Topic C8

• • When a subject is pure, it isn't mixed with anything and is in its natural state.
  • A pure substance will melt or boil at a specific temperature
  • Impurities will lower the melting point and increase the melting range of a substance
  • Impurities will also increase the boiling point and may result in your sample boiling at a number of temperatures
• Paper Chromatography


  An analytical method to separate and identify the substances in a mixture
• Chromatography


  1. Uses two phases

  2. Mobile phase where the molecules move

  3. Stationary phase where the molecules get separated
• There are different types of chromatography, but they all have two phases
• Mobile phase


  The phase where the molecules move
• Stationary phase


  The phase where the molecules get separated
• Chromatography experiment


  1. Substances in the sample constantly move between the mobile and stationary phases

  2. An equilibrium is formed between the two phases
• How quickly chemical substances are separated

  Depends on whether they spend more time in the mobile phase or the stationary phase
• The chemicals that spend more time in the mobile phase will move further up the paper
• As long as all the components spend different amounts of time in the mobile phase, the number of spots may change in different solvents
• A substance will only ever form one spot in any solvent as there is only one substance in the sample
• Paper chromatography


  1. Stationary phase is the chromatography paper

  2. Mobile phase is the solvent (e.g. ethanol or water)
• Time molecules spend in each phase


  Depends on solubility in the solvent and attraction to the paper
• Molecules with higher solubility in the solvent and less attraction to the paper will spend more time in the mobile phase and be carried further up the paper
• Carrying out paper chromatography

  Method is on page 100
• 1. value


  Ratio between the distance travelled by the dissolved substance (the solute) and the distance travelled by the solvent
• Calculating R-value


  R₁ = distance travelled by substance (B) / distance travelled by solvent (A)
• Chromatography is often carried out to see if a certain substance is present in a mixture
• To do this, you run a pure sample of that substance (a reference) alongside the unknown mixture
• If the R-values of the reference and one of the spots in the mixture match, the substance may be present (although you haven't yet proved they're the same)
• The R-value is dependent on the solvent - if you change the solvent the R-value for a substance will change
• You can test both the mixture and the reference in a number of different solvents
• If the R-value of the reference compound matches the R-value of one of the spots in the mixture in all the solvents, then it's likely the reference compound is present in the mixture
• If the spots in the mixture and the spot in the reference only have the same R-value in some of the solvents, then the reference compound isn't present in the mixture
• You can't see the chemicals moving between the two phases, but it does happen
• A spot on a chromatogram is called a chromatogram
• There are tests for chlorine, oxygen, and carbon dioxide
• Tests for 4 Common Gases

  • Chlorine
  • Oxygen
  • Carbon Dioxide
  • Hydrogen
• Chlorine test


  Chlorine bleaches damp Litmus paper, turning it white. (It may turn red for a moment first though that's because a solution of chlorine is acidic.)
• Oxygen test


  If you put a glowing splint inside a test tube containing oxygen, the oxygen will relight the glowing splint.
• Carbon Dioxide test


  Bubbling carbon dioxide through (or shaking carbon dioxide with) an aqueous solution of calcium hydroxide (known as limewater) causes the solution to turn cloudy.
• Hydrogen test


  If you hold a lit splint at the open end of a test tube containing hydrogen, you'll get a "squeaky pop". (The noise comes from the hydrogen burning quickly with the oxygen in the air to form H₂O.)