Chemical analysis

• Pure substances:

  It is a single element or compound, not mixed with any other substance.

  They melt and boil at specific temperatures.

  • This melting and boiling points data can be used to distinguish pure substances from mixtures
• Formulations:

  It is a mixture that has been designed as a useful product.

  Many products are complex mixtures in which each chemical has a particular purpose.

  They are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.

  Examples:

  • Fuels
  • Cleaning agents
  • Paints
  • Medicines
  • Alloys
  • Fertilisers
  • Foods
• Chromatography:

  It is used to separate mixtures and give information to help identify substances.

  Involves a stationary phase and a mobile phase.

  Separation depends on the distribution of substances between the phases.

  Rf value = distance moved by the substance/distance moved by solvent

  Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.

  Compounds in a mixture may separate into different spots depending on the solvent but a pure substance will produce a single spot in solvents.

  
• Test for hydrogen:

  Use a burning splint held at the open end of a test tube of the gas.

  Creates a squeaky pop sound.
• Test for oxygen:

  Use a glowing splint inserted into a test tube of the gas.

  Splint relights in oxygen.
• Test for carbon dioxide:

  Bubble the gas through the limewater [calcium hydroxide[aq]].

  It will turn milky [cloudy].
• Test for chlorine:

  Use damp litmus paper.

  When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
• Flame tests:

  They can be used to identify metal ions.

  Lithium -> Crimson

  Sodium -> Yellow

  Potassium -> Lilac

  Calcium -> Orange-red

  Copper -> Green

  If a sample containing a mixture of ions is used some of the flame colours can be masked.
• Metal hydroxides:

  Aluminum, calcium, and magnesium ions form a white precipitate with NaOH.

  Only aluminum's precipitate dissolves when excess NaOH is added.

  Copper[III] produces a blue precipitate.

  Iron[II] produces a green precipitate.

  Iron[III] produces a brown precipitate.
• Carbonates:

  Carbonates react with dilute acids to create carbon dioxide.
• Halides:

  First add dilute nitric acid, followed by silver nitrate solution.

  Chloride gas gives a white precipitate.

  Bromide gives a cream precipitate.

  Iodide gives a yellow precipitate.
• Sulfates:

  First add dilute hydrochloric acid, followed by barium chloride solution.

  A white precipitate will form when sulfate ions are in this solution.
• Instrumental methods:

  Elements and compounds can be detected and identified using instrumental methods.

  These are accurate, sensitive, and rapid, making them advantageous compared to chemical tests.
• Flame emission spectroscopy:

  An example of an instrumental method used to analyse metal ions in solutions.

  The sample is put into a flame and the light given out is passed through a spectroscope.

  Output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.