AQA A-Level Chemistry Paper 3, June 2018: Question 19

📋 What This Question Tests

This question assesses your recall and understanding of Group 2 element reactions with water:

Distinguishing between the reaction of magnesium with cold liquid water versus gaseous steam.
Predicting the correct chemical products, formulas, and redox outcomes when an alkaline earth metal reacts with water vapor.

C: Magnesium oxide and hydrogen

Mark Scheme: Award 1 mark for option C.

Balanced chemical equation:
Mg(s) + H₂O(g) → MgO(s) + H₂(g)

With Steam (fast & vigorous):
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
Magnesium burns with a bright white flame to produce a white solid ( MgO ) and flammable hydrogen gas.
With Cold Water (very slow):
Mg(s) + 2H₂O(l) → Mg(OH)₂(aq/s) + H₂(g)
Forms magnesium hydroxide, which coats the surface and inhibits further reaction due to low solubility.
Why MgO with steam? At the high temperatures used with steam, any Mg(OH)₂ formed decomposes thermally into MgO and H₂O .

Eliminate the wrong gas: Magnesium is an active metal; when it reacts with water/steam, hydrogen ions are reduced ( 2H⁺ + 2e⁻ → H₂ ). Oxygen is never released! This immediately eliminates B and D.
Identify the physical state: Look closely at the word steam.
- Liquid water → Hydroxide ( Mg(OH)₂ )
- Steam ( H₂O(g) ) → Oxide ( MgO )
This eliminates A, leaving C as the only correct choice.

Confusing steam with liquid water: Many students rush and select option A ( Mg(OH)₂ ), forgetting that hydroxide is only the product when reacting with cold/liquid water.
Assuming oxygen gas is evolved: Students who don't think about redox assume that because oxygen is present in H₂O , it might be evolved as O₂ (options B and D).
Incorrect balancing: Note the 1:1 stoichiometry with steam vs 1:2 with liquid water.

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