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Question 1

Which two changes, when applied at the same time to a fixed amount of gas, cause the greatest increase in volume of the gas?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 1.1: Solids, liquids and gases (Describe the effects of temperature and pressure on the volume of a gas)

▶️ Answer/Explanation

Volume of a gas is inversely proportional to pressure and directly proportional to temperature. To get the greatest increase in volume, you need the largest pressure decrease combined with a temperature increase. Decreasing pressure by 20% while increasing temperature by 10% pushes the volume up the most because both changes work together to expand the gas.
✅ Answer: (C)

Question 2

Solid G is heated until it melts. It is then left to cool to room temperature.

The graph shows the cooling curve for G.

Four statements about the cooling curve between X and Y are listed.

The speed of the molecules of G decreases.
The molecules of G become less closely packed.
Thermal energy is released by the molecules of G to the surroundings.
G changes from solid to liquid.

Which statements are correct?

(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 1.1: Solids, liquids and gases (Explain changes of state in terms of kinetic particle theory, including the interpretation of heating and cooling curves)

▶️ Answer/Explanation

Between X and Y the liquid is simply cooling down, so the molecules move slower and release thermal energy to the surroundings. Statement 2 is wrong because the molecules only become less closely packed during freezing, not during cooling of the liquid. Statement 4 is also wrong since G is already a liquid at this stage, not changing from solid to liquid.
✅ Answer: (B)

Question 3

Which atom has twice as many neutrons as protons?

(A) \(^1\mathrm{H}\)
(B) \(^2\mathrm{H}\)
(C) \(^3\mathrm{H}\)
(D) \(^4\mathrm{He}\)

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 2.2: Atomic structure and the Periodic Table (Define proton number/atomic number as the number of protons in the nucleus of an atom; Define mass number/nucleon number as the total number of protons and neutrons in the nucleus of an atom)

▶️ Answer/Explanation

Hydrogen-3, also known as tritium, has 1 proton and a mass number of 3. That means it contains \(3 – 1 = 2\) neutrons, which is exactly twice the number of protons. The other isotopes don’t fit: \(^1\mathrm{H}\) has 0 neutrons, \(^2\mathrm{H}\) has 1 neutron, and \(^4\mathrm{He}\) has 2 protons and 2 neutrons.
✅ Answer: (C)

Question 4

What is the nucleon number of an atom?

(A) the number of electrons, neutrons and protons in the nucleus
(B) the number of neutrons and protons in the nucleus
(C) the number of neutrons in the nucleus
(D) the number of protons in the nucleus

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 2.2: Atomic structure and the Periodic Table (Define mass number/nucleon number as the total number of protons and neutrons in the nucleus of an atom)

▶️ Answer/Explanation

The nucleon number, also called the mass number, counts the total number of protons and neutrons inside the nucleus. Electrons are not found in the nucleus and have negligible mass, so they are not included. That makes option B the correct definition.
✅ Answer: (B)

Question 5

The diagram shows the positions of elements P and Q in the Periodic Table.

What is the formula of the compound that is formed between P and Q?

(A) PQ
(B) P₂Q
(C) PQ₂
(D) P₂Q₇

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 2.4: Ions and ionic bonds (Describe the formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams)

▶️ Answer/Explanation

From the diagram, P is in Group II so it forms a P²⁺ ion, while Q is in Group VII so it forms a Q⁻ ion. To balance the charges, you need two Q⁻ ions for every one P²⁺ ion, giving the formula PQ₂. This is typical for compounds between Group II metals and Group VII halogens.
✅ Answer: (C)

Question 6

Which statement about ionic compounds is correct?

