C8 - Chemical Analysis

• Pure Substance

  • A single element or compound, not mixed with any other substances
• Boiling points for pure elements and compounds

  • Pure elements + compounds melt and boil at specific temperatures
  • We can distinguish a pure substance from mixtures through its melting point + boiling point
• Formulations

  • Mixtures that have been designed as a useful product
  • Individual substances are MIXED together; they do not create bonds e.g. cleaning agents, paints, medicines, alloys, fertilisers
• Chromatography

  • Can be used to separate mixtures and can give information to help identify substances
• Basic set-up for chromatography

  1. Chromatography paper
  2. Draw a pencil line 1 cm from the bottom
  3. Different inks are placed on the line
  4. Put a small volume of solvent into a beaker
  5. Place the chromatography paper in the solvent BELOW THE PENCIL LINE
  6. The solvent absorbs up the paper and must be STOPPED BEFORE THE SOLVENT REACHES THE TOP
• Rf Values

  • Distance moved by substance / Distance moved by the solvent
• Why are Rf Values useful?

  • Different compounds have different Rf values in different solvents, which can be used to identify the compounds.
• The trend of pure compounds in chromatography

  • Pure compounds will produce a single spot in all solvents
• 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.
• Why is the solvent meant to be below the start line in Chromatography?

  • Prevent the ink from dissolving into the solvent
• Why isn't the start line drawn in ink in Chromatography?

  • If it's drawn in ink, the ink will mix with the solvent