Chapter8: chemical analysis

• A pure substance consists only of one element or one compound
• Pure substances have a sharp melting point but mixtures melt over a range of temperatures
• A formulation is a mixture which has been designed as a useful product
• Examples of formulations:

  Fuels

  Cleaning products

  Paints

  Medicines

  Alloys

  Fertilisers

  Foods
• Paper chromatography is used to separate mixtures of soluble substances and provide information on the possible identity of the substances present in the mixture
• The mobile phase is the solvent that moves through the paper, carrying different substances with it
• The stationary phase is contained on the paper and does not move through it
• The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper
• What is the chromatography required practical (explain)?

  Draw a pencil line across the chromatography paper, 1-2 cm from the bottom

  Use a pipette/capillary tube to add small spots of each ink to the line on the paper

  Place the paper into a container with a suitable solvent in the bottom

  Allow the solvent to move through the paper, but remove the chromatogram before it reaches the top

  Allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
• Hazards & precautions of chromatography?

  Harmful solvent -> skin irritation/ breathing difficulties -> avoid skin contact, wear gloves, use a fume cupboard
• If oxygen is present in a test tube, a glowing splint relights when it is held inside
• If hydrogen is present in a test tube, a lighted splint is held near its mouth ignites with a squeaky pop
• If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white
• Damp litmus paper is bleached white when it is placed in chlorine

  If blue litmus paper is used, the paper turns red then bleaches white
• How to carry out a flame test?


  Dip a clean wire loop into a solid sample of the compound being tested

  Put the loop into the edge of the blue flame from a Bunser burner

  Observe and record the flame colour produced
• Lithium produces a crimson flame
• Potassium produces a lilac flame
• Calcium produces an orange-red flame
• Copper produces a green flame
• Cooper sulfate + sodium hydroxide -> sodium sulfate + copper hydroxide

  CuSO4 (aq) + 2NaOH (aq) -> Na2SO4 (aq) + Cu(OH)2 (s)
• Copper hydroxide forms a blue precipitate
• Aluminium produces a white precipitate
• Calcium produces a white precipitate
• Magnesium produces a white precipitate
• Copper(II) produces a blue precipitate
• Iron(II) produces a green precipitate
• Iron(III) produces a brown precipitate
• If excess hydroxide solution is added to aluminium hydroxide the precipitate dissolves to form a colourless solution
• Carbonate ions are detected using a dilute acid. Bubbles are given off when an acid, usually dilute hydrochloric acid is added to the test compound

  The bubbles are caused by carbon dioxide. Limewater is used to confirm that the gas is carbon dioxide. It turns cloudy when CO2 is bubbled through it
• Add a few drops of dilute hydrochloric acid to the sample

  Add a few drops of dilute barium chloride solution

  A white precipitate forms if sulfate ions are present
• Silver ions react with halide ions to form insoluble precipitates
• Chloride produces a white precipitate
• Bromide produces a cream precipitate
• Iodide produces a yellow precipitate
• Add a few drops of dilute nitric acid to the sample

  Add a few drops of dilute silver nitrate solution

  Observe and record the colour of any precipitate that forms
• Compared to lab tests, instrumental methods of analysis may give improved:

  Speed

  Accuracy

  Sensitivity
• The flame emission spectroscope is a scienctific instrument based on flame testing. Data from a spectroscope can be used to:

  Identify metal ions in a sample

  Determine the concentration of metal ions in dilute solutions