Metal Hydroxides (AQA GCSE Chemistry)
Revision Note
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Sodium hydroxide test
Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide
If only a small amount of NaOH is used then normally the metal hydroxide precipitates
In excess NaOH some of the precipitates may dissolve
For this reason just a few drops of NaOH are added at first and very slowly
If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate which dissolves as quickly as it forms if excess solution is added
A small amount is thus added, very gradually and any colour changes or precipitates formed are noted
Then the NaOH is added in excess and the reaction is observed again
Ca2+ and Mg2+ ions can be distinguished from Al3+ as calcium hydroxide and magnesium hydroxide precipitates do not dissolve in excess NaOH but aluminium hydroxide does
Another test could be used to distinguish between the Ca2+ and Mg2+ ions, for example a flame test
Most transition metals produce hydroxides with distinctive colours
Metal ion | Effect of adding NaOH |
|---|---|
Cu2+ | Light blue precipitate, insoluble in excess NaOH |
Fe2+ | Green precipitate, insoluble in excess NaOH |
Fe3+ | Brown precipitate, insoluble in excess NaOH |
Ca2+ | White precipitate, insoluble so remains in excess NaOH |
Mg2+ | White precipitate, insoluble so remains in excess NaOH |
Al3+ | White precipitate, dissolves in excess NaOH to form a colourless solution |
Examiner Tips and Tricks
Be sure to distinguish between the term “colourless” and “clear”. A solution that loses its colour has become colourless. A clear solution is one that you can see through such as water. Solutions can be clear and have colour e.g: dilute copper sulphate.
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