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CIE iGCSE Co-Ordinated Science C8.2 Group I properties Exam Style Questions Paper 4

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Question

(a) Different substances can be elements, compounds, or mixtures.
Draw a line from each substance to the definition.
(b) The properties of the Group I alkali metals show trends down the group.
Tick \((\checkmark)\) the correct box in each row of Table 5.1 to show the trends down the group.
(c) The Group I element sodium reacts with oxygen to make sodium oxide, Na₂O.
Fig. 5.1 shows the electronic structures of a sodium atom and an oxygen atom.
Draw the electronic structures of the sodium ion and oxide ion in sodium oxide. Include the charges on the ions.
(d) Explain why molten sodium oxide conducts electricity, but solid sodium oxide does not conduct electricity.

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

• Topic C2.1 — Elements, compounds and mixtures
• Topic C8.2 — Group I properties
• Topic C2.4 — Ions and ionic bonds

▶️ Answer/Explanation

(a)

Elements consist of one type of atom only. Compounds are formed by chemical bonds between atoms of different elements. Mixtures are physical combinations of substances that can be separated by physical methods.

(b)

As you go down Group I, the atoms become larger with more electron shells. The outer electron is further from the nucleus and more easily lost, increasing reactivity. Melting point decreases because weaker metallic bonds form between larger ions.

(c)

Sodium loses its outer electron to achieve a stable electronic configuration of (2,8), forming a positive ion. Oxygen gains two electrons to achieve (2,8), forming a negative ion.

(d) idea that conduction depends on movement of ions ; ions cannot move in the solid state but can move in the molten state

In solid ionic compounds, ions are held in fixed positions within the giant lattice structure. When molten, the lattice breaks down and ions become free to move, allowing electrical conduction.

Question

(a) A teacher investigates the reactions of the Group I metals, lithium, potassium and sodium, with water.
Table 2.1 shows their results.
(i) Deduce the order of reactivity of the metals.
___________ most reactive
___________ least reactive
(ii) When potassium reacts with water a flame is seen. State the colour of the flame.
(iii) Complete and balance the symbol equation for the reaction between sodium and water.
\( \ldots\ldots \) Na \( + \) \( \ldots\ldots \) H2O \( \rightarrow \) \( \ldots\ldots\ldots\ldots \) \( + \) \( \ldots\ldots\ldots\ldots \)
(b) Sodium reacts with chlorine to make the ionic compound sodium chloride.
Fig. 2.1 shows the electronic structures of a sodium atom and a chlorine atom.
(i) Draw the electronic structures of the sodium ion and chloride ion in the ionic compound sodium chloride. Show the charges on the ions.
(ii) Describe the structure of sodium chloride.
(iii) Explain why molten sodium chloride conducts electricity, but solid sodium chloride does not conduct electricity.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic C8.2 — Group I properties (Part (a))
• Topic C2.4 — Ions and ionic bonds (Part (b))

▶️ Answer/Explanation

(a)(i)

Most reactive: potassium
Middle: sodium
Least reactive: lithium
Potassium is most reactive as it fizzes violently and produces a flame; lithium only fizzes gently, indicating the least vigorous reaction.

(a)(ii)

The colour of the flame produced when potassium reacts with water is lilac / purple / pink.
This characteristic flame colour is used in flame tests to identify the presence of potassium ions.
It is caused by electrons in potassium atoms being excited and emitting light of a specific wavelength as they return to the ground state.

(a)(iii)

The balanced equation is: \( 2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 \)
Sodium reacts with water to produce sodium hydroxide (an alkali) and hydrogen gas.
The equation must be balanced: 2 sodium atoms on each side, 4 hydrogen atoms and 2 oxygen atoms on each side.

(b)(i)

The sodium ion \(\text{Na}^+\) has electronic structure 2,8 (loses its outer electron), shown inside square brackets with a \(+\) charge.
The chloride ion \(\text{Cl}^-\) has electronic structure 2,8,8 (gains one electron), shown inside square brackets with a \(-\) charge.
Both ions achieve a full outer shell (stable noble gas configuration).

(b)(ii)

Sodium chloride has a lattice structure with a regular arrangement of alternating positive (Na⁺) and negative (Cl⁻) ions.
The ions are held together by strong electrostatic forces of attraction (ionic bonds) in all directions.
This giant ionic lattice structure gives sodium chloride its high melting and boiling points.

(b)(iii)

In molten sodium chloride, the ions are free to move and carry charge, so it conducts electricity.
In solid sodium chloride, the ions are fixed in the lattice and cannot move, so it does not conduct electricity.
Electrical conductivity requires mobile charge carriers (ions or electrons).

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