Compare sodium and magnesium reactions with water
Describe and balance the specified reactions of sodium and magnesium with cold water and steam.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Period 3 trends are qualitative unless supplied data are given. Equations show the stated products and conditions; observations and pH depend on concentration and experimental conditions. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, balanced equations and ionic charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does sodium form with cold water?
What you will learn
- Describe sodium with cold water
- Compare magnesium with cold water and steam
- Write balanced equations with the conditions and products stated
Sodium reacts vigorously with cold water
Sodium reacts vigorously with cold water to form aqueous sodium hydroxide and hydrogen: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). The solution is alkaline because NaOH dissolves and forms OH⁻ ions.
The reaction releases heat and the hydrogen may ignite. This is a hazardous reaction; the lesson describes the equation and observations, not a procedure for carrying it out.
Magnesium reacts much more slowly with cold water
Magnesium reacts very slowly with cold water: Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g). The sparingly soluble hydroxide may form a surface layer that slows further reaction.
With steam, magnesium reacts more readily to form magnesium oxide and hydrogen: Mg(s) + H₂O(g) → MgO(s) + H₂(g). State whether the question gives liquid water or steam; the products differ.
Pause and explain
What solid forms when magnesium reacts with steam?
Worked example
Compare the product formed when magnesium reacts with cold water and with steam.
Show the worked solution
- Cold liquid water forms Mg(OH)₂ and H₂.
- Steam forms MgO and H₂.
- The physical state of water changes the oxide/hydroxide product.
Answer Cold water: Mg + 2H₂O(l) → Mg(OH)₂ + H₂. Steam: Mg + H₂O(g) → MgO + H₂.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Balance sodium reacting with cold water and name the products.
Give me a hint
Form hydroxide and hydrogen, then balance Na and H atoms.
Compare my reasoning
- The unbalanced products are NaOH and H₂.
- Use two Na and two H₂O to make two NaOH.
- Check atoms: 2 Na, 4 H and 2 O on each side.
2Na + 2H₂O → 2NaOH + H₂; products are sodium hydroxide and hydrogen.
Look for these in your work
- Products correct
- Coefficients balanced
- Atoms checked
Write the equation for magnesium reacting with steam and identify the gas.
Give me a hint
Steam reacts to form the oxide rather than the hydroxide.
Compare my reasoning
- Write Mg + H₂O(g) → MgO + H₂.
- Count one Mg, two H and one O on each side.
- Hydrogen is the gaseous product.
Mg(s) + H₂O(g) → MgO(s) + H₂(g); hydrogen gas is formed.
Look for these in your work
- Steam state shown
- Products correct
- Equation balanced
A student gives Mg + 2H₂O → MgO + 2H₂ for reaction with cold water. Diagnose both the condition and balance errors.
Give me a hint
Cold water gives hydroxide; check hydrogen atoms after fixing the product.
Compare my reasoning
- The stated cold-water product is Mg(OH)₂, not MgO.
- Balanced equation: Mg + 2H₂O(l) → Mg(OH)₂ + H₂.
- This equation balances Mg, O and H; one H₂ is formed.
Use Mg + 2H₂O(l) → Mg(OH)₂ + H₂ for cold water. The proposed equation has the wrong product and excess H₂.
Look for these in your work
- Cold-water product corrected
- Balanced equation supplied
- Hydrogen coefficient diagnosed
Common mistakes
- Giving MgO as the cold-water product
- Giving Na₂O as the sodium-and-water product
- Omitting state or condition context
- Presenting a hazardous reaction as a learner experiment
What can you explain now?
What does sodium form with cold water?
Compare with the explanation
Sodium hydroxide and hydrogen
The balanced reaction forms aqueous NaOH and H₂.
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Make it stick
How do sodium, magnesium plus cold water, and magnesium plus steam differ in product and rate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.