(A) Giant lattices are formed when ionic compounds crystallise from solution.
(B) In all ionic compounds, the metal ions have fewer electrons than the non-metal ions.
(C) In a giant lattice, ions of the same charge are closer together than ions of opposite charge.
(D) When ionic compounds melt, strong intermolecular forces are broken.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 2.4: Ions and ionic bonds (Describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions)

▶️ Answer/Explanation

When ionic compounds crystallise from a solution, the ions arrange themselves into a regular, repeating three-dimensional giant lattice. Option B is not always true, option C is wrong because opposite charges attract and are closer together, and option D is incorrect because ionic compounds are held by ionic bonds, not intermolecular forces.
✅ Answer: (A)

Question 7

The equation for the reaction between iron(III) chloride and sodium hydroxide is shown.

\[\mathrm{FeCl}_3(\mathrm{aq}) + 3\mathrm{NaOH}(\mathrm{aq})\rightarrow \mathrm{Fe(OH)}_3(\mathrm{s}) + 3\mathrm{NaCl}(\mathrm{aq})\]

What is the ionic equation for this reaction?

(A) \(\mathrm{Fe}^{3+}(\mathrm{aq}) + 3\mathrm{OH}^{-}(\mathrm{aq})\rightarrow \mathrm{Fe}^{3+}(\mathrm{s}) + 3\mathrm{OH}^{-}(\mathrm{s})\)
(B) \(\mathrm{Fe}^{3+}(\mathrm{aq}) + 3\mathrm{OH}^{-}(\mathrm{aq})\rightarrow \mathrm{Fe(OH)}_3(\mathrm{s})\)
(C) \(3\mathrm{Na}^{+}(\mathrm{aq}) + 3\mathrm{Cl}^{-}(\mathrm{aq})\rightarrow 3\mathrm{Na}^{+}(\mathrm{aq}) + 3\mathrm{Cl}^{-}(\mathrm{aq})\)
(D) \(3\mathrm{Na}^{+}(\mathrm{aq}) + 3\mathrm{Cl}^{-}(\mathrm{aq})\rightarrow 3\mathrm{NaCl}(\mathrm{aq})\)

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 3.1: Formulae (Construct symbol equations with state symbols, including ionic equations)

▶️ Answer/Explanation

The ionic equation shows only the ions that actually take part in forming the precipitate. Sodium ions and chloride ions are spectator ions — they stay dissolved and don’t change. The iron(III) ions react with hydroxide ions to form the solid precipitate Fe(OH)₃, which is shown correctly in option B.
✅ Answer: (B)

Question 8

A compound is analysed and found to contain \(85.7\%\) carbon and \(14.3\%\) hydrogen only. What is its empirical formula?

(A) CH
(B) CH₂
(C) C₂H₄
(D) C₆H

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 3.3: The mole and the Avogadro constant (Calculate empirical formulae and molecular formulae, given appropriate data)

▶️ Answer/Explanation

Divide the percentage by the relative atomic mass: Carbon \(85.7 \div 12 \approx 7.14\), Hydrogen \(14.3 \div 1 = 14.3\). Then divide both by the smaller number (7.14): Carbon gives 1, Hydrogen gives about 2. So the simplest whole number ratio is C:H = 1:2, making the empirical formula CH₂.
✅ Answer: (B)

Question 9

Aqueous barium hydroxide, \(\mathrm{Ba(OH)_2}\), reacts with dilute nitric acid, \(\mathrm{HNO_3}\). The products are aqueous barium nitrate, \(\mathrm{Ba(NO_3)_2}\), and water.

In a titration, \(15.2\text{ cm}^3\) of \(0.75\text{ mol/dm}^3\) aqueous barium hydroxide is required to neutralise \(20.0\text{ cm}^3\) of dilute nitric acid.

What is the concentration of the nitric acid?

