Home Home Astronomy Chemistry Electronics Mathematics Physics Field Trips Home  

Ion Tests

Test for Halide Ions: Cl- Br- I-

Add silver nitrate to a salt dissolved in nitric acid. The colour of the precipitate indicates the halide present:

  • White precipitate = chloride
  • Cream precipitate = bromide
  • Pale yellow precipitate = iodide

NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)

Ag+(aq) + X-(aq) → AgX(s)

The differences between the colours of the precipitates of silver chloride (white) and silver bromide (cream) are difficult to see unless you have both together to compare. To help distinguish them add dilute ammonia solution. Silver chloride dissolves in the ammonia solution, but silver bromide is insoluble. Silver bromide will dissolve in concentrated ammonia solution, but silver iodide is insoluble even in concentrated ammonia solution.

Test for Sulphate Ions: SO42-

Add barium nitrate or barium chloride solution to sulphate compound dissolved in nitric acid. A white precipitate (barium sulphate) indicates a sulphate.

Na2SO4(aq) + Ba(NO3)2(aq) → 2NaNO3(aq) + BaSO4(s)

Na2SO4(aq) + BaCl2(aq) → 2NaCl(aq) + BaSO4(s)

Test for Nitrate Ions: NO3-

Add the nitrate to sodium hydroxide solution, then add powdered aluminium. The nitrate ion is reduced by the aluminium, and ammonia gas is given off. Use damp red litmus paper to test the gas. If the litmus paper turns blue the gas is ammonia and the test for nitrate ions is positive.

The aluminium reduces the nitrate ion (NO3-) to an ammonium ion (NH4+).
The ammonium ion reacts with hydroxide to produce ammonia gas and water (see alkalis).

NH4+(aq) + OH-(aq) ⇌ NH3(g) + H2O(l)

Test for Carbonate Ions: CO32-

Adding dilute acid to the carbonate. If the gas given off turns lime water cloudy, then the test is positive. For example:

2HCl(aq) + Na2CO3(s) → 2NaCl(aq) + H2O(l) + CO2(g)

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

Note that if more CO2 is blown into the cloudy lime water, it will cause the calcium carbonate precipitate to redissolve to form soluble calcium hydrogencarbonate.

CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)

Test for Metal Ions: Cu2+ Fe2+ Fe3+ Zn2+ Al3+ Mg2+ Ca2+

Add sodium hydroxide solution to the metal ion solution. The colour of the precipitate indicates the metal ions present:

  • Light blue precipitate = copper II
  • Dirty green precipitate = iron II
  • Rusty brown precipitate = iron III
  • White precipitate which dissolves in excess ammonia solution or sodium hydroxide = zinc
  • White precipitate which dissolves in excess sodium hydroxide (only) = aluminium
  • White precipitate, not soluble in excess solutions = magnesium or calcium, use a flame test to distinguish