Describe and explain the method for testing a solution for the presence of ammonium ions.

  1. Gently heat the solution with aqueous sodium hydroxide - effervescence suggests the presence of ammonium ions.
  2. Test the gas produced with damp red litmus paper - a colour change to blue confirms the presence of ammonium ions.


Ammonia, produced in step 1, is an alkaline gas so turns red litmus paper blue.

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What is the equation, including state symbols, for the reaction between ammonium ions and hydroxide ions?

NH4+(aq) + OH-(aq) ➔ NH3(g) + H2O(l)

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What is observed when excess aqueous sodium hydroxide and aqueous ammonia are added to separate solutions of aqueous Cu2+ ions?

When either sodium hydroxide or ammmonia are added to Cu2+, a pale blue precipitate of [Cu(H2O)4(OH)2] forms. The precipitate remains insoluble in excess sodium hydroxide, but it redissolves in excess ammonia to form a deep blue solution of Cu(NH3)4(H2O)2]2+.

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What is observed when excess aqueous hydroxide ions and aqueous ammonia are added to separate solutions of aqueous Fe2+ ions?

When either excess hydroxide ions or ammonia are added to Fe2+, a green precipitate of [Fe(H2O)4(OH)2] forms. The precipitate turns brown on exposure to air as [Fe(H2O)4(OH)2] is oxidised to [Fe(H2O)3(OH)3]. 

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What is observed when excess aqueous hydroxide ions and aqueous ammonia are added to separate solutions of aqueous Fe3+ ions?

When either excess hydroxide ions or ammonia are added to Fe3+, a brown precipitate of [Fe(H2O)3(OH)3] forms.

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What is observed when excess aqueous sodium hydroxide is added to a solution of aqueous Al3+ ions?

When sodium hydroxide is added to Al3+, a white precipitate of Al(H2O)3(OH)3 forms. The precipitate redissolves in excess sodium hydroxide to form a colourless solution of [Al(OH)4]-.

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State and explain the colour change of iron(II) hydroxide on exposure to air.

Iron(II) hydroxide changes from green to brown because it is oxidised by oxygen in the air to iron(III) hydroxide.

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What is observed when carbonate ions are added to separate solutions of aqueous Cu2+ and Al3+ ions?

When carbonate ions are added to Cu2+, a blue-green precipitate of CuCO3 forms.

When carbonate ions are added to Al3+, a white precipitate of Al(H2O)3(OH)3 forms, releasing bubbles of CO2 gas.

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