(A) \(0.29\text{ mol/dm}^3\)
(B) \(0.57\text{ mol/dm}^3\)
(C) \(1.14\text{ mol/dm}^3\)
(D) \(2.03\text{ mol/dm}^3\)

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 3.3: The mole and the Avogadro constant (Use experimental data from a titration to calculate the moles of solute, or the concentration or volume of a solution)

▶️ Answer/Explanation

First, find moles of Ba(OH)₂: \(0.75 \times (15.2/1000) = 0.0114\text{ mol}\). The balanced equation is Ba(OH)₂ + 2HNO₃ → Ba(NO₃)₂ + 2H₂O, so moles of HNO₃ = \(0.0114 \times 2 = 0.0228\text{ mol}\). Concentration of HNO₃ = \(0.0228 / (20.0/1000) = 1.14\text{ mol/dm}^3\).
✅ Answer: (C)

Question 10

Which particles are responsible for the transfer of charge in the external circuit during the electrolysis of molten lead(II) bromide?

(A) anions
(B) atoms
(C) cations
(D) electrons

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 4.1: Electrolysis (Describe the transfer of charge during electrolysis to include: the movement of electrons in the external circuit)

▶️ Answer/Explanation

In the external circuit — the wires and power source outside the electrolyte — electric charge is carried by electrons moving from the anode to the cathode. Ions move inside the electrolyte but do not travel through the metal wires, so electrons are the charge carriers in the external circuit.
✅ Answer: (D)

Question 11

Two statements about hydrogen are listed.

Hydrogen is used as a fuel.
When hydrogen burns in the air to form water, thermal energy is produced.

Which conclusion about these statements is correct?

(A) Both statements are correct, and statement 2 explains statement 1.
(B) Both statements are correct, but statement 2 does not explain statement 1.
(C) Statement 1 is correct, but statement 2 is not correct.
(D) Statement 2 is correct, but statement 1 is not correct.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 5.1: Exothermic and endothermic reactions (State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings)

▶️ Answer/Explanation

Hydrogen is indeed used as a fuel, and its combustion with oxygen is highly exothermic, releasing a lot of thermal energy. The fact that burning hydrogen produces so much heat is precisely what makes it a good fuel, so statement 2 directly explains statement 1.
✅ Answer: (A)

Question 12

Concentrated aqueous copper(II) chloride is electrolysed using carbon electrodes.

What is formed at each electrode?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 4.1: Electrolysis (Predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution)

▶️ Answer/Explanation

At the cathode (negative electrode), copper is less reactive than hydrogen so Cu²⁺ ions are reduced to copper metal. At the anode (positive electrode), the solution is concentrated chloride, so Cl⁻ ions are oxidised to chlorine gas instead of oxygen from water. So copper forms at the negative electrode and chlorine at the positive electrode.
✅ Answer: (B)

Question 13

Which row describes an exothermic reaction?

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• TOPIC 5.1: Exothermic and endothermic reactions (State that the transfer of thermal energy during a reaction is called the enthalpy change, ΔH of the reaction. ΔH is negative for exothermic reactions and positive for endothermic reactions)

▶️ Answer/Explanation

Exothermic reactions release energy to the surroundings, so ΔH is negative and the surroundings get hotter. Option D has both a negative sign for ΔH and an increase in the temperature of the surroundings, which perfectly describes an exothermic reaction.
✅ Answer: (D)

Question 14

A reaction pathway diagram is shown.

Which row shows the correct labels for X and Y and the type of reaction taking place?

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

• TOPIC 5.1: Exothermic and endothermic reactions (Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: activation energy, Eₐ; enthalpy change of the reaction, ΔH)

▶️ Answer/Explanation

The energy barrier from reactants up to the peak is the activation energy (X). The overall energy difference between reactants and products is the enthalpy change (Y). Since the products are at a higher energy level than the reactants, energy has been absorbed—this is an endothermic reaction.
✅ Answer: B

Question 15

The rate of reaction between magnesium and dilute hydrochloric acid is increased by increasing the concentration of the acid.

How does this affect the reacting particles?

