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
The 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
\(FeCl_2(aq) + 2NaOH(aq) → 2NaCl(aq) + Fe(OH)_2(s)\)
- A light green precipitate is formed.
Iron(III) Ions
\(FeCl_3(aq) + 3NaOH(aq) → 3NaCl(aq) + Fe(OH)_3(s)\)
- A red-brown precipitate is formed.
Copper(II) Ions
\(CuCl_2(aq) + 2NaOH(aq) → 2NaCl(aq) + Cu(OH)_2(s)\)
- A light blue precipitate is formed.
Magnesium, Calcium and Aluminium Ions
\(MgCl_2(aq) + 2NaOH(aq) → 2NaCl(aq) + Mg(OH)_2(s)\)
\(CaCl_2(aq) + 2NaOH(aq) → 2NaCl(aq) + Ca(OH)_2(s)\)
- Both produce a white precipitate but can then be identified with a Flame Test.
\(AlCl_3(aq) + 3NaOH(aq) → 3NaCl(aq) + Al(OH)_3(s)\)
- 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
The test for Halide anions begins by adding dilute Nitric Acid to remove any possible Carbonates. Then Silver Nitrate is added to form a precipitate of Silver Halide.
\(NaCl(aq) + AgNO_3(aq) → NaNO_3(aq) + AgCl(s)\)
- A white precipitate is formed.
\(NaBr(aq) + AgNO_3(aq) → NaNO3(aq) + AgBr(s)\)
- A cream coloured precipitate is formed.
\(NaI(aq) + AgNO_3(aq) → NaNO3(aq) + AgI(s)\)
- A yellow precipitate is formed.
Sulphates
The test for Sulphate anions begins by adding dilute Hydrochloric Acid to remove any possible Carbonates. Then Barium Chloride is added to form a precipitate of Barium Sulphate. \(Na_2SO_4(aq) + BaCl_2(aq) → 2NaCl(aq) + BaSO_4(s)\)
- A white precipitate is formed.
Carbonates
The test for carbonate anions begins by adding Hydrochloric Acid to the solution. This produces a gas which is collected and bubbled through Limewater to identify if Carbon Dioxide has been produced. If Carbon Dioxide has been produced then the Limewater will turn from transparent to cloudy.
\(Na_2CO_3(aq) + 2HCl(aq) → 2NaCl(aq) + H_2O(l) + CO_2(g)\)
\(CO_2(g) + CaO(aq) → CaCO_3(s)\)
- The Calcium Carbonate makes the Limewater appear cloudy.