Sodium Hydroxide Test (Oxford AQA IGCSE Chemistry)

Metal Hydroxides

• Metal ions 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 therefore 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

Table of precipitate colours with NaOH