purity and separating mixtures

• Relative atomic mass is the mean mass of an atom of an element compared to 1/12th of the mass of a carbon-12 atom
• A chemical formula tells you how many atoms of each element there are in a unit of a substance
• Relative formula mass is the total mass of all the atoms in a chemical formula, It's the mass of one molecule or formula unit of a compound compared to 1/12th the mass of a carbon-12 atom
• An empirical formula shows the simplest whole number ratio of the atoms of each element in a compound
• Calculating empirical formula for butane, C4H10:

  1. find the highest common factor - highest is 2
  2. divide the chemical formula by the highest common factor - C=4/2 H=10/2 C=2 H=5
  3. Write down the empirical formula - C2H5
• A balanced chemical equation shows the formulae and number of units for all the substances in a reaction e.g. CO2

  C + O2 = CO2

  12.0 + 32.0 = 44.0

  (1x12.0) + (2x16.0) = (12.0+16.0+16.0)
• A pure substance consists of just one element or compound
• an impure substance is a mixture as they contain one or more element or compound
• An alloy is a mixture of metal with one or more other elements
• The melting point of a substance is the temp at which it changes from solid or liquid
• the melting point of a pure substance is a single temp
• the melting point of an impure substance is higher than the melting point of a pure substance and often melts over a range
• A solution forms when a solute dissolves in a solvent. e.g. salt in water the solute is salt
• when a substance dissolves its particles separate and become completely mixed with the particles of the solvent
• filtration separates an insoluble solid from a liquid
• How does crystallisation work?

  If you heat a solution and allow the solvent to evaporate slowly you get crystals
• simple distillation separates a soluble solute from a solvent, it relies on the solvent having a much lower boiling point than the solute
• fractional distillation separates 2 or more substances from a mixture in the liquid state
• each substance separated by fractional distillation is called a fraction
• chromatography relies on 2 different chemical phases

  • a stationary phase that does not move
  • a mobile phase that does move
• in paper chromatography the stationary phase is absorbent paper, the mobile phase is a solvent in liquid state
• thin layer chromatography works the same but the stationary phase is a thin layer of silica or alumina powder
• gas chromatography: stationary phase is silica or alumina powder packed into a metal column, the mobile phase is an unreactive carrier gas
• TLC has some advantages:

  • it is quicker
  • it is more sensitive so a smaller sample cam be used
  • there is a larger range of stationary phases and solvents to chose from
• Filtration summary

  • separates solid from liquid
  • insoluble substance forms residue
  • solube substance goes through filter paper as filtrate
• crystallisation summary

  • produces dry sample of soluble substance
  • solution is heated
  • solvent evaporates
• simple distillation summary

  • separates solvent from a solution
  • evaporation -> cooling -> condensing
• fractional distillation summary

  • separates a liquid from mixture of liquids
  • evaporation -> cooling at different temps -> condensing
• chromatography summary

  • stationary phase
  • mobile phase
  • substances distributed between phases
  • distance travelled by substance/distance travelled by solvent