0620_m24_qp_12-hina

Question 1

Which statement about a solid, a liquid or a gas is correct?
A. A solid has a fixed shape and can be compressed.
B. A liquid takes the shape of the container it is in and can be compressed.
C. A solid has no fixed shape and cannot be compressed.
D. A gas takes the shape of the container it is in and can be compressed.

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

• Topic 1.1: Solids, liquids and gases — State the distinguishing properties of solids, liquids and gases (Core)

▶️ Answer/Explanation

A solid has both a fixed shape and a fixed volume, and it cannot be compressed because its particles are closely packed. A liquid takes the shape of its container but has a fixed volume and is very difficult to compress. A gas takes both the shape and volume of its container, and its particles are far apart, making it easy to compress. Therefore, only option D correctly describes a gas.
✅ Answer: D

Question 2

Which diagram represents a mixture of compounds?

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

• Topic 2.1: Elements, compounds and mixtures — Describe the differences between elements, compounds and mixtures (Core)

• Topic 2.4/2.5: Ions and ionic bonds / Simple molecules and covalent bonds — Representing compounds as combinations of different atoms bonded together

▶️ Answer/Explanation

A mixture of compounds contains two or more different compounds physically mixed together. In particle diagrams, each compound is shown as bonded groups of different atom types (different colours/shapes) that are not chemically joined to each other. Option C shows two distinct types of bonded groups (different pairings of atoms) present together, which represents a mixture of two different compounds. Option A is a single compound (only one type of bonded group). Option B is a mixture of elements (only single atoms of two types, not bonded as compounds). Option D is a single element (all atoms identical).
✅ Answer: (C)

Question 3

Four ions are listed.

Which pair of ions have the same electronic configuration?
A $N^{3-}$ and $Li^+$
B $Al^{3+}$ and $N^{3-}$
C $Cl^-$ and $Al^{3+}$
D $Li^+$ and $Cl^-$

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

• Topic 2.2: Atomic structure and the Periodic Table — Determine the electronic configuration of elements and their ions with proton number 1 to 20.
• Topic 2.4: Ions and ionic bonds — Describe the formation of positive ions (cations) and negative ions (anions).

▶️ Answer/Explanation

$N^{3-}$ (atomic number 7, plus 3 electrons) has 10 electrons, configuration 2,8. $Al^{3+}$ (atomic number 13, minus 3 electrons) has 10 electrons, also 2,8. Thus they are isoelectronic. $Li^+$ has 2 electrons (1s²), $Cl^-$ has 18 electrons (2,8,8). Only pair B matches.
✅ Answer: B

Question 4

Which statement about isotopes is correct?

A. Atoms with different numbers of electrons are isotopes of each other.
B. Atoms with the same mass numbers are isotopes of each other.
C. Isotopes of the same element have different numbers of neutrons.
D. Isotopes of the same element have different numbers of protons.

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

• Topic 2.3: Isotopes — Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons.

▶️ Answer/Explanation

Isotopes are defined as atoms of the same element (same number of protons) but with different numbers of neutrons, resulting in different mass numbers. Option A is wrong because changing the number of electrons creates ions, not isotopes. Option B is wrong because atoms with the same mass number could be different elements (isobars). Option D is wrong because isotopes of the same element must have the same number of protons. Therefore, only option C correctly describes isotopes.
✅ Answer: (C)

Question 5

Which statement about the ions formed by the elements in Group VII of the Periodic Table is correct?

A. All the ions have the same charge of 1–.
B. All the ions have the same number of electron shells.
C. Each ion is formed by losing one electron.
D. Each ion has seven electrons in its outer electron shell.

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

• Topic 2.4: Ions and ionic bonds — Describe the formation of positive ions, known as cations, and negative ions, known as anions (Core)
• Topic 8.1: Arrangement of elements — Describe the relationship between group number and the charge of the ions formed from elements in that group (Core)
• Topic 8.3: Group VII properties — Describe the Group VII halogens, chlorine, bromine and iodine, as diatomic non-metals (Core)

▶️ Answer/Explanation

All Group VII (halogen) atoms have seven electrons in their outer shell. To achieve a stable noble gas electronic configuration, each atom gains one electron to form a negative ion with a charge of 1–. Option B is incorrect because ions of heavier halogens have more electron shells. Option C is incorrect because electrons are gained, not lost. Option D describes the atom, not the ion.
✅ Answer: (A)

Question 6

Which row describes the properties of potassium bromide?