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

• TOPIC 6.2: Rate of reaction (Describe collision theory in terms of: number of particles per unit volume; frequency of collisions between particles)

▶️ Answer/Explanation

Increasing concentration packs more acid particles into the same volume, so collisions happen more often. However, the temperature hasn’t changed, so the individual particles don’t gain any extra kinetic energy. The proportion of particles that have enough activation energy therefore remains exactly the same.
✅ Answer: B

Question 16

The reaction between aqueous iron(III) ions, \(\mathrm{Fe}^{3+}\), and aqueous thiocyanate ions, \(\mathrm{SCN}^{-}\), is reversible.

The equation for the reaction is shown.

\[\mathrm{Fe}^{3+}(\mathrm{aq}) + \mathrm{SCN}^{-}(\mathrm{aq})\rightleftharpoons \mathrm{FeSCN}^{2+}(\mathrm{aq})\]

\[\text{yellow}\qquad \text{colourless}\qquad \text{red}\]

At equilibrium, the concentrations of \(\mathrm{Fe}^{3+}\) ions, \(\mathrm{SCN}^{-}\) ions, and \(\mathrm{FeSCN}^{2+}\) ions are identical, and the mixture appears orange.

When a few drops of aqueous thiocyanate ions are added to the mixture, a new equilibrium forms.

How does the colour of the mixture change?

A. It becomes colourless.
B. It becomes a lighter orange.
C. It becomes more yellow.
D. It becomes more red.

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

• TOPIC 6.3: Reversible reactions and equilibrium (Predict and explain, for a reversible reaction, how the position of equilibrium is affected by changing concentration)

▶️ Answer/Explanation

Adding more SCN⁻ shifts the equilibrium position to the right to use up the extra reactant, producing more FeSCN²⁺. Since FeSCN²⁺ is red, the mixture becomes redder. The orange colour deepens towards red as the concentration of the red product increases.
✅ Answer: D

Question 17

The equation for the reaction between iron(II) sulfate and bromine is shown.

\[6\mathrm{FeSO}_4 + 3\mathrm{Br}_2\rightarrow 2\mathrm{Fe}_2(\mathrm{SO}_4)_3 + 2\mathrm{FeBr}_3\]

Which row identifies the oxidising agent and the reducing agent?

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

• TOPIC 6.4: Redox (Define an oxidising agent as a substance that oxidises another substance and is itself reduced; Define a reducing agent as a substance that reduces another substance and is itself oxidised; Identify oxidising agents and reducing agents in redox reactions)

▶️ Answer/Explanation

Iron in FeSO₄ goes from oxidation state +2 to +3 in the products—it loses electrons and is oxidised, so FeSO₄ is the reducing agent. Bromine goes from 0 in Br₂ to –1 in FeBr₃—it gains electrons and is reduced, so Br₂ is the oxidising agent.
✅ Answer: A

Question 18

The equation represents a redox reaction.

\[2\mathrm{Al} + \mathrm{Fe}_2\mathrm{O}_3\rightarrow 2\mathrm{Fe} + \mathrm{Al}_2\mathrm{O}_3\]

What is the change in the oxidation number of iron in this reaction?

A. from \(+2\) to \(+3\)
B. from \(+2\) to \(0\)
C. from \(+3\) to \(+2\)
D. from \(+3\) to \(0\)

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

• TOPIC 6.4: Redox (Identify redox reactions by changes in oxidation number using: the sum of the oxidation numbers in a compound is zero; the oxidation number of elements in their uncombined state is zero)

▶️ Answer/Explanation

In Fe₂O₃, oxygen has an oxidation number of –2, so the total for three oxygens is –6. To balance this, the two iron atoms must have a total of +6, meaning each iron is +3. In the product Fe, the element is uncombined, so its oxidation number is 0. Iron therefore changes from +3 to 0, meaning it is reduced.
✅ Answer: D

Question 19

Aqueous ammonia forms an equilibrium with water.

\[\mathrm{NH_3 + H_2O\rightleftharpoons NH_4^+ + OH^-}\]

In this equilibrium, which particle acts as an acid and which particle acts as a base?

