Carbonates — AQA GCSE Chemistry
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Carbonates explained
Carbonates covers 2 specification points, including Carbonates react with dilute acids to form… and Carbon dioxide can be identified with limewater..
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Open a point below for its explanation and revision guidance.
Carbonates react with dilute acids to form carbon dioxide gas.
Metal carbonates contain the carbonate ion, CO₃²⁻. When a carbonate reacts with a dilute acid, such as hydrochloric, sulfuric or nitric acid, a salt, water and carbon dioxide are produced. For example, calcium carbonate plus dilute hydrochloric acid gives calcium chloride, water and carbon dioxide: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The carbon dioxide is seen as effervescence, and it turns limewater milky. The reaction is a neutralisation in which the carbonate ion accepts hydrogen ions, releasing carbon dioxide and water. Students should be able to name the salt formed from the acid used and the metal in the carbonate, and to describe a simple test-tube method for observing the gas.
Carbon dioxide can be identified with limewater.
Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂(aq). When carbon dioxide is bubbled through limewater, the solution turns milky or cloudy because a white precipitate of calcium carbonate forms: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). This is the standard laboratory test for carbon dioxide. The gas is usually delivered through a delivery tube into a test tube partly filled with limewater. The cloudiness is caused by tiny solid particles of calcium carbonate suspended in the liquid. Students should describe the test, state the result and explain the chemical change. They should also recognise that excess carbon dioxide can later dissolve the precipitate, but the initial milky appearance is the key observation.
Your focus
- Describe the reaction between a named carbonate and a dilute acid, including the products formed.
- Write a balanced equation for a carbonate–acid reaction.
- Explain how carbon dioxide production can be observed and tested in the laboratory.
Show all 6 objectives
- Describe the limewater test for carbon dioxide and the result observed.
- Write word and balanced symbol equations for the reaction between carbon dioxide and limewater.
- Explain why limewater turns milky when carbon dioxide is bubbled through it.
Carbonates exam tips
Marking Points
- State that a carbonate reacts with a dilute acid to produce a salt, water and carbon dioxide.
- Identify carbon dioxide as the gas responsible for effervescence or bubbling during the reaction.
- Name the salt formed from the metal in the carbonate and the acid used, for example calcium chloride from calcium carbonate and hydrochloric acid.
- Write a balanced symbol equation for a named carbonate and dilute acid, including state symbols where required.
- Describe how the gas can be collected or tested, for example using limewater to confirm carbon dioxide.
- Explain that the carbonate ion reacts with hydrogen ions from the acid, releasing carbon dioxide and water.
- State that limewater is calcium hydroxide solution and is used to test for carbon dioxide.
- Describe bubbling the gas through limewater and observing a milky or cloudy white precipitate.
- Identify the white precipitate as calcium carbonate.
- Write the word equation calcium hydroxide + carbon dioxide → calcium carbonate + water.
- Write the balanced symbol equation Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
- Explain that the cloudiness is caused by insoluble calcium carbonate particles forming.
Examiner Tips
- 💡Link the observation of bubbling to the production of carbon dioxide rather than to hydrogen or oxygen.
- 💡When asked to name a salt, identify the metal from the carbonate and the negative ion from the acid before writing the name.
- 💡Practise balancing equations for group 1 and group 2 carbonates with hydrochloric, sulfuric and nitric acids.
- 💡Use the phrase milky or cloudy when describing the positive result with limewater.
- 💡Include the formula Ca(OH)₂ for limewater when writing equations.
- 💡If asked to compare gases, remember that hydrogen gives a squeaky pop and oxygen relights a glowing splint, while carbon dioxide turns limewater milky.
Common Mistakes
- Writing that carbon dioxide is the only product; the correction is that a salt and water are also formed.
- Naming the salt from the acid alone without considering the metal in the carbonate; the correction is to combine the metal ion with the acid's negative ion.
- Assuming all carbonates react in exactly the same ratio with every acid; the correction is to balance the equation for the specific carbonate and acid used.
- Confusing limewater with lime or limestone; the correction is that limewater is calcium hydroxide solution, while limestone is mainly calcium carbonate.
- Stating that limewater burns or glows; the correction is that the test depends on a white precipitate forming, not on a flame.
- Writing that the precipitate is calcium oxide; the correction is that the precipitate is calcium carbonate.