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

• Topic 2.4: Ions and ionic bonds — Describe the properties of ionic compounds: (a) high melting points and boiling points (b) good electrical conductivity when aqueous or molten and poor when solid (Core); Explain in terms of structure and bonding the properties of ionic compounds (Supplement)

▶️ Answer/Explanation

Potassium bromide (KBr) is an ionic compound. Ionic compounds have high melting points (due to strong electrostatic forces in the giant lattice) and conduct electricity only when molten or in aqueous solution (because ions are then free to move), but not when solid (ions are fixed in place). Reviewing the rows: Row A (high melting point, conducts when solid) is wrong; Row B (low melting point, conducts when molten) is wrong for KBr; Row C (low melting point, no conductivity) is typical of covalent substances; Row D (high melting point, conducts when molten, not when solid) matches all properties of KBr.
✅ Answer: (D)

Question 7

Which statement explains why graphite is used as a lubricant?

A. Each carbon atom in graphite forms three bonds.
B. The bonding in graphite is covalent.
C. The carbon atoms are arranged in hexagons.
D. There are weak forces of attraction between the layers of carbon atoms

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

• Topic 2.6: Giant covalent structures — Describe the giant covalent structures of graphite and diamond; Relate the structures and bonding of graphite and diamond to their uses, limited to: (a) graphite as a lubricant and as an electrode.

▶️ Answer/Explanation

Graphite consists of layers of carbon atoms arranged in hexagons. While strong covalent bonds hold the atoms together within each layer, only weak intermolecular forces (London forces) act between the layers. These weak forces allow the layers to slide easily over each other, which makes graphite slippery and ideal for use as a lubricant.
✅ Answer: (D)

Question 8

What is the balanced equation for the reaction between magnesium and dilute sulfuric acid?

A. Mg + H₂SO₄ → MgSO₄ + H₂

B. Mg + 2H₂SO₄ → Mg(SO₄)₂ + 2H₂

C. 2Mg + H₂SO₄ → Mg₂SO₄ + H₂

D. 2Mg + 3H₂SO₄ → Mg₂(SO₄)₃ + 3H₂

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

• Topic 7.1: The characteristic properties of acids and bases — Describe the characteristic properties of acids in terms of their reactions with metals (Core)
• Topic 9.4: Reactivity series — Describe the reactions of metals with dilute hydrochloric acid (Core)

▶️ Answer/Explanation

Magnesium is a reactive metal and displaces hydrogen from dilute sulfuric acid. One Mg atom reacts with one H₂SO₄ molecule to form one MgSO₄ formula unit and one H₂ molecule. This gives the balanced equation Mg + H₂SO₄ → MgSO₄ + H₂. Options B, C and D are incorrect because they have incorrect balancing or incorrect formula units (e.g., Mg(SO₄)₂ does not represent the correct sulfate of magnesium).
✅ Answer: (A)

Question 9

The relative atomic mass, A_r, of an element is the average mass of the isotopes of that element compared to another particle. Which particle is used for this comparison?

A. a proton
B. an atom of ¹²C
C. an atom of ⁴⁰Ca
D. an atom of ¹H

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

• Topic 3.2: Relative masses of atoms and molecules — Describe relative atomic mass, A_r as the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C (Core).

▶️ Answer/Explanation

By definition, relative atomic mass (A_r) is the average mass of the isotopes of an element compared to 1/12th of the mass of one atom of carbon-12 (¹²C). This standard was adopted internationally because carbon-12 is a stable isotope and the standard provides consistent atomic mass values for all elements. Therefore, the correct particle for comparison is an atom of ¹²C.
✅ Answer: (B)

Question 10

The equations for two reactions are shown.

Which row shows the value of x, the value of y and the equations that are for redox reactions?

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

• Topic 6.4: Redox — Define oxidation as gain of oxygen and reduction as loss of oxygen (Core); Identify redox reactions as reactions involving gain and loss of oxygen (Core); Define oxidation in terms of loss of electrons and reduction in terms of gain of electrons (Supplement).

▶️ Answer/Explanation

In reaction 1, Fe₂O₃ loses oxygen to become Fe (reduction), and CO gains oxygen to become CO₂ (oxidation), so it is a redox reaction (x = yes). In reaction 2, there is no change in oxidation states (all are +1, -1, +1, -1, +1, -1 before and after) and no oxygen transfer, so it is not redox (y = no). Only option D matches ‘yes’ for reaction 1, ‘no’ for reaction 2, and correctly states that both reaction equations are balanced as shown.
✅ Answer: (D)

Question 11

Concentrated aqueous sodium chloride is electrolysed using graphite electrodes. What is the product formed at the cathode?