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

• TOPIC 7.1: Acids, bases and salts (Define acids as proton donors and bases as proton acceptors)

▶️ Answer/Explanation

Water donates a proton (H⁺) to ammonia, forming OH⁻—that makes H₂O the acid. Ammonia accepts that proton to become NH₄⁺, so NH₃ is acting as a base. This is a Brønsted–Lowry acid-base pair.
✅ Answer: C

Question 20

Which row identifies an acidic oxide, a basic oxide and an amphoteric oxide?

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

• TOPIC 7.2: Oxides (Classify oxides as acidic, including SO₂, or basic, including CuO and CaO, related to metallic and non-metallic character; Classify Al₂O₃ and ZnO as amphoteric oxides)

▶️ Answer/Explanation

Sulfur dioxide (SO₂) is a non-metal oxide, so it is acidic. Sodium oxide (Na₂O) is a metal oxide from Group I, so it is basic. Aluminium oxide (Al₂O₃) is amphoteric—it reacts with both acids and bases. Option D correctly lists them in this order.
✅ Answer: D

Question 21

A Group I metal (lithium or potassium) is reacted with a Group VII element (chlorine or iodine).

Which compound is formed when the Group I metal of highest density reacts with the Group VII element of lowest density?

A. potassium chloride
B. lithium chloride
C. potassium iodide
D. lithium iodide

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

• TOPIC 8.2: Group I properties (Describe the Group I alkali metals with general trends down the group, limited to: increasing density)
• TOPIC 8.3: Group VII properties (Describe the Group VII halogens with general trends down the group, limited to: increasing density)

▶️ Answer/Explanation

Going down Group I, density increases, so potassium is denser than lithium. Going down Group VII, density also increases, so chlorine has a lower density than iodine. The Group I metal of highest density is potassium, and the Group VII element of lowest density is chlorine, giving potassium chloride.
✅ Answer: A

Question 22

Element X is in the same group of the Periodic Table as silicon.

An atom of element X has more protons than a silicon atom.

Which element could be X?

A. aluminium
B. carbon
C. germanium
D. phosphorus

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

• TOPIC 8.1: Arrangement of elements (Explain similarities in the chemical properties of elements in the same group of the Periodic Table in terms of their electronic configuration; Explain how the position of an element in the Periodic Table can be used to predict its properties)

▶️ Answer/Explanation

Silicon is in Group IV. Carbon is above silicon with fewer protons, while germanium is below silicon with more protons. Aluminium is in Group III and phosphorus is in Group V—neither shares the same group. Only germanium fits both conditions: same group as silicon and more protons.
✅ Answer: C

Question 23

Which statement about the elements in Group VIII is correct?

A. They all form diatomic molecules.
B. They all have eight electrons in their outer electron shells.
C. They all react with oxygen to form oxides.
D. They all are gases at room temperature.

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

• TOPIC 8.5: Noble gases (Describe the Group VIII noble gases as unreactive, monatomic gases and explain this in terms of electronic configuration)

▶️ Answer/Explanation

All noble gases (Group VIII/Group 0) exist as monatomic gases at room temperature. Helium has only 2 electrons in its outer shell, so statement B is wrong. They do not form diatomic molecules, and they are too unreactive to form oxides under normal conditions.
✅ Answer: D

Question 24

Which element is a transition element?

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

• TOPIC 8.4: Transition elements (Describe the transition elements as metals that: have high densities; have high melting points; form coloured compounds)

▶️ Answer/Explanation

Transition elements typically have very high melting points, high densities, and form coloured compounds. Option D has an extremely high melting point (1907°C), a high density (7.2 g/cm³), and a green oxide—all classic signs of a transition metal like chromium. The other options have low melting points or white oxides typical of Groups I/II.
✅ Answer: D

Question 25

Four test-tubes contain dilute hydrochloric acid. Each test-tube has one piece of a different metal added to it. The metals are copper, iron, magnesium and zinc.

