4.2.5 halides & sulfates

• Testing for halides - add dilute nitric acid then silver nitrate:

  • add dilute nitric acid to solution. removes carbonate ions that could disrupt test results by forming a precipitate with the silver ions added later
  • add silver nitrate to the same solution. if halides are present, they will form a precipitate with the silver ions

  Ag(+)(aq) + Br(-)(aq) -> AgBr(s) (silver bromide is cream)

  Ag(+)(aq) + Cl(-)(aq) -> AgCl (silver chloride is white)

  Ag(+)(aq) + I(-)(aq) -> AgI(s) (silver iodide is pale yellow)
• Fill the gaps

  A) dilute nitric acid

  B) silver nitrate

  
• Test for sulfates - add dilute hydrochloric acid then barium chloride:

  • add dilute hydrochloric acid. removes carbonate ions that could disrupt test results by forming a precipitate with barium ions added next
  • add barium chloride to the same solution. if sulfates are present, the white precipitate barium sulfate will form

  Ba(2+)(aq) + SO4(2-)(aq) -> BaSO4(s)
• Chemical analysis - advantages:

  • greater sensitivity and accuracy
  • quicker at producing results
  • able to analyse tiny samples
• Chemical analysis - disadvantages:

  • instruments are often expensive
  • you need to go through special training to operate the instruments
  • results are often only useful when compared to data from known substances
• Fill the gaps

  A) sensitivity

  B) small

  C) known substances

  D) special training

  
• Instrumental methods are more sensitive and accurate than traditional methods.