A. chlorine
B. hydrogen
C. oxygen
D. sodium

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

• Topic 4.1: Electrolysis — Identify the products formed at the electrodes and describe the observations made during the electrolysis of concentrated aqueous sodium chloride (Core).

▶️ Answer/Explanation

In the electrolysis of concentrated aqueous sodium chloride, the cathode attracts positive ions (Na⁺ and H⁺ from water). Hydrogen ions (H⁺) are preferentially discharged over sodium ions because hydrogen is lower in the reactivity series. The reduction of H⁺ produces hydrogen gas. Chlorine is produced at the anode, oxygen comes from hydroxide discharge (dilute conditions), and sodium metal is not formed in aqueous solution.
✅ Answer: (B)

Question 12

Which row describes the changes that occur when metals burn in oxygen?

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

• Topic 6.4: Redox — Define oxidation as gain of oxygen and reduction as loss of oxygen (Core); Define oxidation in terms of loss of electrons and increase in oxidation number (Supplement). Also relates to metals reacting with oxygen (Topic 9.1).

▶️ Answer/Explanation

When a metal burns in oxygen, it gains oxygen to form a metal oxide, so the metal is oxidized. Since oxidation is the loss of electrons, the metal atoms lose electrons to become positive ions. Reduction is the gain of electrons, but here the metal is being oxidized, not reduced. Thus, only the statements “Metal is oxidized” and “Metal loses electrons” are correct.
✅ Answer: (C)

Question 13

When calcium carbonate is heated strongly, carbon dioxide gas is produced. Which words describe the type of change that occurs?
A. endothermic and chemical
B. endothermic and physical
C. exothermic and chemical
D. exothermic and physical

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

• Topic 5.1 (Core): State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings.
• Topic 7.3 (Core): Describe the preparation, separation and purification of soluble salts by reaction of an acid with an excess insoluble carbonate.
• Topic 6.1 (Core): Identify physical and chemical changes, and describe the differences between them (a new substance, carbon dioxide, is formed).

▶️ Answer/Explanation

The thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂) requires continuous heating to proceed, meaning it absorbs thermal energy from the surroundings, making it an endothermic process. Furthermore, the reaction produces a new chemical substance (carbon dioxide gas) and the starting material is permanently changed, which are the defining features of a chemical change. Therefore, the change is both endothermic and chemical.
✅ Answer: (A)

Question 14

Which row about a hydrogen–oxygen fuel cell is correct?

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

• Topic 4.2: Hydrogen-oxygen fuel cells — State that a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product (Core); Describe the advantages and disadvantages of using hydrogen-oxygen fuel cells in comparison with gasoline/petrol engines in vehicles (Supplement)

▶️ Answer/Explanation

In a hydrogen–oxygen fuel cell, hydrogen gas is supplied to the anode (negative electrode) where it is oxidised, and oxygen gas is supplied to the cathode (positive electrode) where it is reduced. The only chemical product is water, and the cell produces electricity directly without combustion, making it more efficient and cleaner than gasoline/petrol engines. Therefore, the correct row shows hydrogen at the anode, oxygen at the cathode, and water as the product.
✅ Answer: A

Question 15

Which list contains only chemical changes?
A melting, evaporating, dissolving
B rusting, freezing, cracking
C neutralisation, polymerisation, combustion
D boiling, condensing, distillation

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

• Topic 6.1: Physical and chemical changes — Identify physical and chemical changes, and describe the differences between them (Core)

▶️ Answer/Explanation

A chemical change produces new substances with different properties. Melting, evaporating, dissolving, freezing, boiling, condensing, and distillation are physical changes (states change or separation). Rusting (oxidation of iron), cracking (breaking large hydrocarbons), neutralisation (acid+base → salt+water), polymerisation (forming polymers), and combustion (burning) are all chemical changes. Therefore, only option C contains three chemical changes.
✅ Answer: C

Question 16

Excess calcium carbonate is added to $50 cm^3$ of dilute hydrochloric acid of different concentrations in two separate experiments. The volume of gas produced in experiment 1 and in experiment 2 is measured every 30 seconds. The results are shown.

Which diagram represents the data collected?

