Testing for Unknowns

Testing Inorganic Unknowns

Metal Ions: Flame Tests

Halide Ions: Silver Nitrate Test

·          test must be done in solution
if starting from a solid, it must first be dissolved in pure water

·          add dilute nitric acid to the samples
this is to remove/react any interfering ions (carbonate or hydroxide ions) that may give a false positive result/precipitation

·          add silver nitrate solution to the samples

Sulphate Ions:

·          test must be done in solution
if starting from a solid, it must first be dissolved in pure water

·          acidify the sample with dilute hydrochloric acid

·          add some barium chloride (BaCl₂)

·          if a sulphate is present, a white precipitate will form (insoluble barium sulphate)

Carbonate Ions:

·          add a dilute acid to the compound (usually dilute hydrochloric acid)

·          if carbonate ions are present, they will react with the acid and produce carbon dioxide, which will be observed as bubbles/effervescence

·          to test the gas being produced, collect a sample and test with limewater. carbon dioxide turns limewater cloudy

Testing Organic Unknowns

Alkenes:

·          shake an alkene with bromine water, or bubble a gaseous alkene through bromine water

·          alkenes decolourise bromine water, so the solution will become colourless (from orange/brown)

Aldehydes:

·          to test for aldehydes, you can use Fehling’s solution, Tollen’s reagent or Benedict’s solution.