WJEC A-Level Chemistry AS Unit 1, June 2025: Question 6
2 marks · Easy difficulty · Short Answer
Identify the s-block element that forms the least soluble sulfate and the element whose compounds produce an apple-green flame test.
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How to answer it
s-Block Elements: Solubility Trends & Flame Tests
This question assesses your recall of core Group 2 trends and diagnostic tests from the s-block specification:
- Solubility trends of Group 2 sulfates compared to Group 1 sulfates.
- Characteristic flame test colours for s-block metal cations.
- Careful extraction of elements from a provided reference table.
Part (a)
Solubility of s-Block Sulfates
✅ Correct Answer
Barium (or Ba )
💡 Key Knowledge
Solubility trends in Group 2 (alkaline earth metals):
- Hydroxides: Solubility increases down the group (Mg(OH)₂ is sparingly soluble; Ba(OH)₂ is soluble).
- Sulfates: Solubility decreases down the group (MgSO₄ is soluble; BaSO₄ is completely insoluble).
- All Group 1 (alkali metal) sulfates (Na₂SO₄, K₂SO₄, Rb₂SO₄, Cs₂SO₄) are readily water-soluble.
🧠 Exam Technique
Notice the question says "s-block elements", presenting both Group 1 and Group 2. Always remember that all Group 1 salts are soluble. Therefore, the least soluble sulfate must belong to Group 2, right at the bottom: Barium (Ba).
❌ Common Errors
- Mixing up trends: Confusing sulfate solubility (decreases down Group 2) with hydroxide solubility (increases down Group 2), incorrectly choosing Magnesium (Mg).
- Naming the compound: Writing BaSO₄ or "barium sulfate" instead of the requested element name/symbol.
Part (b)
Flame Test Identification
✅ Correct Answer
Barium (or Ba )
💡 Key Knowledge
Must-know AS Level flame colours for s-block ions:
| Element / Ion | Flame Test Colour |
|---|---|
| Lithium (Li⁺) | Crimson / Red |
| Sodium (Na⁺) | Persistent yellow-orange |
| Potassium (K⁺) | Lilac |
| Rubidium (Rb⁺) | Red / Purple-red |
| Caesium (Cs⁺) | Blue / Violet |
| Calcium (Ca²⁺) | Brick-red / Orange-red |
| Strontium (Sr²⁺) | Scarlet / Crimson-red |
| Barium (Ba²⁺) | Apple-green |
| Magnesium (Mg²⁺) | No colour (energy outside visible spectrum) |
🧠 Exam Technique
Students are often hesitant when the exact same answer applies to two consecutive sub-parts. Do not second-guess yourself: Ba is both the least soluble sulfate and the producer of an apple-green flame!
❌ Common Errors
- Confusing shades of green/red: Misattributing apple-green to copper (Cu²⁺ gives a blue-green flame, but Cu is d-block, not in the provided grid).
- Giving ions: Writing Ba²⁺ rather than stating the element itself. While examiners often accept symbols, giving the plain element name or standard symbol avoids losing marks on strict mark schemes.
Quick Revision Summary: Barium at a Glance
- Position: Group 2, Period 6 (s-block metal).
- Barium Sulfate (BaSO₄): Dense, bright white precipitate; insoluble in water and dilute acids. Used as a medical "barium meal" for X-ray imaging because it is opaque to X-rays and non-toxic due to near-zero solubility.
- Flame Test: Characteristic pale apple-green flame emission.
Topics
Inorganic Chemistry · 1.6 The Periodic Table
Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.