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

• Topic 6.2: Rate of reaction — Describe the effect on the rate of reaction of changing the concentration of solutions (Core); Interpret data, including graphs, from rate of reaction experiments (Core)

▶️ Answer/Explanation

Detailed Solution: Increasing the concentration of hydrochloric acid increases the number of acid particles per unit volume, leading to more frequent successful collisions with calcium carbonate and a faster initial rate of reaction (steeper slope). Both experiments use excess calcium carbonate and the same volume of acid, so the total volume of carbon dioxide gas produced will be the same at completion (equal plateau) because the amount of acid (limiting reagent) is the same. Graph B correctly shows a steeper initial gradient for experiment 2 (higher concentration) with both curves leveling off at the same final gas volume.

✅ Answer: (B)

Question 17

An experiment is set up as shown. The mass of the conical flask and its contents is measured at 30-second intervals.

Which statement about the reaction and changes to the reaction conditions is correct?
A. Adding $10 cm^3$ of water to the $50 cm^3$ of acid increases the rate of the reaction.
B. Increasing the size of the pieces of calcium carbonate increases the rate of the reaction.
C. Increasing the temperature increases the rate of the reaction.
D. The mass of the conical flask and its contents increases as carbon dioxide is formed.

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

• Topic 6.2: Rate of reaction — Describe the effect on the rate of reaction of changing the temperature; Describe practical methods for investigating the rate of a reaction including change in mass (Core).

▶️ Answer/Explanation

Increasing the temperature provides the reacting particles with more kinetic energy, leading to more frequent and more energetic collisions that overcome the activation energy, thus increasing the rate of reaction. Adding water would dilute the acid, slowing the reaction. Larger pieces of calcium carbonate have a smaller surface area, which decreases the rate. The mass of the flask decreases over time because the produced carbon dioxide gas escapes into the atmosphere.
✅ Answer: (C)

Question 18

Which reaction is reversible?

A. reaction of aqueous sodium hydroxide with dilute hydrochloric acid
B. formation of anhydrous copper(II) sulfate from hydrated copper(II) sulfate
C. oxidation of methane to form carbon dioxide and water
D. combustion of sulfur to form sulfur dioxide

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

• Topic 6.3: Reversible reactions and equilibrium — State that some chemical reactions are reversible as shown by the symbol ⇌ (Core); Describe how changing the conditions can change the direction of a reversible reaction for the effect of heat on hydrated compounds (Core).

▶️ Answer/Explanation

The reaction is reversible because heating hydrated copper(II) sulfate (CuSO₄·5H₂O) drives off water to form white anhydrous copper(II) sulfate (CuSO₄), and adding water to the anhydrous form reverses the process, reforming the blue hydrated crystals and releasing heat. Options A, C, and D go to completion (neutralization, complete combustion) and are not easily reversed under simple condition changes.
✅ Answer: (B)

Question 19

Silver oxide reacts with magnesium to make silver and magnesium oxide.

Which substance is oxidised in this reaction?
A magnesium
B magnesium oxide
C silver
D silver oxide

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

• Topic 6.4: Redox — Define oxidation in terms of loss of electrons and an increase in oxidation number; Identify oxidising agents and reducing agents in redox reactions (Supplement).

▶️ Answer/Explanation

In the reaction, magnesium (Mg) starts with an oxidation state of 0 and ends as Mg²⁺ in MgO, meaning it loses electrons. Oxidation is defined as the loss of electrons (or an increase in oxidation state). Therefore, magnesium is the substance that gets oxidised. Silver oxide is reduced as silver gains electrons, making magnesium the reducing agent.
✅ Answer: (A) magnesium

Question 20

Compound X dissolves in water to form an aqueous solution. Methyl orange is added to aqueous compound X. The methyl orange turns red. What is compound X?

A. sodium carbonate
B. copper(II) oxide
C. potassium oxide
D. sulfur dioxide

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

• Topic 7.1: The characteristic properties of acids and bases — Describe the characteristic properties of acids in terms of their effect on indicators like methyl orange (Core).

▶️ Answer/Explanation

Methyl orange turns red in acidic solutions. Among the options, only sulfur dioxide (D) dissolves in water to form sulfurous acid (H₂SO₃), which is acidic. Sodium carbonate is alkaline, copper(II) oxide is insoluble and does not form an alkaline solution, and potassium oxide forms a strongly alkaline solution.
✅ Answer: (D)

Question 21

Dilute hydrochloric acid reacts with aqueous sodium hydroxide in a neutralisation reaction. Which two ions are involved in this neutralisation reaction?

