Properties of Period 3 elements and their oxides (A-level only) — AQA A-Level Chemistry
Test yourself on Properties of Period 3 elements and their oxides (A-level only) with AQA A-Level practice questions.
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Properties of Period 3 elements and their oxides (A-level only) explained
Sodium reacts vigorously with cold water: 2Na + 2H₂O → 2NaOH + H₂.
Read the full explanation
The metal melts into a ball, skates over the surface and fizzes, and because sodium hydroxide is very soluble the solution ends up strongly alkaline, around pH 13-14. Magnesium reacts only very slowly with cold water, Mg + 2H₂O → Mg(OH)₂ + H₂, and the pH reaches only about 9-10 because magnesium hydroxide is sparingly soluble, so few OH⁻ ions get into solution. With steam magnesium behaves quite differently: heated magnesium burns in steam with a bright white flame to give the oxide, Mg + H₂O → MgO + H₂, leaving a white solid. Reactivity with water falls from Na to Mg as the metal becomes harder to oxidise.
Your focus
- Write balanced equations for sodium with cold water and for magnesium with both cold water and steam, naming the different products.
- Predict the approximate pH of each resulting solution and justify the difference using the solubilities of NaOH and Mg(OH)₂.
- Describe what is seen in each reaction, distinguishing a white solid formed in steam from a precipitate formed in solution.
Properties of Period 3 elements and their oxides (A-level only) exam tips
Marking Points
- one mark for 2Na + 2H₂O → 2NaOH + H₂ correctly balanced
- one mark for Mg + H₂O → MgO + H₂ as the reaction of magnesium with steam
- one mark for the observation with magnesium and steam, a white solid or a bright white flame
- one mark for linking the higher pH of the sodium product to sodium hydroxide being far more soluble than magnesium hydroxide
Examiner Tips
- 💡State the temperature or phase of the water; magnesium with cold water and magnesium with steam give different products.
- 💡Quote an approximate pH when comparing the solutions: about 13-14 for sodium, about 9-10 for magnesium.
- 💡Balance the equation before describing observations, because the equation mark is usually independent of the observation mark.
Common Mistakes
- writing white precipitate for the magnesium oxide formed in steam, which examiners reject because no solution is present
- offering effervescence as the observation for magnesium in steam instead of the white solid or white flame
- giving Mg + 2H₂O → Mg(OH)₂ + H₂ for the steam reaction, which is the cold water product, not the steam product
- claiming magnesium does not react with cold water at all rather than saying it reacts very slowly
- predicting pH 14 for the magnesium hydroxide solution by forgetting it is only sparingly soluble