Testing for Aqueous Ions

Contents

Key Stage 4

Meaning

Testing for aqueous ions is a set of techniques used to identify the ions in a compound.

About Testing For Aqueous Ions

Aqueous ions can be identified through chemical reactions which produce:

• A precipitate - Which can indicate Metal Ions, Halide ions or Sulphate ions.
• A gas - Carbon Dioxide gas indicates Carbonate ions.

Metal Ions are Positive Ions and referred to as Cations because they would collect at the Cathode in an electrolysis reaction.

There are many different Negative Ions and these are referred to as Anions because they would collect at the Anode in an electrolysis reaction.

Testing for Aqueous Metal Cations

To test for aqueous metal cations is done by adding Sodium Hydroxide to the solution. The following reactions occur to produce identifiable precipitates:

Iron(II) Ions

<chem>FeCl2(aq) + 2NaOH(aq) -> 2NaCl(aq) + Fe(OH)2(s)</chem>

A light green precipitate is formed.

Iron(III) Ions

<chem>FeCl3(aq) + 3NaOH(aq) -> 3NaCl(aq) + Fe(OH)3(s)</chem>

A red-brown precipitate is formed.

Copper(II) Ions

<chem>CuCl2(aq) + 2NaOH(aq) -> 2NaCl(aq) + Cu(OH)2(s)</chem>

A light blue precipitate is formed.

Magnesium, Calcium and Aluminium Ions

<chem>MgCl2(aq) + 2NaOH(aq) -> 2NaCl(aq) + Mg(OH)2(s)</chem>

<chem>CaCl2(aq) + 2NaOH(aq) -> 2NaCl(aq) + Ca(OH)2(s)</chem>

Both produce a white precipitate but can then be identified with a Flame Test.

<chem>AlCl3(aq) + 3NaOH(aq) -> 3NaCl(aq) + Al(OH)3(s)</chem>

A white precipitate is formed but can be distinguished form Magnesium and Calcium by adding excess Sodium Hydroxide which causes the precipitate to dissolve.

Testing for Aqueous Anions

Halides

First dilute Nitric Acid is added to remove any possible Carbonates.