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

• Topic 7.1: The characteristic properties of acids and bases — Describe the neutralisation reaction between an acid and an alkali to produce water, H⁺(aq) + OH⁻(aq) → H₂O(l) (Core)

▶️ Answer/Explanation

In a neutralisation reaction between an acid and an alkali (base), the hydrogen ions (H⁺) from the acid combine with the hydroxide ions (OH⁻) from the alkali to form water. Hydrochloric acid (HCl) dissociates to provide H⁺, and sodium hydroxide (NaOH) dissociates to provide OH⁻. The spectator ions (Na⁺ and Cl⁻) do not participate in the essential chemical change that defines neutralisation.
✅ Answer: (B)

Question 22

The table shows some properties of some of the elements in Group I of the Periodic Table.

Rubidium is also an element in Group I of the Periodic Table. Which row describes the properties of rubidium?

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

• Topic 8.2: Group I properties — Describe the Group I alkali metals, lithium, sodium and potassium, as relatively soft metals with general trends down the group, limited to: (a) decreasing melting point (b) increasing density (c) increasing reactivity (Core); Predict the properties of other elements in Group I, given information about the elements (Core)

▶️ Answer/Explanation

Down Group I (the alkali metals), the melting point decreases, the density increases, and the reactivity increases as you move from lithium to sodium to potassium. Rubidium is below potassium in the group, so it should have a lower melting point than potassium (39°C), a higher density than potassium (0.86 g/cm³), and be more reactive than potassium (which reacts vigorously with water). Comparing the rows, Row B correctly shows a low melting point (39°C), high density for a metal (1.53 g/cm³), and ‘very fast’ reactivity.
✅ Answer: (B)

Question 23

Copper(II) sulfate crystals are blue. They are made by adding an excess of copper(II) oxide to sulfuric acid. The mixture is heated and stirred. The mixture is then filtered and the filtrate is allowed to evaporate, leaving blue crystals.

Why is filtration necessary?

A. to remove soluble impurities
B. to remove sulfuric acid
C. to remove the blue crystals
D. to remove unreacted copper(II) oxide

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

• Topic 7.3: Preparation of salts — Describe the preparation, separation and purification of soluble salts by reaction of an acid with excess metal, excess insoluble base, or excess insoluble carbonate (Core).

▶️ Answer/Explanation

In this preparation, copper(II) oxide is added in excess to ensure all the sulfuric acid reacts completely. Since copper(II) oxide is an insoluble base, any unreacted excess remains as a solid in the mixture. Filtration is the technique used to separate this insoluble solid residue (unreacted copper(II) oxide) from the desired soluble product, copper(II) sulfate solution. The filtrate contains the pure salt solution, which is then evaporated to obtain blue crystals. Therefore, filtration is necessary to remove the excess unreacted copper(II) oxide.
✅ Answer: D

Question 24

Which barium salts are soluble in water?

1 barium carbonate
2 barium chloride
3 barium nitrate
4 barium sulfate

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

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

• Topic 7.3: Preparation of salts — Describe the general solubility rules for salts: chlorides are soluble, except lead and silver; sulfates are soluble, except barium, calcium and lead; carbonates are insoluble, except sodium, potassium and ammonium (Core)

▶️ Answer/Explanation

According to general solubility rules, all chlorides and nitrates are soluble. Barium chloride and barium nitrate therefore dissolve in water. Conversely, carbonates are generally insoluble (except with Group I or ammonium), and sulfates are insoluble when combined with barium, calcium, or lead. Hence, barium carbonate and barium sulfate are insoluble.
✅ Answer: (C) 2 and 3

Question 25

Which statement about the properties of elements in Group I or in Group VII is correct?

A. Bromine displaces iodine from an aqueous solution of potassium iodide.
B. Chlorine, bromine and iodine are diatomic gases at room temperature.
C. Lithium, sodium and potassium are soft non-metals.
D. Lithium, sodium and potassium have an increasing number of electrons in their outer shells.

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

• Topic 8.2: Group I properties — Describe the Group I alkali metals, lithium, sodium and potassium, as relatively soft metals with general trends down the group, limited to: (a) decreasing melting point (b) increasing density (c) increasing reactivity.
• Topic 8.3: Group VII properties — Describe and explain the displacement reactions of halogens with other halide ions.

▶️ Answer/Explanation

Option A is correct because bromine is more reactive than iodine, so it will displace iodine from potassium iodide solution (K⁺I⁻). Option B is false as iodine is a grey-black solid at room temperature, not a gas. Option C is false because Group I metals are soft metals, not non-metals. Option D is false because all Group I elements have exactly one electron in their outer shell; the number of occupied shells increases down the group, but the outer electron count remains constant.
✅ Answer: (A)

Question 26

Rubidium and strontium are both in Period 5 of the Periodic Table. Rubidium is in Group I. Strontium is in Group II. Which statement about these elements is correct?
A. Each element has five electrons in its outer electron shell.
B. The atomic number of rubidium is greater than the atomic number of strontium.
C. Rubidium forms the $Rb^+$ ion; strontium forms the $Sr^{2+}$ ion.
D. Electrolysis of molten rubidium chloride and of molten strontium chloride produces hydrogen.

