Strontium and magnesium are both Group 2 elements. When each metal is heated in oxygen, it forms a metal oxide. Explain why strontium oxide dissolves in water to form an alkaline solution, and why strontium reacts more vigorously with oxygen than magnesium does.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Strontium oxide is a basic oxide.
When heated in oxygen, Sr reacts to give SrO, which is a basic oxide.
SrO dissolves in water to give Sr(OH)₂.
Sr(OH)₂ dissociates in water, releasing OH⁻ ions, which makes the solution alkaline.
Strontium has more electron shells than magnesium; its outer electrons are further from the nucleus.
Because the outer electrons are further from the nucleus, they are lost more easily, so strontium reacts more vigorously with oxygen than magnesium.
When heated in oxygen, Sr reacts to give SrO, which is a basic oxide.
SrO dissolves in water to give Sr(OH)₂.
Sr(OH)₂ dissociates in water, releasing OH⁻ ions, which makes the solution alkaline.
Strontium has more electron shells than magnesium; its outer electrons are further from the nucleus.
Because the outer electrons are further from the nucleus, they are lost more easily, so strontium reacts more vigorously with oxygen than magnesium.
Examiner tips
- Use the term "basic oxide" to gain the first point. Show the dissolution reaction SrO + H₂O → Sr(OH)₂ for the second. Explain dissociation to give OH⁻ for the third. Mention electron shells for the fourth and the ease of electron loss for the fifth.
Common mistakes
- Writing SrO + H₂O → SrO·H₂O instead of Sr(OH)₂. Forgetting to state that the hydroxide dissociates to give OH⁻. Confusing the reason for reactivity with electronegativity rather than electron shell distance.
Mark scheme (5 marks)
- Strontium oxide is a basic oxide (accept: metal oxide is basic/alkaline)
- Strontium oxide reacts with water to form strontium hydroxide
- Strontium hydroxide dissociates / ionises in water, releasing OH⁻ ions (which makes the solution alkaline)
- Strontium has more electron shells / its outer electrons are further from the nucleus than magnesium
- So the outer electrons in strontium are lost more easily / strontium loses electrons more readily, making it more reactive
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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