Which test-tube contains iron and dilute hydrochloric acid?

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

• TOPIC 9.4: Reactivity series (Describe the reactions, if any, of magnesium, zinc, iron, copper, silver and gold with dilute hydrochloric acid and explain these reactions in terms of the position of the metals in the reactivity series)

▶️ Answer/Explanation

In the reactivity series, magnesium reacts very vigorously with dilute acid, zinc reacts moderately, iron reacts slowly with gentle bubbling, and copper does not react at all. The test-tube showing a slow but steady production of gas bubbles without extreme violence corresponds to iron in dilute hydrochloric acid.
✅ Answer: A

Question 26

Brass is a mixture of copper and zinc.

Which statement describes and explains the relative strength of brass compared to copper?

A. Brass is stronger than copper because the layers of atoms in brass cannot easily slide over each other.
B. Brass is stronger than copper because the intermolecular forces are greater in brass than in copper.
C. Brass is weaker than copper because the layers of atoms slide more easily over each other in brass than in copper.
D. Brass is weaker than copper because the intermolecular forces are weaker in brass than in copper.

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

• TOPIC 9.3: Alloys and their properties (Explain in terms of structure how alloys can be harder and stronger than the pure metals because the different sized atoms in alloys mean the layers can no longer slide over each other)

▶️ Answer/Explanation

Brass is an alloy—zinc atoms are a different size from copper atoms, so they disrupt the regular layers in the metallic lattice. When a force is applied, the layers cannot easily slide over each other because of this distortion, making brass harder and stronger than pure copper. The mention of intermolecular forces is incorrect for metallic bonding.
✅ Answer: A

Question 27

A sample of calcium carbonate is heated strongly, and substances U and V are formed.

Substance U is a white solid. Substance V is a colourless gas.

What are substances U and V?

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

• TOPIC 9.6: Extraction of metals (State the symbol equations for the extraction of iron from hematite: CaCO₃ → CaO + CO₂)

▶️ Answer/Explanation

Calcium carbonate undergoes thermal decomposition when heated strongly: CaCO₃ → CaO + CO₂. Calcium oxide (CaO) is a white solid, and carbon dioxide (CO₂) is a colourless gas. This is the same reaction that occurs in the blast furnace when limestone is heated.
✅ Answer: C

Question 28

A sample of water taken from a river contains harmful microbes.

What is a source of harmful microbes in the river water?

A. sewage
B. fertilisers
C. detergents
D. plastics

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

• TOPIC 10.1: Water (State that water from natural sources may contain substances, including: sewage; State that sewage contains harmful microbes which cause disease)

▶️ Answer/Explanation

Sewage contains human and animal waste teeming with bacteria, viruses, and other harmful microbes. Fertilisers, detergents, and plastics are chemical or physical pollutants—they do not introduce harmful microbes into the water.
✅ Answer: A

Question 29

Anhydrous copper(II) sulfate and anhydrous cobalt(II) chloride change colour when water is added to them.

Which row shows the correct colour changes?

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

• TOPIC 10.1: Water (Describe chemical tests for the presence of water using anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate)

▶️ Answer/Explanation

Anhydrous copper(II) sulfate is white and turns blue when water is added. Anhydrous cobalt(II) chloride is blue and turns pink with water. So the correct sequence is white to blue for copper sulfate and blue to pink for cobalt chloride.
✅ Answer: C

Question 30

Which gas contributes to increased global warming?

A. argon
B. carbon dioxide
C. nitrogen
D. oxygen

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

• TOPIC 10.3: Air quality and climate (State the adverse effect of these air pollutants, limited to: carbon dioxide: higher levels of carbon dioxide leading to increased global warming, which leads to climate change)

▶️ Answer/Explanation

Carbon dioxide is a greenhouse gas that absorbs infrared radiation and traps heat in the atmosphere, contributing significantly to global warming. Argon, nitrogen, and oxygen are not greenhouse gases and do not contribute to this effect.
✅ Answer: B

Question 31

Which statement describes how greenhouse gases increase global warming?

