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Science

Transition elements

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Precipitation

Transition metal hydroxides are insoluble in water.

If a solution of any soluble transition metal compound is mixed with sodium hydroxide solution then we get a displacement reaction. The sodium is the more reactive metal, and displaces the transition metal from its compound. The transition metal hydroxide is produced as a result. As this is insoluble in water it appears as a solid in the liquid. A solid produced in a liquid in this way is called a precipitate.

Copper sulfate solution + Sodium hydroxide solution -> Solid Copper hydroxide in  colourless Sodium sulfate solution

The colour of the precipitate indicates which transition metal ion was in the initial solution.

Examples

Transition metal ions in solutionPrecipitate formed when mixed with sodium hydroxide solutionColour of the precipitate
Cu2+copper hydroxideblue
Fe2+iron(II)hydroxidegrey/green
Fe3+iron(III)hydroxideorange/red

Higher tier only

You need to be able to construct the balanced symbol equations for the thermal decomposition reactions and the precipitation reactions described in this section.

Examples

FeCO3 → FeO + CO2

CuCO3 → CuO + CO2

MnCO3 → MnO + CO2

ZnCO3 → ZnO + CO2

And

Cu2+ + 2OH- → Cu(OH)2

Fe2+ + 2OH- → Fe(OH)2

Fe3+ + 3OH- → Fe(OH)3

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