Chemical Analysis


Pure Substances and Formulations

  • In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
  • In everyday language, a pure substance can mean a substance that has had nothing added to it
  • Pure substances have specific melting and boiling points 
  • Impure substances melt and boil over a range of different temperatures
  • A formulation is a mixture that has been designed as a useful product.
  • In formulations, the quantity of each component is carefully measured to ensure that the product has the required properties.
  • Examples include: fuels, cleaning agents, paints, medicines, alloys, fertilisers, and foods.
1 of 6

Paper Chromatography

  • This is a physical process, which means no chemical reactions take place and no new substances are made 
  • It allows us to separate substances based on their solubilities
  • Stationary Phase: The paper, Mobile Phase: Solvent
  • Method:
  • Use a ruler to draw a horizontal line on the chromatography paper using a pencil
  • Use a capillary tube to put a small spot of each food colouring (A-D) and the unknown
  • Pour water (solvent) into a beaker to a depth of 1cm.
  • Attach the paper to a glass rod using tape and place it in the beaker
  • Pencil line must be above the water so the ink does not wash off
  • Put a lid over the beaker to reduce evaporation of the solvent
  • Water will travel up the paper and we should mark where it stops
  • Calculate the Rf = Distance moved by the chemical / Distance moved by the solvent
  • Rf tells us the identity of the chemical
  • Chemicals strongly attracted to the stationary phase will not move very far
2 of 6

Testing for Gases

  • To test for hydrogen, we insert a burning splint and should hear a squeaky pop, because of hydrogen burning quickly
  • To test for oxygen, we insert a glowing splint and should see the splint relight
  • To test for carbon dioxide, we use limewater (an aqueous solution of calcium hydroxide) and should see it turn cloudy
  • To test for chlorine, we insert damp litmus paper into the tube and should see it bleached and turn white
3 of 6

Flame Tests and Flame Emission Spectroscopy

  • First, place a small amount of our chemical onto a nichrome wire 
  • Place the end of this into a blue bunsen burner flame
  • The colour of the flame can show us the metal ion that is present
  • Lithium produces a crimson flame
  • Sodium produces a yellow flame
  • Potassium produces a lilac flame
  • Calcium produces an orange-red flame
  • Copper produces a green flame
  • Colour of the flame test can be difficult to distinguish if there's a low concentration of it
  • A sample can contain a mixture of metal ions that can mask the colour of the flame
  • We can use flame emission spectroscopy ( instrumental method) instead, which can tell us the concentration of the ion as well
  • A sample of the metal ion in solution is placed into a flame and the light it gives out can be seen through the spectroscope
  • The spectroscope can convert the light into a line spectrum
  • Instrumental methods are rapid, sensitive and accurate
4 of 6

Metal Hydroxide Precipitates

  • If we add sodium hydroxide solution to calcium, aluminium and magnesium ions they will all produce white precipitates
  • We can't distinguish between these, so we add excess sodium hydroxide the aluminium precipitate redissolves
  • We then use a flame test to work out which one is calcium
  • Copper II ions react with sodium hydroxide to form a blue precipitate of copper (II) hydroxide
  • Iron II ions react with sodium hydroxide to form a green precipitate iron (II) hydroxide
  • Iron III ions react with sodium hydroxide to form a brown precipitate iron (III) hydroxide
5 of 6

Identifying Non-metal Ions

  • Testing for Carbonate Ions:
  • Add dilute acid to the sample
  • If the carbonate is present, the acid will react with the carbonate to make carbon dioxide
  • We should see fizzing
  • Bubble the gas through limewater, if it turns cloudy the carbonate is present
  • Testing for Halide Ions:
  • Add dilute nitric acid to the sample
  • Add dilute silver nitrate solution
  • They should produce a precipitate of the silver halide. Chlorine: White, Iodine: Yellow, Bromide: Cream  
  • Testing for Sulfate Ions:
  • Add dilute hydrochloric acid to our sample
  • Add barium chloride solution
  • If sulfate ions are present, we should see a white precipitate formed
6 of 6


No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Testing and analysing substances resources »