flame test to identify cations (potassium = lilac); Describe tests to identify the carbonate anion by reaction with dilute acid and testing for CO₂ gas (Core)

▶️ Answer/Explanation

A lilac flame in a flame test is characteristic of the potassium ion (K⁺) — this rules out sodium compounds (options A and B, which give a yellow flame). The gas that turns limewater milky/cloudy is carbon dioxide (CO₂), which is produced when a carbonate reacts with an acid: CO₃²⁻ + 2HCl → CO₂ + H₂O + 2Cl⁻. This rules out sulfate (option C). The compound with both K⁺ (lilac flame) and CO₃²⁻ (produces CO₂ with acid) is potassium carbonate (K₂CO₃), option D.
✅ Answer: (D)