A. Greenhouse gases absorb thermal energy from the Sun, reducing the Earth’s temperature.
B. Greenhouse gases reflect thermal energy back into space, cooling the Earth’s surface.
C. Greenhouse gases absorb and re-emit thermal energy radiated from the Earth’s surface.
D. Greenhouse gases release thermal energy from the Earth into space.

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

• TOPIC 10.3: Air quality and climate (Describe how the greenhouse gases carbon dioxide and methane cause global warming, limited to: the absorption, reflection and emission of thermal energy; reducing thermal energy loss to space)

▶️ Answer/Explanation

The Earth absorbs sunlight and re-radiates it as infrared (thermal) energy. Greenhouse gases in the atmosphere absorb this outgoing thermal energy and re-emit it in all directions, including back toward the surface. This traps heat and causes the planet to warm up.
✅ Answer: C

Question 32

Nitrogen monoxide is produced in a car engine when petrol is burned.

The gases from the car engine are passed through a catalytic converter.

In the catalytic converter, the nitrogen monoxide, NO, reacts with carbon monoxide, CO, to form nitrogen and carbon dioxide.

Which statement is not correct?

A. Carbon monoxide is oxidised in the catalytic converter.
B. Carbon monoxide is produced by the complete combustion of petrol.
C. Nitrogen monoxide is formed by the reaction of nitrogen and oxygen.
D. Nitrogen monoxide is reduced in the catalytic converter.

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

• TOPIC 10.3: Air quality and climate (Explain how oxides of nitrogen form in car engines and describe their removal by catalytic converters, e.g. 2CO + 2NO → 2CO₂ + N₂)

▶️ Answer/Explanation

Complete combustion of petrol produces only carbon dioxide and water. Carbon monoxide is produced by incomplete combustion, where there is insufficient oxygen. All other statements are correct: CO is oxidised to CO₂, NO is reduced to N₂, and NO forms from nitrogen and oxygen at high engine temperatures.
✅ Answer: B

Question 33

The displayed formula of but-2-ene is shown.

Which displayed formulae represent structural isomers of but-2-ene?

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

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

• TOPIC 11.1: Formulae, functional groups and terminology (Define structural isomers as compounds with the same molecular formula, but different structural formulae, including C₄H₈ as CH₃CH₂CH=CH₂ and CH₃CH=CHCH₃)

▶️ Answer/Explanation

But-2-ene (CH₃CH=CHCH₃) has molecular formula C₄H₈. Its structural isomers include but-1-ene (CH₂=CHCH₂CH₃) and 2-methylpropene. Structure 2 is likely butane (C₄H₁₀), which is an alkane—not an isomer of an alkene. So only structures 1 and 3 are correct structural isomers.
✅ Answer: C

Question 34

Which row describes two characteristics of members of the same homologous series?

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

• TOPIC 11.1: Formulae, functional groups and terminology (Describe the general characteristics of a homologous series as: having the same functional group; displaying a trend in physical properties; sharing similar chemical properties)

▶️ Answer/Explanation

Members of the same homologous series share the same functional group and similar chemical properties, but their physical properties (like boiling point) show a gradual trend as the chain length increases. They differ by a CH₂ unit, not CH₃, and they do not have the same physical properties.
✅ Answer: A

Question 35

Compound W is a hydrocarbon that contains single bonds only.

Which homologous series contains W?

A. alkenes
B. alkanes
C. carboxylic acids
D. alcohols

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

• TOPIC 11.4: Alkanes (State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons)

▶️ Answer/Explanation

Hydrocarbons with only single bonds are saturated. Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂, containing only C–C single bonds. Alkenes have a C=C double bond, while carboxylic acids and alcohols contain oxygen and are not hydrocarbons.
✅ Answer: B

Question 36

Methane forms chloromethane in a photochemical reaction using ultraviolet light.

