Topic 8: Chemical Analysis

• pure substance

  a substance made up of only one element or compound
• The melting and boiling points of pure substances are fixed, whereas a mixture will melt over a wider range of temperatures and tend to have a lower melting point and higher boiling point than a pure substance.
• As each substance has a specific melting and boiling point, an unknown substance can be melted or boiled and its melting or boiling point can be compared with those of known chemicals to find what chemicals the substance is made of. To find its purity, you can see if it melts or boils at the exact recorded melting or boiling point or if it melts or boils over a wide range of temperatures.
• formulation/composite

  a mixture that has been designed as a useful product, made by carefully measuring quantities of components then mixing them to ensure that the product has the right properties. eg. medicine tablets, paint, washing up liquid
• chromatography

  technique used to separate and identify a mixture's components by allowing the molecules in the mixture to distribute themselves between a stationary and a mobile phase
• How does chromatography work?

  1. Pencil baseline is drawn 2cm from the bottom of the paper so that the mixture doesn't dissolve in the solvent. Pencil is used because pen ink will spread and smudge in the solvent.
  2. Pencil spots at equal distances from each other are marked and spots of mixtures are drawn on top of them and labelled
  3. The paper is put in a beaker and 1cm of solvent is poured into the beaker so that the baseline doesn't touch it

  Molecules that spend more time in the mobile phase are better solvents as they are more soluble so are carried along faster.
• A pure substance only produces one spot on a chromatogram, whereas an impure substance produces multiple
• Rf = distance moved by substance ÷ distance moved by solvent
• Tests for Gases:

  • Oxygen: light a splint and blow out the flame leaving a glowing end. Place the glowing splint into the gas: it will relight if the gas is oxygen
  • Hydrogen: light a splint and place it into hydrogen gas to produce a 'squeaky pop' sound
  • Carbon dioxide: when bubbled through limewater, it will change colour from clear to cloudy
  • Chlorine: blue litmus paper will go slightly red (produces hydrochloric acid) and then white after being dampened and placed in chlorine gas - experiment is poisonous
• Tests for Cations:

  • Flame tests: substance is put into hot flame using a colourless nichrome wire that has been cleaned into hydrochloric acid before being put in the solution. The flame will change colour dependent on the metal: lithium turns the flame crimson, sodium yellow, potassium lilac, calcium orange/brick red, copper green. (only works with pure ions)
  • Precipitation reactions: metal solution added to sodium hydroxide to produce solid precipitate: iron 2 = green, iron 3 = orange/brown, copper = blue, aluminium/calcium/magnesium = white
  • Excess sodium hydroxide dissolves aluminium
• Ionic Equations for Precipitation Reactions:

  • Copper: Cu2+(aq) + 2OH-(aq) = Cu(OH)2(s)
  • Copper chloride: CuCl2(aq) + NaOH(aq) = Cu(OH)2(s) + NaCl(aq)
  • Iron(II): Fe2+(aq) + 2OH-(aq) = Fe(OH)2(s)
  • Iron(III): Fe3+(aq) + 3OH-(aq) = Fe(OH)3(s)
  • Magnesium: Mg2+(aq) + 2OH-(aq) = Mg(OH)2(s)
  • Calcium: Ca2+(aq) + 2OH-(aq) = Ca(OH)2(s)
  • Aluminium: Al3+(aq) + 3OH-(aq) = Al(OH)3(s) ; excess NaOH dissolves aluminium
• Group 1 do not form precipitates
• Identifying Carbonate Ions (CO32-):

  • Adding acid produces a salt, water and carbon dioxide
  • Add hydrochloric acid to test sample and bubble any gas produced through limewater - if it is a carbonate then the carbon dioxide will have turned the limewater cloudy
  • Ionic equation: 2H+(aq) + CO32-(aq) = salt + H2O(l) + CO2(g)
• Identifying Halide Ions (silver nitrate test):

  • Add dilute nitric acid to sample to react with any carbonate and sulfite ions that are impurities and remove them
  • When silver nitrate is added, chloride produces a white precipitate, bromide produces cream and iodide produces yellow
  • Ionic equation for chloride (same for other halides): Ag+(aq) + Cl-(aq) = AgCl(s)
• Identifying Sulfate Ions (SO42-):

  • Adding barium chloride produces a white precipitate (barium sulfate)
  • Dilute hydrochloric acid is added first to remove any carbonate or sulfite ions as they are impurities that could confuse the results by reacting with barium chloride
  • Ionic equation: Ba2+(aq) + SO42-(aq) = BaSO4(s)
• instrumental analysis

  using technology and machines to analyse and identify chemicals
• Advantages of Instrumental Analysis:

  • Developments are improving at a rapid rate due to modern technology
  • More accurate, sensitive and faster
  • Useful when the amount of a sample is small

  Disadvantages:

  • Can involve expensive apparatus which may require specialised training
• Flame Emission Spectroscopy:

  • Identifies metal ions in a sample
  • Electrons use energy to move to a higher energy level when heated
  • They then fall back, releasing energy as a flame
  • Analysing the light with a spectroscope shows its wavelength and reveals the metal as each metal gives a unique pattern of coloured lines
  • Can be used to test water supplies for metal ions, as some like mercury can be toxic at low levels
  • Ions can be detected at concentrations as low as 1x10-9g
  • Also used in steel industry to mix steel with other elements
• physical test

  a test of a substance's physical properties eg. melting
• chemical test

  a test involving a chemical reaction eg. reacting a known chemical with an unknown chemical
• Increasing Accuracy of Paper Chromatography:

  • Put a lid over the beaker to stop the solvent from evaporating
  • Keep the types of solvent and paper the same when repeating the experiment
• A limitation of flame tests is that if you have a mixture with multiple cations then the colour of your flame will mix.