8-Chemical analysis

• chemical analysis - analytical chemistry

  • is about the instruments and methods used to separate, identify and quantify different substances
• formulations

  • formulations are mixtures that have been prepared using a specific formula
  • they contain precise amounts of different components, each component has a particular function
  • they can be - fuels, cleaning agents, paint, medicine, alloys fertilisers, food and drink
• pure substances

  • pure substance is one that only contains only one type of element or compound - it cannot be a mixture
  • they melt and boil at specific temperatures
  • impure substances melt and boil over a range of different temperatures
• testing - you can freeze or boil the substance to test if it is pure or impure:

  • pure - will melt or freeze at specific temperature
  • impure - will start to melt or freeze at one temperature and will finally complete physical reaction at a different temperature
• • physical test - physical properties
  • chemical test - with other chemicals, chemically
• separation of substances

  • crystallisation
  • chromatography
  • filtration
  • distillation
• chromatography is a chemical analysis technique used to separate substances in a mixture
• paper chromatography is used to separate mixtures of soluble substances in liquids
• paper chromatography - chromatogram

  1. draw a line in pencil (baseline) on filter paper at the bottom of the sheet
  2. add sample of ink on the line to test if it has different substances in it
  3. add a small volume of solvent in a beaker (water or ethanol)
  4. use clips to attach the paper to a stick and place it in the beaker so that the line is not submerged
  5. wait for the solvent to travel up the paper, the different dyes will dissolve and move up
• chromatography

  • mobile phase is a substance that the molecules can move in (liquid or a gas) - solvent
  • stationary phase is substance or materials that the molecules can't move in (solid or thick liquid) - paper
  • chemicals that are more soluble spend more time in the mobile phase so move faster and travel further up
  • chemicals that are less soluble spend more time in the stationary phase so move slower and travel less
  • insoluble substances will stay on the baseline
• Rf value = distance travelled by substance divided by distance travelled by solvent
• chromatography - pure substances

  • pure substance has one type of chemical in it
  • it won't separate out at all, there will just be a single spot

  
• • the Rf value for a substance is specific to a particular mobile phase (solvent) and stationary phase (paper)
  • if you change either the mobile or stationary phase, you'll get a different Rf value
• test for gases - chlorine

  1. fill a test tube with chlorine
  2. dampen blue litmus paper and put it in the test tube
  3. the litmus paper will turn from blue to white

  • the paper may briefly turn red before it is bleached because chlorine dissolves in the water and forms hydrochloric acid which is acidic
  • you must wear a mask or do the experiment in a fume cupboard as it is poisonous
• test for gases - oxygen

  1. fill a test tube with oxygen
  2. place a glowing splint into the tube
  3. the glowing splint will relight

  • the splint is lighted because burning requires oxygen
• test for gases - hydrogen

  1. fill a test tube with hydrogen
  2. place a burning/ lit splint in the test tube
  3. you will hear a squeaky pop

  • the heat energy provided by the flame causes the hydrogen to burn with the oxygen in the air to form water
• test for gases - carbon dioxide

  1. fill a test tube with carbon dioxide
  2. fill another test tube with limewater (calcium hydroxide)
  3. bubble carbon dioxide through the limewater
  4. limewater will turn cloudy

  • carbon dioxide and calcium hydroxide react to form calcium carbonates and water
  • it turns cloudy because calcium carbonate is a solid and the solid particles make the solution appear cloudy