AQA A-Level Chemistry Paper 3, June 2025: Question 14

1 mark · Easy difficulty · Multiple Choice

Identify the correct and complete observation when a piece of strontium is added to an excess of water.

Practise this question

Question

Question 14 asks: 'A piece of strontium is added to an excess of water. Which observation is correct and complete?' Four multiple choice options are given: A: Colourless solution; B: Colourless solution and effervescence; C: Dense white precipitate; D: Dense white precipitate and effervescence.
Question text

14 A piece of strontium is added to an excess of water.

Which observation is correct and complete?

[1 mark]

A Colourless solution

B Colourless solution and effervescence

C Dense white precipitate

D Dense white precipitate and effervescence

Mark scheme

Show the mark scheme Mark scheme entry for question 14 shows the correct answer as option B ('Colourless solution and effervescence') worth 1 mark with assessment objective AO1.

14 B 1 (AO1) Colourless solution and effervescence

How to answer it

AQA A-Level Chemistry • Inorganic Chemistry • Group 2

Reactions of Group 2 Elements with Water

What this question tests

This multiple-choice question assesses your fundamental knowledge of the Group 2 Alkaline Earth Metals, specifically:

  • The chemical reaction between Group 2 metals and water: products formed and visible signs of reaction.
  • The trend in solubility of Group 2 hydroxides down the group (Mg to Ba).
  • Distinguishing between complete observations (gas evolution AND state/appearance of the solution formed).

Question 14 Breakdown

A piece of strontium is added to an excess of water. Which observation is correct and complete?

✅ Correct Answer: B

Colourless solution and effervescence

Mark Scheme: 1 mark (AO1) — B

When strontium reacts with cold water, it produces strontium hydroxide and hydrogen gas:

Sr(s) + 2H₂O(l) → Sr(OH)₂(aq) + H₂(g)

  • Effervescence (fizzing/bubbles): caused by the vigorous production of hydrogen gas, H₂(g).
  • Colourless solution: Sr(OH)₂ is soluble in excess water, yielding a completely clear, colourless alkaline solution.

💡 Key Knowledge: Group 2 Hydroxides

The solubility of Group 2 hydroxides, M(OH)₂, increases down the group:

  • Mg(OH)₂: Sparingly soluble / practically insoluble (forms a white precipitate or suspension, e.g. milk of magnesia).
  • Ca(OH)₂: Slightly soluble (forms a faintly cloudy solution; saturated solution is clear limewater).
  • Sr(OH)₂: Soluble (dissolves to form a colourless solution in excess water).
  • Ba(OH)₂: Very soluble (dissolves readily to give a strongly alkaline colourless solution).

🧠 Exam Technique & Strategy

Break every observation question into two independent criteria:

  1. Is there a gas evolved?
    Any reactive metal reacting with water to produce H₂ gas must show effervescence (fizzing). This immediately eliminates option A and option C.
  2. What is the physical state of the metal hydroxide?
    Look at the position of strontium in Group 2 (below Mg and Ca). Hydroxides become more soluble downwards. Since Ca(OH)₂ is already partially soluble, Sr(OH)₂ is sufficiently soluble in excess water that no precipitate forms. This eliminates option D.

❌ Common Misconceptions

  • Confusing hydroxide and sulfate solubility trends: Students frequently mix up the two trends:
    • Hydroxides: solubility increases down the group (Mg insoluble → Ba soluble).
    • Sulfates: solubility decreases down the group (Mg soluble → Ba insoluble dense white ppt).
    Choosing D happens when a student mistakenly applies the sulfate trend to the hydroxide.
  • Incomplete observations: Choosing A misses the formation of hydrogen gas, despite recognizing that Sr(OH)₂ dissolves. The question explicitly demands a complete observation.

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, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.