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Chemical Analysis

Metal hydroxides

  • 3 things to know
  • Paper 2

Overview

Precipitating metal hydroxides with sodium hydroxide solution, the white precipitates of aluminium, calcium and magnesium and how excess tells them apart, the coloured precipitates of copper(II), iron(II) and iron(III), and their equations.

What you need to know

Answer each one before you reveal it.

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Key term

Which reagent is added to a solution in order to identify metal ions by precipitation?

Key term

Why does adding sodium hydroxide solution to a solution of a metal salt often produce a solid?

Key term

Name the three metal ions that give a white precipitate with sodium hydroxide solution.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Which reagent is added to a solution in order to identify metal ions by precipitation?

    Model answer

    Sodium hydroxide solution.

  2. 2

    Why does adding sodium hydroxide solution to a solution of a metal salt often produce a solid?

    Model answer

    Many metal hydroxides are insoluble, so they come out of solution as a precipitate.

  3. 3

    Name the three metal ions that give a white precipitate with sodium hydroxide solution.

    Model answer

    Aluminium ions, calcium ions and magnesium ions.

  4. 4

    How can aluminium ions be distinguished from calcium and magnesium ions using sodium hydroxide solution?

    Model answer

    All three give a white precipitate at first, but only the aluminium hydroxide dissolves again when excess sodium hydroxide is added.

  5. 5

    A solution gives a white precipitate with sodium hydroxide solution and the precipitate remains when more is added. Which ions could be present?

    Model answer

    Calcium ions or magnesium ions.

  6. 6

    A white precipitate forms with sodium hydroxide solution and then disappears as more is added. Which ion is present?

    Model answer

    Aluminium ions.

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