chemical analysis

• Pure substance


  A single element or compound, not mixed with any other substance
• Formulation


  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
• Examples of formulations


  • Fuels
  • Cleaning agents
  • Paints
  • Medicines
  • Alloys
  • Fertilisers
  • Foods
• Pure substance (in everyday language)

  A substance that has had nothing added to it, so it is unadulterated and in its natural state
• 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
• Melting point of a pure substance


  A fixed temperature
• Melting point of a mixture or impure substance


  A range of temperatures, not a sharp melting point
• Melting point curve of a pure solid


  1. Solid starts melting at this point

  2. The temperature does not rise when the solid is melting because the heat is absorbed to break the bonds between the solid particles

  3. The temperature will start to rise again when all the solid has melted
• Chromatography


  A technique used to separate mixtures and can give information to help identify substances
• Paper chromatography


  1. A solvent moves through the paper carrying different compounds different distances

  2. The distance the substance moves depends on their attraction for the paper and their solubility in the solvent

  3. If a compound is more soluble in the solvent it will move further up the paper

  4. If a compound is not soluble in the solvent it will not move up the paper

  
• Required practical method for chromatography


  1. Draw a pencil line 1 cm above the bottom of the paper and mark spots for each sample, equally spaced along line

  2. Add a tiny drop of each substance to a different spot

  3. Add solvent to beaker with a lid so that is no more than 1cm in depth

  4. Place the paper into the beaker, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal

  5. When the level of the solvent reaches about 1 cm from the top of the paper, remove the paper and mark the solvent level with a pencil

  6. Calculate the Rf values of the observed spots
• Rf value


  The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent

  
• Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
• A pure compound will produce a single spot in all solvents
• Hydrogen gas test


  Hydrogen burns rapidly with a pop sound
• Oxygen gas test


  A glowing splint is relighted in oxygen
• Carbon dioxide gas test


  When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy)
• Chlorine gas test


  Damp litmus paper is bleached and turns white
• Flame tests


  1. A nichrome wire or damp splint is dipped in the substance to be tested then put in Bunsen flame and the colour is observed

  2. The wire should be cleaned in acid before/between use

  3. lithium compounds– crimson red flame

  4. sodium compounds - yellow flame

  5. potassium compounds - lilac flame

  6. calcium compounds – orange red flame

  7. copper compounds -green flame
• Sodium hydroxide solution test

  1. Copper(II) forms a blue precipitate

  2. iron(II) forms a dirty green precipitate

  3. iron(III) forms a brown precipitate

  4. Aluminium, calcium and magnesium ions form white precipitates but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution to form a colourless solution
• Carbonates (CO3^2-) react with dilute acids to form carbon dioxide, which turns limewater milky
• Halide ion test with silver nitrate


  1. Silver chloride is a white precipitate

  2. silver bromide is a cream precipitate

  3. silver iodide is a yellow precipitate
• Sulfate ion test with barium chloride


  Produces a white precipitate
• Instrumental methods


  Accurate, sensitive and rapid techniques used to detect and identify elements and compounds, particularly useful when the amount of sample is very small
• Flame emission spectroscopy


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

  2. The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations
• Each element has its own unique line spectra (fingerprint)