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

• Topic 2.2: Atomic structure and the Periodic Table — Determine electronic configuration; State that the number of outer shell electrons is equal to the group number in Groups I to VII (Core)
• Topic 2.4: Ions and ionic bonds — Describe the formation of positive ions, known as cations (Core)
• Topic 4.1: Electrolysis — Define electrolysis; Identify products formed at electrodes during electrolysis of molten compounds (Core)
• Topic 8.1: Arrangement of elements — Describe the relationship between group number and the charge of the ions formed from elements in that group (Core)

▶️ Answer/Explanation

Rubidium (Group I) has 1 outer electron and forms Rb⁺; strontium (Group II) has 2 outer electrons and forms Sr²⁺, making option C correct. Option A is false because Group I and II have 1 and 2 outer electrons respectively. Option B is false: atomic number increases across a period, so strontium (38) has a higher atomic number than rubidium (37). Option D is false: electrolysis of molten chlorides of reactive metals produces the metal and chlorine, not hydrogen (hydrogen comes from aqueous solutions or acids).
✅ Answer: (C)

Question 27

A company needs a metal with a low density which resists corrosion and is a good electrical conductor. Which metal should it use?

A. aluminium
B. iron
C. magnesium
D. sodium

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

• Topic 9.2: Uses of metals — Describe the uses of metals in terms of their physical properties, including: aluminium in the manufacture of aircraft because of its low density; aluminium in the manufacture of overhead electrical cables because of its low density and good electrical conductivity; aluminium in food containers because of its resistance to corrosion.

▶️ Answer/Explanation

Aluminium has a low density (about 2.7 g/cm³), it naturally forms a protective oxide layer that resists corrosion, and it is an excellent conductor of electricity. Iron has high density and rusts; magnesium corrodes more easily; sodium is too soft, reactive, and a poor choice for structural applications. Therefore, aluminium uniquely satisfies all three required properties.
✅ Answer: A

Question 28

Which statement about stainless steel is correct?
A All atoms in stainless steel are the same size.
B Stainless steel is a mixture of copper and zinc.
C Stainless steel is an iron compound.
D Stainless steel is stronger than pure iron.

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, including stainless steel as a mixture of iron and other elements such as chromium, nickel and carbon (Core); State that alloys can be harder and stronger than the pure metals and are more useful (Core)

▶️ Answer/Explanation

Stainless steel is an alloy of iron with other elements such as chromium, nickel and carbon. The different atom sizes disrupt the regular lattice, making it harder for layers to slide, so stainless steel is stronger and harder than pure iron. Option A is false because the atoms are of different sizes; B describes brass (copper+zinc); C is false because it is a mixture, not a compound.
✅ Answer: (D)

Question 29

Which statements explain why a water tap made of steel is electroplated with copper?
1 It improves the conductivity of the tap.
2 It improves the tap’s resistance to corrosion.
3 It increases the density of the tap.

A 1 and 2

B 1 and 3

C 2 only

D 3 only

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

• Topic 4.1: Electrolysis — State that metal objects are electroplated to improve their appearance and resistance to corrosion (Core)

▶️ Answer/Explanation

Electroplating steel with copper primarily provides a decorative layer and protects the steel from rusting by acting as a barrier, which improves corrosion resistance (statement 2). While copper does conduct electricity, a water tap does not require improved electrical conductivity (statement 1 is irrelevant), and electroplating does not significantly increase the tap’s density (statement 3 is false). Therefore, only statement 2 is correct.
✅ Answer: (C) 2 only

Question 30

The table shows the results of separately heating four metals with oxides of different metals. The four metals are iron, copper, magnesium and Y. The results are shown.

What is the order of reactivity of the metals, least reactive first?

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

• Topic 9.4: Reactivity series — Deduce an order of reactivity from a given set of experimental results (Core); Describe the relative reactivities of metals in terms of their tendency to form positive ions, by displacement reactions (Supplement)

▶️ Answer/Explanation

A more reactive metal can displace a less reactive metal from its oxide. Magnesium reacts with all three oxides, showing it is most reactive. Y reacts with iron(II) oxide and copper(II) oxide but not magnesium oxide, so it is less reactive than magnesium. Iron reacts with copper(II) oxide only, so it is less reactive than Y. Copper reacts with none, so it is least reactive. Thus the order least reactive first is copper → iron → Y → magnesium.
✅ Answer: (A)

Question 31

Which metal is most easily obtained from its ore?

