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4.3.4Electrochemistry

Test for Carbonates — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 20 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Carrying Out the Test

4.3.4 Test for Carbonates

The carbonate is reacted with acid, and the gas produced is tested with limewater.

1
Place a small amount of the solid or solution in a test tube and add dilute acid, such as hydrochloric acid.
2
Quickly connect the test tube to a second tube containing limewater, so that none of the gas escapes.
3
Observe the effervescence, which shows that a gas is being released from the sample.
4
Record that the limewater turns cloudy white, which confirms the gas is carbon dioxide and so the sample contains a carbonate.

Part 2 of 5Equations for the Carbonate Test

Acid step
  • Carbonate ions react with hydrogen ions to form carbon dioxide and water: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).
Limewater step
  • Carbon dioxide reacts with calcium hydroxide to form a precipitate: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l).
Effervescence
  • The bubbles seen during the reaction are carbon dioxide gas escaping from the solution.
Precipitate
  • Calcium carbonate is insoluble, so it makes the limewater turn cloudy and milky white.

Part 3 of 5Limits and Confirmation

Limits and Confirmation
Effervescence alone
  • Bubbling alone is not enough, because other gases can also be produced, so the limewater test is needed.
Gas delivery
  • The tubes must be joined quickly, because carbon dioxide can escape and give a false negative result.
Carbonate group
  • All carbonates contain the carbonate ion, CO₃²⁻, so the test works for any carbonate compound.
Balancing
  • In the acid step, the carbonate ion accepts two hydrogen ions from the acid.

Part 4 of 5Worked Example

Question

  • A white solid is added to dilute hydrochloric acid.
  • It fizzes, and the gas given off turns limewater cloudy.
  • What ion is in the solid, and what is the equation for the reaction with acid?

  1. Fizzing with acid and cloudy limewater shows that the gas is carbon dioxide.
  2. Carbon dioxide from acid shows the solid contains a carbonate ion, CO₃²⁻.
  3. Write the ionic equationCO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).

Answer

  • The solid contains carbonate ions, CO₃²⁻;
    • CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).

Part 5 of 5Exam Checklist & Tips

4.3.4 – Key Points for the Exam

  • Carbonate ion The CO₃²⁻ ion, found in compounds such as calcium carbonate.
  • Effervescence The fizzing caused by a gas being released from a reaction.
  • Limewater A solution of calcium hydroxide that turns cloudy in carbon dioxide.
  • Ionic equation An equation that shows only the particles that take part in the reaction.

Common Mistakes

  • Stating that fizzing alone proves a carbonate, without testing the gas with limewater.
  • Failing to connect the tubes quickly, so the carbon dioxide escapes before reaching the limewater.
  • Writing the carbonate ion as CO₃⁻ instead of the correct CO₃²⁻.
  • Forgetting that the second stage needs limewater, which shows the gas is carbon dioxide.

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