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Group II Elements and Their Compounds

The oxides and their reactions with water and dilute acids

  • 3 things to know
  • Unit AS 2

Overview

Saying that the Group II oxides are basic and why calcium oxide reacts only slowly with dilute sulfuric acid.

What you need to know

Answer each one before you reveal it.

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Key term

Write the equation for the reaction of magnesium oxide with water.

Key term

Why is the mixture formed when magnesium oxide is shaken with water only weakly alkaline?

Key term

Write the equation for the reaction of calcium oxide with water and describe what is seen.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is meant by saying that the Group II oxides are basic?

    Model answer

    They are metal oxides that neutralise acids, reacting with them to give a salt and water only.

  2. 2

    Write the equation for the reaction of magnesium oxide with water.

    Model answer

    The equation is MgO(s) + H₂O(l) → Mg(OH)₂(s).

  3. 3

    Why is the mixture formed when magnesium oxide is shaken with water only weakly alkaline?

    Model answer

    Magnesium hydroxide is only very slightly soluble, so few hydroxide ions pass into solution and the pH reaches only about 10.

  4. 4

    Write the equation for the reaction of calcium oxide with water and describe what is seen.

    Model answer

    The equation is CaO(s) + H₂O(l) → Ca(OH)₂(s), and the mixture becomes very hot, steams and crumbles to a dry white powder.

  5. 5

    State and explain the trend in the pH of the solutions formed when the oxides are added to water.

    Model answer

    The pH rises on going down the group, because the hydroxides become more soluble and so release more hydroxide ions into the solution.

  6. 6

    Which gives the more alkaline solution with water, magnesium oxide or barium oxide?

    Model answer

    Barium oxide gives the more alkaline solution, because barium hydroxide is much the more soluble of the two hydroxides.

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