Strontium (Sr) is a Group 2 metal. When strontium burns in oxygen it forms strontium oxide (SrO). When strontium oxide is added to water, strontium hydroxide solution is formed. Explain what happens during these two reactions, using your knowledge of oxidation, reduction and the reactivity series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Strontium (Sr) is a Group 2 metal found below calcium in the periodic table. It reacts vigorously with oxygen when burned, forming strontium oxide (SrO). When strontium oxide is added to water, the solution formed turns universal indicator dark blue.
Model answer (5 marks)
When strontium burns in oxygen it is oxidised:
1. Sr(s) → Sr²⁺ + 2e⁻ (loss of two electrons)
2. O₂(g) + 4e⁻ → 2O²⁻ (gain of electrons)
3. 2Sr²⁺ + 2O²⁻ → SrO(s) (formation of strontium oxide).
Strontium oxide is a basic oxide. When it dissolves in water it reacts with water:
4. SrO(s) + H₂O(l) → Sr(OH)₂(aq).
The hydroxide ions give the solution a pH > 7, so the universal indicator turns dark blue.
Strontium is lower in Group 2 than calcium, so it is more reactive. Reactivity increases down the group because the outermost electrons are in a shell further from the nucleus, are shielded by more inner shells and are therefore lost more easily. Consequently strontium reacts more vigorously with oxygen, forming SrO more readily than calcium or magnesium would.
1. Sr(s) → Sr²⁺ + 2e⁻ (loss of two electrons)
2. O₂(g) + 4e⁻ → 2O²⁻ (gain of electrons)
3. 2Sr²⁺ + 2O²⁻ → SrO(s) (formation of strontium oxide).
Strontium oxide is a basic oxide. When it dissolves in water it reacts with water:
4. SrO(s) + H₂O(l) → Sr(OH)₂(aq).
The hydroxide ions give the solution a pH > 7, so the universal indicator turns dark blue.
Strontium is lower in Group 2 than calcium, so it is more reactive. Reactivity increases down the group because the outermost electrons are in a shell further from the nucleus, are shielded by more inner shells and are therefore lost more easily. Consequently strontium reacts more vigorously with oxygen, forming SrO more readily than calcium or magnesium would.
Examiner tips
- Use the word ‘oxidised’ and show electron transfer in the first reaction.
- Mention that SrO is a basic oxide and give the equation for its hydrolysis.
- State that the solution is alkaline (pH > 7) and the indicator colour change.
- Explain why strontium is more reactive – lower in the group, more shells, outer electron further from nucleus.
Common mistakes
- Saying strontium is ‘reduced’ instead of oxidised.
- Forgetting to show the hydrolysis of SrO to Sr(OH)₂.
- Claiming the solution is acidic instead of alkaline.
Mark scheme (5 marks)
- When strontium burns in oxygen, strontium is oxidised (gains oxygen / loses electrons)
- Strontium oxide is a basic/alkaline oxide, so when it dissolves in water it forms a solution with a pH greater than 7 / an alkaline solution
- Strontium is more reactive than calcium (because it is lower in the reactivity series / lower in Group 2)
- Reactivity increases down Group 2 because atoms have more electron shells / the outer electron is further from the nucleus / lost more easily
- Because strontium is more reactive, it burns more vigorously / reacts more readily with oxygen / forms strontium oxide more easily than calcium or magnesium would
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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