Which statement about this reaction is correct?

A. Methane is mixed with hydrogen chloride gas, and ultraviolet light prevents a combustion reaction.
B. Methane is mixed with hydrogen chloride gas, and ultraviolet light provides the activation energy for the reaction.
C. Methane is mixed with chlorine gas, and ultraviolet light prevents a combustion reaction.
D. Methane is mixed with chlorine gas, and ultraviolet light provides the activation energy for the reaction.

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

• TOPIC 11.4: Alkanes (Describe the substitution reaction of alkanes with chlorine as a photochemical reaction, with ultraviolet light providing the activation energy, Eₐ)

▶️ Answer/Explanation

The reaction is CH₄ + Cl₂ → CH₃Cl + HCl. It requires chlorine gas, not hydrogen chloride. UV light breaks the Cl–Cl bond to form chlorine radicals, providing the activation energy needed to start the substitution reaction. The UV light does not prevent combustion—it initiates the reaction.
✅ Answer: D

Question 37

Which statement about polymers is correct?

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

• TOPIC 11.8: Polymers (Describe the formation of poly(ethene) as an example of addition polymerisation using ethene monomers; Identify the repeat units and/or linkages in addition polymers)

▶️ Answer/Explanation

Polymers are long-chain molecules made from many repeating monomer units joined by covalent bonds. In addition polymerisation, monomers join together without the loss of any atoms. The diagram illustrates the formation of a polymer chain from alkene monomers, showing the correct repeat unit structure for an addition polymer.
✅ Answer: A

Question 38

The steps used to separate a mixture of a soluble solid and an insoluble solid are listed.

A solution is made by adding the mixture to a suitable …..1….. .
The mixture is stirred and then poured through a filter funnel lined with filter paper.
The insoluble solid is collected on the filter paper as the …..2….. .

Which words complete gaps 1 and 2?

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

• TOPIC 12.1: Experimental design (Describe a: solvent as a substance that dissolves a solute; residue as a substance that remains after filtration; filtrate as a liquid or solution that has passed through a filter)

▶️ Answer/Explanation

The mixture is added to a suitable solvent (like water) to dissolve the soluble solid. After filtration, the insoluble solid left on the filter paper is called the residue. The liquid that passes through is the filtrate. So gap 1 is solvent, gap 2 is residue.
✅ Answer: D

Question 39

Which items of apparatus are used to separate a mixture of liquids with different boiling points?

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

• TOPIC 12.4: Separation and purification (Describe and explain methods of separation and purification using fractional distillation)

▶️ Answer/Explanation

To separate a mixture of liquids with different boiling points, you need a fractional distillation setup. This includes a round-bottom flask, a fractionating column packed with glass beads, a thermometer, a condenser with a water jacket, and a receiving flask. Diagram A correctly shows this complete apparatus arrangement.
✅ Answer: A

Question 40

In paper chromatography, what is the equation for the \(R_{\mathrm{f}}\) value?

A. \(R_{\mathrm{f}} = \dfrac{\text{distance travelled by solvent}}{\text{distance travelled by substance}}\)
B. \(R_{\mathrm{f}} = \dfrac{\text{distance travelled by locating agent}}{\text{distance travelled by substance}}\)
C. \(R_{\mathrm{f}} = \dfrac{\text{distance travelled by substance}}{\text{distance travelled by locating agent}}\)
D. \(R_{\mathrm{f}} = \dfrac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}\)

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

• TOPIC 12.3: Chromatography (State and use the equation for Rf: distance travelled by substance / distance travelled by solvent)

▶️ Answer/Explanation

The Rf (retention factor) is calculated by dividing the distance moved by the substance (the spot) from the baseline by the distance moved by the solvent front from the baseline. Option D gives this correct formula. The solvent always travels further, so Rf is a fraction less than 1.
✅ Answer: D