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CIE iGCSE Co-Ordinated Science C8.4 Transition elements Exam Style Questions Paper 4

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Question

This question is about metals.
(a) Potassium is a metal in Group I of the Periodic Table.
Fig. 5.1 shows the electronic structure of three elements.
(i) State which diagram A, B or C, shows the electronic structure of a Group I metal.
(ii) A student wants to confirm that a compound contains potassium.
The student uses a flame test.
State what the student observes if the compound contains potassium.
(b) Iron is a transition element.
Iron(II) sulfate contains iron(II) ions, \(Fe^{2+}\).
Sodium hydroxide solution is used to test for iron(II) ions.
The iron(II) ions react with \(OH^-\) ions from the sodium hydroxide solution. A precipitate of iron(II) hydroxide, \(Fe(OH)_2\), is made.
(i) State the colour of the precipitate of iron(II) hydroxide.
(ii) Construct the balanced ionic equation for the formation of \(Fe(OH)_2\). Include state symbols.
(c) Magnesium reacts with oxygen to make magnesium oxide.
(i) Fig. 5.2 shows the electronic structure of a magnesium atom.
The proton number (atomic number) of magnesium is 12.
Draw a diagram to show the electronic structure of an oxygen atom.
The proton number (atomic number) of oxygen is 8.
(ii) When magnesium reacts with oxygen, magnesium ions and oxide ions are made.
Fig. 5.3 shows the electronic structure of an oxide ion.
Draw a diagram to show the electronic structure of a magnesium ion.
(iii) Explain why magnesium oxide has a high melting point.
(d) Potassium oxide is also an ionic compound.
Potassium ions, \(K^+\), combine with oxide ions, \(O^{2-}\), to form potassium oxide.
Determine the formula of potassium oxide.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C8.2 — Group I properties (Part a)
• Topic C8.4 — Transition elements (Part b)
• Topic C2.4 — Ionic bonding (Parts c, d)

▶️ Answer/Explanation

(a)(i) B

Group I metals have exactly one electron in their outermost shell.
Diagram B shows this arrangement (2, 1), matching a Group I element such as lithium.

(a)(ii) Lilac (purple) flame

Potassium compounds give a characteristic lilac flame when heated in a flame test.
This is used to confirm the presence of potassium ions in a compound.

(b)(i) Green

Iron(II) hydroxide, \(Fe(OH)_2\), forms as a green precipitate.
This distinguishes it from iron(III) hydroxide, which is reddish-brown.

(b)(ii) \(Fe^{2+}(aq) + 2OH^{-}(aq) \rightarrow Fe(OH)_2(s)\)

One iron(II) ion combines with two hydroxide ions.
Charges balance: \(2+\) and \(2 \times (1-) = 2-\), giving a neutral solid product.

(c)(i) 

Oxygen has atomic number 8, so it has 8 electrons.
These are arranged as 2 electrons in the first shell and 6 electrons in the second (outer) shell.

(c)(ii) 

Magnesium atom (2,8,2) loses its 2 outer electrons to form \(Mg^{2+}\).
The resulting ion has the electronic structure 2,8, shown with a 2+ charge on the bracket.

(c)(iii) Strong ionic bonds require a lot of energy to break

There is a strong electrostatic attraction between the oppositely charged \(Mg^{2+}\) and \(O^{2-}\) ions.
A large amount of energy is needed to overcome this strong ionic bonding, giving magnesium oxide a high melting point.

(d) \(K_2O\)

Charges must balance: \(K^+\) and \(O^{2-}\).
Two \(K^+\) ions (total charge 2+) balance one \(O^{2-}\) ion, giving the formula \(K_2O\).

Question

Copper is a transition metal. Transition metals form coloured compounds.

(a) Write down two other properties of transition metals that are not properties of all metals.

▶️Answer/Explanation

1. High density
2. Act as catalysts
Additional properties:
– Variable oxidation states
– Form complex ions
– High melting points (except mercury)

Example: Copper has density 8.96 g/cm³ (vs 2.7 for aluminum) and copper(II) sulfate is blue

(b) Copper carbonate, CuCO3, reacts with dilute hydrochloric acid, HCl.
Copper chloride, CuCl2, is made.

CuCO3 + 2HCl → CuCl2 + H2O + CO2

(i) Copper chloride contains copper ions, Cu2+, and chloride ions, Cl–. Describe the test and its positive result for chloride ions.

▶️Answer/Explanation

Test: Add nitric acid followed by silver nitrate solution
Result: White precipitate forms

Chemical equation:
Ag+(aq) + Cl–(aq) → AgCl(s)↓
Note: Nitric acid is added first to remove carbonate impurities that would also give precipitate

(ii) In an experiment, 4.0 g of copper carbonate reacts with excess dilute hydrochloric acid.
Calculate the maximum mass of copper chloride that can be made. [Ar; C, 12; Cl, 35.5; Cu, 64; O, 16]

▶️Answer/Explanation

4.4 g

Calculation:
1. Calculate molar masses:
– CuCO3 = 64 + 12 + (3×16) = 124 g/mol
– CuCl2 = 64 + (2×35.5) = 135 g/mol
2. Moles of CuCO3 = 4.0 g ÷ 124 g/mol = 0.03226 mol
3. 1:1 mole ratio → 0.03226 mol CuCl2 produced
4. Mass = 0.03226 mol × 135 g/mol = 4.354 g → 4.4 g (2 sig figs)

(iii) In another experiment, 8.8 g of carbon dioxide gas is made. Calculate the volume of carbon dioxide gas in cm3 at 25°C.
The molar gas volume at 25°C is 24 dm3.
[Ar; C, 12; O, 16]

▶️Answer/Explanation

4800 cm3

Calculation steps:
1. Molar mass CO2 = 12 + (2×16) = 44 g/mol
2. Moles of CO2 = 8.8 g ÷ 44 g/mol = 0.2 mol
3. Volume at 25°C = 0.2 mol × 24 dm3/mol = 4.8 dm3
4. Convert to cm3: 4.8 × 1000 = 4800 cm3

Key points:
– 1 dm3 = 1000 cm3
– Temperature must be in Kelvin for exact calculations (25°C = 298K)

(c) Explain why copper is a conductor of electricity. Use ideas about metallic bonding.

▶️Answer/Explanation

1. Copper has delocalized electrons that can move freely through the metal lattice
2. These mobile electrons carry charge when potential difference is applied

Metallic bonding details:
– Cu atoms form a “sea of electrons”
– Outer shell electrons (4s1 in copper) become delocalized
– Applied voltage causes net electron flow

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