A. aluminium
B. copper
C. calcium
D. iron

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

• Topic 9.6: Extraction of metals — Describe the ease in obtaining metals from their ores, related to the position of the metal in the reactivity series.

▶️ Answer/Explanation

Metals that are low in the reactivity series (such as copper, silver, gold) can be obtained easily, often by heating their ores alone or with carbon, because they are less stable as compounds. Aluminium and calcium are high in the reactivity series and require electrolysis for extraction, while iron requires a blast furnace with carbon monoxide. Therefore, copper is the easiest to obtain from its ore among the options.
✅ Answer: (B)

Question 32

Why is distilled water used in chemical experiments rather than tap water?
A. Distilled water contains fewer chemical impurities.
B. Distilled water has a better colour.
C. Distilled water has a higher boiling point.
D. Distilled water is a better solvent.

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

• Topic 10.1: Water — Explain that distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities (Core)

▶️ Answer/Explanation

Distillation removes most dissolved ions, organic matter, and other contaminants present in tap water. Using distilled water prevents unwanted side reactions or interferences that impurities could cause in chemical experiments. Therefore, the key reason is that distilled water contains fewer chemical impurities.
✅ Answer: A

Question 33

Which type of compound is also the name of a homologous series?

A. carbonate
B. carboxylic acid
C. halide
D. hydroxide

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

• Topic 11.1: Formulae, functional groups and terminology — Identify a functional group as an atom or group of atoms that determine the chemical properties of a homologous series; State that a homologous series is a family of similar compounds with similar chemical properties due to the presence of the same functional group.

▶️ Answer/Explanation

A homologous series is a family of organic compounds with the same functional group and similar chemical properties. Carboxylic acids (with the functional group -COOH) form a specific homologous series. In contrast, carbonates, halides, and hydroxides are classes of compounds (often ionic) but are not homologous series names.
✅ Answer: (B) carboxylic acid

Question 34

The fractional distillation of petroleum is shown.

$Fractional distillation column diagram for petroleum with fractions from refinery gas at the top to bitumen at the bottom$

Which fraction is the least volatile?
A bitumen
B diesel oil
C gasoline
D refinery gas

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

• Topic 11.3: Fuels — Describe how the properties of fractions obtained from petroleum change from the bottom to the top of the fractionating column, limited to: decreasing chain length, higher volatility, lower boiling points, lower viscosity (Core).

▶️ Answer/Explanation

Volatility decreases as chain length and boiling point increase from the top to the bottom of the fractionating column. Bitumen is collected at the very bottom, having the longest hydrocarbon chains and highest boiling point, making it the least volatile fraction.
✅ Answer: (A) bitumen

Question 35

Which formula represents an unsaturated hydrocarbon?
A $C_3H_6$

B $C_3H_8$

C $C_4H_{10}$

D $C_5H_{12}$

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

• Topic 11.1: Formulae, functional groups and terminology — State that an unsaturated compound has molecules in which one or more carbon-carbon bonds are not single bonds.
• Topic 11.5: Alkenes — State that the bonding in alkenes includes a double carbon-carbon covalent bond and that alkenes are unsaturated hydrocarbons.

▶️ Answer/Explanation

Unsaturated hydrocarbons contain at least one carbon-carbon double or triple bond and follow the general formula $C_nH_{2n}$ for alkenes (one double bond). For $C_3H_6$, n=3 gives $C_3H_{6}$, which matches the alkene general formula, indicating unsaturation. The other options ($C_3H_8$, $C_4H_{10}$, $C_5H_{12}$) fit the saturated alkane formula $C_nH_{2n+2}$ and contain only single bonds.
✅ Answer: (A) $C_3H_6$

Question 36

The structure of an organic compound is shown.

The compound is tested separately with thymolphthalein and with aqueous bromine. Which row describes the final colour observed for each test?

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

• Topic 7.1: The characteristic properties of acids and bases — Describe alkalis in terms of their effect on thymolphthalein (Core)
• Topic 11.5: Alkenes — Describe the test to distinguish between saturated and unsaturated hydrocarbons by their reaction with aqueous bromine (Core)

▶️ Answer/Explanation

The given structure shows ethanoic acid (CH₃COOH), a weak carboxylic acid. Thymolphthalein is colourless in acidic solutions, so the colour remains colourless. Aqueous bromine reacts with unsaturated C=C bonds (decolourising bromine), but ethanoic acid contains no double bonds; the orange/brown colour of bromine persists.
✅ Answer: (C)

Question 37

Which statement describes methane?
A It is an alcohol.
B It is an unsaturated molecule.
C It contains carbon, hydrogen and oxygen atoms only.
D Its molecules contain four single covalent bonds.

