3.1.11 - testing for ions

• qualitative tests are used to identify the ions in an unknown solution , but it does not show how much of each ion is present
• a precipitation reaction is where an insoluble solid is formed , usually from 2 aqueous solutions
• identifying anions

  anions are negative ions . examples of anions used are carbonate ions , sulfate ions and halide ions ( chloride , bromide and iodide ions )
• carbonate compounds that contain CO32- ions react with acids
• the method for identifying carbonate ions is that you add a dilute strong acid to the suspected carbonate . collect any gas formed and pass it through limewater .
• if carbonate ions are present , there should be fizzing and a colourless gas produced . The gas should also turn limewater cloudy
• sulfate ions react with barium ions to form an insoluble salt ( BaSO4)
• to test for sulfate ions , react diulte hydrochloric acis and barium chloride to the suspected sulfate .
• if sulfate ions are present , a white precipitate of barium sulfate is produced
• you should carry out tests in order of carbonate test , sulfate test and then halide test . This is because barium ions form an insoluble precipitate of BaCO3 , if you didnt rule out carbonate ions you wouldnt know if the precipitate of this was carbonate or barium sulfate .

  silver ions form an insoluble precipitate of Ag2SO4 , if you didnt rule out sulfate ions you would know if this is silver sulfate or silver halide
• halide ions react with silver ions to form different coloured silver halide precipitates . AgX where X is the halide .
• halides used are chlorine , bromine and iodide
• • to identify halide ions , dissolve suspected halide in water ,
  • then add an aqueous solution of silver nitrate .
  • then note the colour of any precipitate formed .
  • if the colour is hard to distinguish , add aqueous ammonia ( first dilute then concentrated) .
  • note the solubility of the precipitate in aqueous ammonia
• if halide ions are present :

  • for chlorine - silver chloride : a white precipitate , soluble in dilute NH3
  • . for bromine - silver bromide : a cream precipitate , soluble in concentrated NH3 only
  • . for iodine - silver iodide: a yellow precipitate , insoluble in dilute and concentrated NH3
• identifying cations: cations are positive ions , often containing metal ions . example of cations are ammonium ions , NH4+
• ammonium ions react with hydroxide ions to produce ammonia and water
• the method to find ammonium ions :

  1. add sodium hydroxide solution to the suspected ammonium compound and WARM gently
  2. test any gas evolved with RED LITMUS PAPER
• if ammonium ions are found , the ammonia gas will turn red litmus paper BLUE , ammonia gas also has a distinctive smell ( its hazardous)
• you can use ionic equations to show reactions