Write Group 2 reactions with oxygen and water
Describe and balance the specified reactions of Group 2 metals with oxygen and water.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Group trends are qualitative unless data are supplied. Show physical states and balance equations; distinguish reaction from dissolution and precipitation. This sequence does not replace supervised practical experience. Written practice is self-checked
Before you begin
- Use Group 2 formulas, ionic charges, state symbols, balanced equations and acid reaction patterns.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What oxide formula does a Group 2 metal form with oxygen in the standard syllabus model?
What you will learn
- Write oxide-forming equations
- Compare the metals' reactions with cold water
- Keep steam and liquid-water products distinct
Oxygen forms the Group 2 oxide
For the syllabus oxide product, a Group 2 metal forms MO: 2M(s) + O₂(g) → 2MO(s). For example, 2Mg + O₂ → 2MgO. State the specified product and conditions; do not replace coefficients with subscripts.
The elements react more readily with oxygen down the group. Fresh surfaces tarnish as oxide forms. Reaction vigor depends on particle size, oxygen supply and heating, so compare observations made under comparable conditions.
Water observations and products depend on the metal and water state
Magnesium reacts only very slowly with cold water, producing magnesium hydroxide and hydrogen: Mg + 2H₂O(l) → Mg(OH)₂ + H₂. With steam, it reacts more readily to form MgO and H₂: Mg + H₂O(g) → MgO + H₂.
Calcium, strontium and barium react with cold water to form the hydroxide and hydrogen: M + 2H₂O(l) → M(OH)₂ + H₂. The reactions become more vigorous down the group. These equations describe syllabus products; they are not instructions for a practical demonstration.
Pause and explain
What solid forms when magnesium reacts with steam?
Worked example
Write the balanced equation for calcium reacting with cold water and compare it with magnesium reacting with steam.
Show the worked solution
- Calcium with liquid water forms Ca(OH)₂ and H₂.
- Balance the equation as Ca + 2H₂O → Ca(OH)₂ + H₂.
- Magnesium with steam forms MgO and H₂: Mg + H₂O(g) → MgO + H₂.
Answer Ca + 2H₂O(l) → Ca(OH)₂ + H₂; 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 the equation for barium reacting with oxygen to form barium oxide.
Give me a hint
Use BaO as product and balance the diatomic oxygen molecule.
Compare my reasoning
- O₂ supplies two oxygen atoms.
- Make two BaO units to use both oxygen atoms.
- Use two Ba atoms: 2Ba + O₂ → 2BaO.
2Ba(s) + O₂(g) → 2BaO(s).
Look for these in your work
- Specified product retained
- Ba balanced
- O balanced
Write the equation for magnesium reacting slowly with cold water and name the gas.
Give me a hint
Use the hydroxide product for liquid water, not the oxide product for steam.
Compare my reasoning
- The solid product is Mg(OH)₂.
- Hydrogen gas is released.
- Two water molecules balance Mg + 2H₂O → Mg(OH)₂ + H₂.
Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g); the gas is hydrogen.
Look for these in your work
- Cold-water product correct
- Equation balanced
- Gas named
A question changes the reactant from liquid water to steam for magnesium. Predict the product change and write the equation.
Give me a hint
The water state distinguishes the hydroxide-forming cold-water reaction from the steam reaction.
Compare my reasoning
- With steam, magnesium forms MgO rather than Mg(OH)₂.
- Hydrogen is still produced.
- The balanced equation is Mg + H₂O(g) → MgO + H₂.
Magnesium forms MgO and hydrogen with steam: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
Look for these in your work
- State condition recognised
- Product distinguished
- Equation balanced
Common mistakes
- Giving MgO as the product with cold liquid water
- Giving Mg(OH)₂ as the stated steam product
- Omitting H₂
- Claiming all Group 2 metals react equally vigorously
What can you explain now?
What oxide formula does a Group 2 metal form with oxygen in the standard syllabus model?
Compare with the explanation
MO
M²⁺ and O²⁻ combine in a 1:1 ratio.
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Make it stick
How do the products differ for Mg with cold water and steam, and what trend applies to Ca, Sr and Ba with cold water?
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.