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

• Topic 11.1 & 11.4: Formulae, functional groups and terminology; Alkanes — State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons (Core).

▶️ Answer/Explanation

Methane (CH₄) is the simplest alkane, a saturated hydrocarbon containing only carbon and hydrogen atoms. Each carbon atom forms four single covalent bonds with four hydrogen atoms. It is not an alcohol (A) as it lacks an -OH group, not unsaturated (B) as it has no double bonds, and contains no oxygen (C).
✅ Answer: D

Question 38

Which row explains why plastics such as poly(ethene) cause pollution?

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

• Topic 11.8: Polymers — Describe the environmental challenges caused by plastics, limited to: (a) disposal in landfill sites (b) accumulation in oceans (c) formation of toxic gases from burning

▶️ Answer/Explanation

Plastics like poly(ethene) are non-biodegradable and cause pollution through multiple pathways. Burning plastics releases toxic gases (e.g., dioxins, carbon monoxide) harmful to health. Disposal in landfill sites is problematic because plastics occupy space permanently and do not decompose. Accumulation in oceans harms marine life through ingestion and entanglement. Row D correctly identifies all three major environmental challenges: toxic gases from burning, issues with landfill sites, and ocean pollution.
✅ Answer: D

Question 39

Two experiments are described.

experiment 1: A large mass of copper(II) sulfate is stirred into a beaker of water. After a few minutes, undissolved crystals are visible on the bottom of the beaker.
experiment 2: Sea water is distilled. Distilled water and solid impure salt are separated into two containers.
Which statement is correct?
A. In experiment 1, the undissolved crystals are the filtrate.
B. In experiment 1, the water is the solute.
C. In experiment 2, sea water boils at 100 °C.
D. In experiment 2, the impure salt is a residue.

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

• Topic 12.1: Experimental design — Define residue as a substance that remains after evaporation, distillation, filtration or any similar process (Core).
• Topic 12.4: Separation and purification — Describe and explain methods of separation using simple distillation (Core).

▶️ Answer/Explanation

In experiment 1, the undissolved copper(II) sulfate crystals are the solid left behind if the mixture were filtered, but the description does not mention filtration, so statement A is incorrect; water is the solvent, not the solute, so B is false. In experiment 2, sea water contains dissolved salts, so its boiling point is above 100 °C, making C incorrect. During distillation, the solid impure salt remains in the distillation flask as the residue, and the distilled water is the distillate. Therefore, statement D is correct.
✅ Answer: D

Question 40

An aqueous sample of X is heated with aqueous sodium hydroxide and small pieces of aluminium. A gas is produced which turns damp red litmus paper blue. Aqueous sodium hydroxide is added to a second aqueous sample of X. A pale green precipitate is observed. What is X?

A. ammonium nitrate
B. chromium(II) chloride
C. iron(II) nitrate
D. iron(II) sulfate

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

• Topic 12.5: Identification of ions and gases — Tests for aqueous cations (hydroxide precipitates) and tests for gases (ammonia using damp red litmus paper).
• Topic 7.1: The characteristic properties of acids and bases — Reaction of ammonium salts with alkalis to produce ammonia.

▶️ Answer/Explanation

The gas produced turns damp red litmus paper blue, identifying it as ammonia (NH₃). This confirms the presence of an ammonium salt (NH₄⁺) in the sample. The pale green precipitate formed with aqueous sodium hydroxide is characteristic of iron(II) ions (Fe²⁺). Combining both results, the compound must contain ammonium (NH₄⁺) and iron(II) (Fe²⁺). Among the options, only ammonium iron(II) sulfate would fit, but it’s not listed. However, iron(II) nitrate (C) cannot be correct as it contains no ammonium. Let’s re-evaluate: The question asks “What is X?” and the test with Al/NaOH produces gas (ammonia) from nitrate (NO₃⁻) reduction, not from ammonium. Nitrate is reduced to ammonia by Al/NaOH. A pale green precipitate with NaOH alone indicates Fe²⁺. Therefore, X contains Fe²⁺ and NO₃⁻, which is iron(II) nitrate.
✅ Answer: (C) iron(II) nitrate