Predict Group 2 oxide reactions with water and acids
Write equations for Group 2 oxides reacting with water, hydrochloric acid and sulfuric acid.
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 is the usual formula for a Group 2 metal oxide?
What you will learn
- Relate oxide formula MO to hydroxide formation
- Balance oxide–acid equations
- Distinguish water reaction from acid neutralisation
Oxides form hydroxides with water
The general reaction is MO(s) + H₂O(l) → M(OH)₂. Magnesium oxide reacts only slightly with water; calcium oxide reacts readily and releases heat as it forms calcium hydroxide. The extent of reaction and hydroxide solubility are separate questions.
Do not infer that every oxide dissolves completely. A low-solubility hydroxide may be present as a suspension, while soluble hydroxide enters solution and makes it alkaline.
Acids neutralise the basic oxide
With hydrochloric acid, MO + 2HCl → MCl₂ + H₂O. With sulfuric acid, MO + H₂SO₄ → MSO₄ + H₂O. Check that the metal, oxygen, hydrogen, chloride or sulfate groups balance.
An insoluble sulfate such as BaSO₄ may form as a solid. State symbols or observations should reflect the salt's solubility when the question provides enough information.
Pause and explain
What is the salt when BaO reacts with sulfuric acid?
Worked example
Write the equations for calcium oxide reacting with water and with hydrochloric acid.
Show the worked solution
- Water converts CaO to Ca(OH)₂.
- Two HCl molecules supply two chloride ions for CaCl₂.
- The remaining H and O form one H₂O in the acid reaction.
Answer CaO + H₂O → Ca(OH)₂; CaO + 2HCl → CaCl₂ + H₂O.
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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.
Write the equation for magnesium oxide reacting with water and state one observation limit.
Give me a hint
Use Mg(OH)₂ as product; remember MgO reacts only slightly with water.
Compare my reasoning
- MgO and H₂O supply one Mg, two H and two O.
- Mg(OH)₂ has the same atoms.
- The reaction is slight; do not claim all solid dissolves.
MgO(s) + H₂O(l) → Mg(OH)₂(s); only a limited amount of the sparingly soluble hydroxide dissolves.
Look for these in your work
- Product formula correct
- Equation balanced
- Extent qualified
Balance calcium oxide reacting with dilute hydrochloric acid.
Give me a hint
The calcium chloride product needs two chloride ions.
Compare my reasoning
- Write CaO + HCl → CaCl₂ + H₂O.
- Use 2HCl to supply two Cl atoms.
- Check H and O in H₂O.
CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l).
Look for these in your work
- Coefficient 2 on HCl
- Atoms balanced
- Neutralisation products correct
Write the reaction of barium oxide with dilute sulfuric acid and predict the sulfate's state.
Give me a hint
Use a 1:1 barium sulfate formula and recall its low solubility.
Compare my reasoning
- BaO + H₂SO₄ forms BaSO₄ and H₂O.
- All atoms balance with coefficients of 1.
- BaSO₄ is insoluble and is written as a solid.
BaO(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O(l).
Look for these in your work
- Salt formula correct
- Equation balanced
- Precipitate state justified
Common mistakes
- Using M₂O for a Group 2 oxide
- Writing MCl without the second chloride
- Confusing oxide reaction with water and neutralisation by acid
- Assuming all sulfate products are soluble
What can you explain now?
What is the usual formula for a Group 2 metal oxide?
Compare with the explanation
MO
M²⁺ and O²⁻ balance in a 1:1 ratio.
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Make it stick
How does MO react with water, HCl and H₂SO₄, and how can sulfate solubility affect the observed product?
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