Test Tube Reactions

• Aldehydes and Ketones (Fehling's Solution)

  • Just aqueous solution of CuSO4
  • Add compound to be tested and Fehling's Solution to two separate test tubes
  • Place them in a water bath and heat them till same temperature (30 - 35 degrees Celsius)
  • Aldehydes: alkaline solution of blue Cu (II) ions is reduced to red Cu (I) ions by aldehydes
  • Ketones: cannot be oxidised - no reaction
  • Positive result: for aldehydes; blue solution makes brick red precipitate
  • Ionic equation - RCHO + 2Cu2+ + 5OH- -> RCOO- + Cu2O + 3H20
• Aldehydes and Ketones (Tollen's Reagent)

  • Compound to be tested and Tollen's reagent added to separate test tubes and heated to same temperature (30-35 degrees celsius) in a water bath
  • Aldehydes: formation of a silver mirror from the reduction of the Ag+ ions to metallic silver as they are oxidised to make carboxylic acids
  • Ketones: cannot be oxidised so no result
  • Ionic equation - RCHO + 2Ag- + 3OH- -> RCOO- + 2Ag +2H20
• Alcohol Oxidation State

  • Orange acidified potassium dichromate
  • K2Cr2O7
  • Turned green by primary and secondary alcohols: bright orange Cr in 6+ oxidation state reduced to green 3+ Cr
  • Primary alcohol -> aldehyde -> carboxylic acid
  • Secondary alcohol -> ketone
  • Tertiary alcohols cannot be oxidised
• Halide Ions

  • Add HNO3 (acidifies solution to remove any CO2- ions which would give a false positive for Cl; not HCl as Cl would also give a false positive )
  • Add AgNO3
  • Cl- -> white
  • Br- -> cream
  • I- -> yellow
  • NH4 to tell AgBr and AgCl apart: AgCl redissolves in dilute ammonia, AgBr redissolves in concentrated ammonia
  • AgI is too insoluble to redissolve
• Sulfate Ions

  • Add HCl to solution (acidify solution to remove any CO2- ions which would form a white precipitate of BaCO2 and give a false positive)
  • Add BaCl2
  • Presence of sulfates gives a white precipiate
  • Ba2+ (aq) + SO4 2- (aq) -> BaSO4 (s)
• CO3 2- or HCO3-

  • Add a few drops of dilute HNO3 to the solid/solution being tested
  • Fizzing or bubbles indicates the presence of carbonate through production of CO2
  • 2H+ + CO3 2- -> CO2 + H2O
  • H+ + HCO3- -> CO2 + H2O
• Hydroxide ions


  • Forms insoluble precipitate using range of metal ions
  • Alkaline, turns red litmus or universal indicator blue/purple
  • Mg2+ - white ppt
  • Cu2+ - blue ppt
  • Fe3+ - brown ppt
  • Ionic equation is metal ion + hydroxide ion -> metal hydroxide
• White metal precipitates

  • Mg(OH)2
  • Ca(OH)2
  • SrSO4
  • BaSO4