chemical analysis (c8)

• Pure substance

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

  • They melt and boil at specific temperatures
  • Melting and boiling point data can be used to distinguish pure substances from mixtures
• Pure substance (everyday language)

  Substance that has had nothing added to it, so it is unadulterated and in its natural state
• Formulation

  Mixture that has been designed as a useful product
• Many products are complex mixtures in which each chemical has a particular purpose
• Test for hydrogen

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

  2. Creates a 'squeaky pop' sound (hydrogen burns rapidly)
• Test for oxygen

  1. Uses a glowing splint inserted into a test tube of the gas

  2. Splint relights in oxygen
• Test for carbon dioxide

  1. Bubble the gas through the limewater (calcium hydroxide (aq))

  2. It will turn milky (cloudy)
• Test for chlorine

  1. Use damp litmus paper

  2. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white
• Making formulations

  Mixing the components in carefully measured quantities to ensure the product has the required properties
• Chromatography

  Used to separate mixtures and 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
• Flame tests

  Can be used to identify metal ions
• If a sample contains a mixture of ions, some flame colours can be masked (you won't be able to see them)
• Rf value

  Distance moved by substance ÷ distance moved by solvent
• Metal hydroxides

  • Aluminium, calcium and magnesium ions form a white precipitate with NaOH
  • Only aluminium's precipitate dissolves when excess NaOH is added
  • Copper(II) produces a blue precipitate
  • Iron(II) produces a green precipitate
  • Iron(III) produces a brown precipitate
• Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
• Carbonates

  • Carbonates react with dilute acids to create carbon dioxide
  • This gas can be bubbled through limewater, if the limewater goes cloudy, the gas is CO2
• Identifying halides

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

  2. Chloride gives a white precipitate

  3. Bromide gives a cream precipitate

  4. Iodide gives a yellow precipitate
• 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
• Identifying sulfates

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

  2. 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
• Paper Chromatography

  Analytical technique separating compounds by their relative speeds in a solvent as it spreads through paper
• Flame emission spectroscopy

  • Example of an instrumental method used to analyse metal ions in solutions
  • 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
• Pigment

  Solid, coloured substance
• The more soluble a substance is, the further up the paper it travels
• Paper chromatography separates different pigments in a coloured substance
• flame tests

  lithium - crimson

  sodium - yellow

  potassium - lilac

  calcium - orange-red

  copper - green