AQA A-Level Chemistry Paper 3, 2019: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify the correct observation when barium metal is added to an excess of water.

Practise this question

Question

Question 15 asks: 'What is the correct observation when barium metal is added to an excess of water?' with four multiple choice options: A 'Forms a colourless solution only', B 'Forms a colourless solution and effervesces', C 'Forms a white precipitate only', and D 'Forms a white precipitate and effervesces'.
Question text

15 What is the correct observation when barium metal is added to an excess of water?

[1 mark]

A Forms a colourless solution only

B Forms a colourless solution and effervesces

C Forms a white precipitate only

D Forms a white precipitate and effervesces

Mark scheme

Show the mark scheme Mark scheme for question 15 showing the correct answer is B, awarded 1 mark.

15 B 1

How to answer it

Reaction of Barium with Water & Group 2 Solubility Trends

📌 What this question tests

This question assesses knowledge of Group 2 (alkaline earth metals) chemistry, specifically:

  • The redox reaction between Group 2 metals and water to produce metal hydroxides and hydrogen gas.
  • The trend in the solubility of Group 2 hydroxides down the group (increasing solubility from Mg to Ba).
  • Linking chemical products ( H₂(g) and aqueous ions) to macroscopic observations (effervescence and clear colourless solution).

Question 15 Analysis

Multiple Choice: Reaction of Barium Metal with Excess Water [1 Mark]

✅ Correct Answer: B

Forms a colourless solution and effervesces

Mark Breakdown:
• 1 mark for selecting B.

The reaction equation is:
Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)

  • Effervescence: Caused by the rapid production of hydrogen gas ( H₂ ).
  • Colourless solution: Barium hydroxide, Ba(OH)₂ , is highly soluble in water and dissociates completely into colourless Ba²⁺(aq) and OH⁻(aq) ions.

💡 Key Knowledge: Group 2 Trends

Keep these two opposite solubility trends crystal clear:

  • Hydroxides, M(OH)₂: Solubility increases down the group:
    • Mg(OH)₂ : Sparingly soluble (white precipitate)
    • Ca(OH)₂ : Slightly soluble (limewater)
    • Sr(OH)₂ : Moderately soluble
    • Ba(OH)₂ : Soluble (forms a colourless aqueous solution)
  • Sulfates, MSO₄: Solubility decreases down the group:
    • MgSO₄ : Soluble
    • BaSO₄ : Insoluble (dense white precipitate used in sulfate testing)

❌ Common Errors & Confusions

  • Confusing Hydroxide vs Sulfate Solubility (choosing C or D): Many students remember that barium forms a distinctive white precipitate in qualitative analysis ( BaSO₄ ) and mistakenly assume all barium compounds form white precipitates in water.
  • Forgetting the Gas Produced (choosing A): Overlooking the reduction of hydrogen in water to H₂(g) leads students to omit "effervesces". Any Group 2 metal reacting with water releases hydrogen gas.
  • Confusing Barium with Magnesium: Magnesium reacts very slowly with cold water to form a sparingly soluble white suspension of Mg(OH)₂ , whereas barium reacts vigorously to form a clear solution.

🧠 Exam Technique: Two-Step Elimination

Break observational multiple-choice questions into independent visual features:

  1. Step 1: Is a gas produced?
    Metal + water → Hydroxide + Hydrogen gas.
    Because H₂(g) forms, there must be fizzing/effervescence.
    Eliminate options A and C immediately.
  2. Step 2: Is the hydroxide soluble or insoluble?
    Hydroxide solubility increases down Group 2. Barium is near the bottom of Group 2, so Ba(OH)₂ is soluble and forms a colourless solution, not a white precipitate.
    Eliminate option D.
  3. Result: Only B satisfies both observations.

Topics

Inorganic Chemistry · 3.2.2 Group 2, The Alkaline Earth Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.