Thermal Decomposition and Precipitation

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Thermal Decomposition

When a substance breaks down into at least two new other substances when heated.

Transition metal carbonates (transition metals with CO3 in them) break down when heated.

They break down into metal oxide and carbon dioxide, usually resulting in a colour change.

Copper(II) carbonate (green) --> Copper oxide (black) + Carbon dioxide

CuCO3 --> CuO + CO2

Can check CO2 by bubbling through limewater. If CO2 present, limewater goes cloudy.

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Two solutions react and insoluble solid forms in solution.

Solid is said to precipitate out and solid is called precipitate.

Some soluble transition metal compounds react with sodium hydroxide to form insoluble hydroxide which precipitates out.

CuSO4 + 2NaOH --> Cu(OH)2 + Na2SO4

Copper(II) sulphate + Sodium hydroxide --> Copper(II) hydroxide + Sodium sulphate

FeSO4 + 2NaOH --> Fe(OH)2 + Na2SO4

Iron(II) sulphate + Sodium hydroxide --> Iron(II) hydroxide + Sodium sulphate

Fe2(SO4)3 + 6NaOH --> 2Fe(OH)3 + 3Na2SO4

Iron(III) sulphate + Sodium hydroxide --> Iron(III) hydroxide + Sodium sulphate

In ionic form: Cu2+ + 2OH- --> Cu(OH)2

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Use Precipitation to Test Transition Metal Ions

Some insoluble transition metals have distinctive colours.

Copper(II) hydroxide is blue solid.

Iron(II) hydroxide is grey/green solid.

Iron(III) hydroxide is orange/brown solid.

You can use facts to test which transition metal ions a solution contains.

For example, if sodium hydroxide added to unknown soluble salt, orange/brown precipitate forms, so you know you have iron(III) hydroxide and Fe3+ ions in solution

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