Write Group 2 carbonate reactions with water and acids
Describe carbonate behavior in water and predict carbon dioxide formation with dilute acids.
Section 10 · Group 2
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 gas is released when a carbonate reacts with a dilute acid?
What you will learn
- Describe the lack of appreciable carbonate reaction with water
- Write carbonate reactions with both dilute acids
- Explain the barium sulfate coating observation
Carbonates do not appreciably react with water in this syllabus treatment
Group 2 carbonates are solids with low solubility in water. When added to water they do not produce a characteristic acid–base reaction like an oxide; describe the solid as remaining largely undissolved rather than inventing a new product equation.
Keep this separate from carbonate decomposition on heating, which forms a metal oxide and carbon dioxide.
Acids release carbon dioxide from a carbonate
With dilute hydrochloric acid: MCO₃ + 2HCl → MCl₂ + CO₂ + H₂O. With dilute sulfuric acid: MCO₃ + H₂SO₄ → MSO₄ + CO₂ + H₂O.
The hydrogen ions react with carbonate to form carbon dioxide and water. If the sulfate is sparingly soluble or insoluble, a solid layer can form; BaSO₄ may coat BaCO₃ and slow further contact. Observed reaction rate depends on surface and acid conditions.
Pause and explain
What happens when a Group 2 carbonate is added to water in the syllabus model?
Worked example
Write equations for calcium carbonate with dilute hydrochloric acid and barium carbonate with dilute sulfuric acid.
Show the worked solution
- Both reactions form CO₂ and H₂O from carbonate plus acid.
- Ca²⁺ pairs with two Cl⁻ in CaCl₂.
- Ba²⁺ pairs with sulfate in solid BaSO₄.
Answer CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O; BaCO₃ + H₂SO₄ → BaSO₄(s) + CO₂ + H₂O.
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Test this topic →From guided practice to a new situation
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 magnesium carbonate reacting with dilute hydrochloric acid.
Give me a hint
Form MgCl₂, CO₂ and H₂O, then balance chloride and hydrogen.
Compare my reasoning
- MgCO₃ + HCl forms MgCl₂ + CO₂ + H₂O.
- Two HCl supply two Cl atoms for MgCl₂.
- Those two H atoms form H₂O.
MgCO₃ + 2HCl → MgCl₂ + CO₂ + H₂O.
Look for these in your work
- Products correct
- Coefficient on HCl correct
- Atoms balanced
Write the equation for strontium carbonate with dilute sulfuric acid.
Give me a hint
The sulfate has a 1:1 ratio with Sr²⁺.
Compare my reasoning
- The salt formula is SrSO₄.
- Carbonate plus acid also forms CO₂ and H₂O.
- One H₂SO₄ balances the equation.
SrCO₃ + H₂SO₄ → SrSO₄ + CO₂ + H₂O.
Look for these in your work
- Sulfate formula correct
- Gas and water products included
- Equation balanced
Barium carbonate shows brief fizzing in dilute sulfuric acid, then slows. Explain a likely cause and write the products.
Give me a hint
Consider CO₂ formation and the sulfate's low solubility at the solid surface.
Compare my reasoning
- The carbonate–acid reaction releases CO₂, causing fizzing.
- BaSO₄ forms as an insoluble solid.
- A BaSO₄ layer can reduce contact between acid and remaining BaCO₃.
BaCO₃ + H₂SO₄ → BaSO₄(s) + CO₂ + H₂O; the insoluble BaSO₄ layer can slow continued reaction.
Look for these in your work
- Fizzing linked to CO₂
- Precipitate identified
- Surface effect explained
Common mistakes
- Omitting carbon dioxide from an acid–carbonate reaction
- Using one HCl per carbonate formula
- Writing M₂CO₃ for M²⁺
- Assuming insoluble sulfate products cannot affect reaction observations
What can you explain now?
What gas is released when a carbonate reacts with a dilute acid?
Compare with the explanation
Carbon dioxide
Acid converts carbonate into CO₂ and water.
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Make it stick
How do Group 2 carbonates behave in water, with hydrochloric acid, and with sulfuric acid?
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