AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    Carbonates: Carbon dioxide can be identified with limewater.

    Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂(aq).

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 2 of 2

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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. They should also recognise that excess carbon dioxide can later dissolve the precipitate, but the initial milky appearance is the key observation.

    What earns the marks

    • 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.

    Common mistakes and how to put them right

    • Confusing limewater with lime or limestone.

      Put it right: Limewater is calcium hydroxide solution, while limestone is mainly calcium carbonate.

    • Stating that limewater burns or glows.

      Put it right: The test depends on a white precipitate forming, not on a flame.

    • Writing that the precipitate is calcium oxide.

      Put it right: The precipitate is calcium carbonate.

    Exam tips

    • 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.

    Try the lesson

    The start of the Carbonates: Carbon dioxide can be identified with limewater. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which reagent is used to identify carbon dioxide gas?

    Listen

    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. They should also recognise that excess carbon dioxide can later dissolve the precipitate, but the initial milky appearance is the key observation. A strong answer earns marks for these points. 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. A common slip: Confusing limewater with lime or limestone. Instead, limewater is calcium hydroxide solution, while limestone is mainly calcium carbonate. Exam tip: Use the phrase milky or cloudy when describing the positive result with limewater.

    See it

    A top-down flowchart. The first box says carbon dioxide bubbled through limewater. An arrow leads from it to a box saying limewater turns milky or cloudy, which leads to a box saying a white precipitate of calcium carbonate forms, which leads to a box saying the cloudiness is caused by tiny solid calcium carbonate particles suspended in the liquid. A second arrow from the first box leads to a box saying limewater is calcium hydroxide solution, Ca(OH)₂(aq), which leads to a box giving the word equation calcium hydroxide + carbon dioxide → calcium carbonate + water, which leads to a box giving the balanced symbol equation Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
    Shows the limewater test for carbon dioxide, from bubbling the gas through limewater to the milky white precipitate of calcium carbonate and the accompanying word and symbol equations.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box says carbon dioxide bubbled through limewater. An arrow leads from it to a box saying limewater turns milky or cloudy, which leads to a box saying a white precipitate of calcium carbonate forms, which leads to a box saying the cloudiness is caused by tiny solid calcium carbonate particles suspended in the liquid. A second arrow from the first box leads to a box saying limewater is calcium hydroxide solution, Ca(OH)₂(aq), which leads to a box giving the word equation calcium hydroxide + carbon dioxide → calcium carbonate + water, which leads to a box giving the balanced symbol equation Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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