C8 - Chemical Analysis

• Pure Substances


  In chemistry, a pure substance is a single element or compound, not mixed with any other substance.Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
• Formulations

  A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties. Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
• Chromatography


  Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases.The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its Rf value.
• Chromatography - Rf values

  Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
• Test for hydrogen


  The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
• Test for oxygen


  The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
• Test for carbon dioxide

  The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
• Test for chlorine

  The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
• Flame Tests

  Flame tests can be used to identify some metal ions (cations). Lithium, sodium, potassium, calcium and copper compounds produce distinctive colours in flame tests:

  • lithium compounds result in a crimson flame
  • sodium compounds result in a yellow flame
  • potassium compounds result in a lilac flame
  • calcium compounds result in an orange-red flame
  • copper compounds result in a green flame.

  If a sample contains a mixture of ions, some flame colours can be masked
• Test for metal hydroxides

  Sodium hydroxide solution can be used to identify some metal ions (cations).Solutions of aluminium, calcium and magnesium ions form white precipitates when sodium hydroxide solution is added but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.Solutions of copper(II), iron(II) and iron(III) ions form coloured precipitates when sodium hydroxide solution is added.Copper(II) forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.
• Test for carbonates

  Carbonates react with dilute acids to form carbon dioxide gas. Carbon dioxide can be identified with limewater.
• Test for halides

  Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid. Silver chloride is white, silver bromide is cream and silver iodide is yellow.
• Test for sulfates


  Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
• What are instrumental methods?

  Elements and compounds can be detected and identified using instrumental methods. Instrumental methods are accurate, sensitive and rapid.
• Flame emission spectroscopy

  Flame emission spectroscopy is 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. The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.