OCR GCSE Chemistry Notes | C2

  • Pure substance: a single element or compound, not mixed with any substance.
  • Pure substances melt and boil at specific temperatures so compare data.
  • Relative formula mass: add up the mass numbers that make a compound.
  • Empirical formula steps:
  • Relative atomic mass: ratio of the average mass of an atom compared to 1/12th of a carbon-12 atom.
  • Relative molecular mass: ratio of the average mass of a molecule of an element or compound to 1/12th of a carbon-12 atom.
  • Relative formula mass: weighted average of the masses of the formula units relative to 1/12th of a carbon-12 atom.
  • Mixtures: two or more elements not chemically combined
  • Formulation: a mixture for a specific purpose e.g. paint
  • Alloys:
  • Filtration: use a funnel and filter paper to separate precipitates from a solution.
  • Crystallisation: to separate a soluble salt from a solution; heat, evaporate, allow solution to cool, dry and form crystals.
  • Simple distillation: separate a solvent from a solution; one of them evaporates and escapes and condenses separately.
  • Fractional distillaton:
  • Chromatography: used to separate mixtures and identify substances.
    • Stationary phase e.g. paper
    • Mobile phase e.g. solvent
  • Rf Value: distance of substance/ distance of solvent (under 1).
  • TLC (thin layer chromatography): stationary phase is a thin, inert substance.
  • We can also use chromatography to check if something is pure as it will only produce one spot.
  • Gas chromatography:
    • Stationary phase: liquid/ solid on a solid support
    • Mobile phase: inert carrier gas
    • We want to see how soluble a susbstance is in the gas.