AQA A-Level Chemistry Paper 3, June 2025: Question 30

📋 What This Question Tests

This question assesses knowledge of AQA A-Level Chemistry Section 3.2.4 (Period 3 Elements):

The redox reactions of sodium, magnesium, and chlorine with liquid water.
The relative solubilities of the resulting hydroxides and the acidity/alkalinity of the resulting solutions.
Understanding pH trends across the period (from strongly alkaline to acidic).
Careful interpretation of ordered data: specifically, decreasing order (highest pH → lowest pH).

Question 30

D | sodium → magnesium → chlorine

Mark Award: 1 mark (AO1 recall & understanding)

"Decreasing pH" means starting at the highest pH (most alkaline) and ending at the lowest pH (most acidic):

Sodium (pH 13–14) > Magnesium (pH ~9) > Chlorine (pH 1–2)

Element	Equation with H₂O(l)	Approx. pH
Sodium	2Na + 2H₂O → 2NaOH + H₂	13 – 14 (Strong alkali, fully soluble)
Magnesium	Mg + 2H₂O → Mg(OH)₂ + H₂	~9 (Weak alkali, sparingly soluble)
Chlorine	Cl₂ + H₂O ⇌ HCl + HClO	1 – 2 (Acidic, forms HCl and HClO)

Define the direction first: "Decreasing pH" = High pH → Low pH. Annotate the question with an arrow pointing from 14 down to 1.
Find the extremes:
- Sodium forms NaOH, which is a strong, completely soluble base (pH ~13–14). The list must start with sodium. This immediately eliminates A, B, and C!
- Chlorine forms hydrochloric acid (strong) and chloric(I) acid (weak), giving an acidic solution (pH < 7). It must be at the end of the decreasing list.
Verify the middle element: Mg(OH)₂ is only sparingly soluble, releasing fewer OH⁻ ions into solution than NaOH, meaning pH ~9. This sits correctly between sodium and chlorine.

Selecting Option A (Inverted Order): The most common error in this question is reading "decreasing pH" as "increasing acidity" or simply confusing the direction. Option A arranges the elements in order of increasing pH (lowest to highest).
Confusing Mg with water vs Mg with steam:
- With cold water (very slow): Mg + 2H₂O(l) → Mg(OH)₂ + H₂
- With steam (vigorous): Mg + H₂O(g) → MgO + H₂
Remember that in cold liquid water, magnesium hydroxide is formed, which has low solubility (K_sp is small), producing only a weakly alkaline solution.
Forgetting Chlorine's Disproportionation: Cl₂ reacts reversibly with water to form two acids: HCl (hydrochloric acid) and HClO (chloric(I) acid). Students sometimes mistakenly think chlorine forms a neutral dissolved molecular solution like O₂ or N₂.

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