GCSE Chemistry

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GCSE Chemistry
2012-08-21 11:43:21
unit two chromatography

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  1. chromatography
    a scientific method of finding what separate substances are in a mixture by making it flow, as a liquid or gas, through a material such as paper which different substances pass through at different speeds
  2. retention time
    the time taken for a substance to reach the detector at the end of a gas chromatography column 
  3. mass spectroscopy
    an analytical technique that involves breaking molecules into chraged fragments and measuring their mass/charge ratio . Also know as mass spectrometry 
  4. paper chromatography is used to 
    analyse coloured substances, such as the coloured pigments in plants and artificial colours used as food additives 
  5. why does paper chromatography work
    because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.
  6. Chromatography is used by scientists to
    • Analyze – examine a mixture, its components,  and their relations to one another
    • Identify – determine the identity of a mixture or  components based on known components
    • Purify – separate components in order to isolate one of interest for further study
    • Quantify – determine the amount of the a mixture  and/or the components present in the sample
  7. Real-life examples of uses for chromatography :
    • Pharmaceutical Company – determine amount of   each chemical found in new product
    • Hospital – detect blood or alcohol levels in a patient’s blood stream
    • Law Enforcement – to compare a sample found at  a crime scene to samples from suspects
    • Environmental Agency – determine the level of   pollutants in the water supply
    • Manufacturing Plant – to purify a chemical   needed to make a product
  8. Chromatography separates the components of a mixture by their
    distinctive attraction to the mobile phase and the stationary phase.
  9. explain how the mobile phase and stationary phase are interlinked
    • Compound is placed on stationary phase
    • Mobile phase passes through the stationary phase
    • Mobile phase solubilizes the components
    • Mobile phase carries the individual components a certain distance through the stationary phase, depending on their attraction to both of the phases
  10. in paper chromatography what is the stationary phase
    solvent (such as water)
  11. in paper chromatography what is the mobile pahse
    paper strip
  12. how can artificial colourings added to foods be analysed using paper chromatography
    a sample of the colour from the food is placed on a piece of chromatography paper along with samples of known dyes , then a solvent is added. the solvent soaks up the paper taking the dyes with it . The more soluble the colouring is , the further up the paper it travels . Different colourings move different distances up the paper
  13. The more soluble the colouring is
    the further up the paper it travels
  14. why do we hang chromatography paper in water during paper chromatography
    because the water is the solvent and travels up the paper taking the substance with it
  15. why must the solvent be below the spots during paper chromatography
    so the substances don't dissolve in the water
  16. why was the line drawn in pencil
    so the substances dont dissolve in the water
  17. why must you wait until the water is near the top of the paper before taking it out to dry
    to allow the dyes to seperate completely
  18. why do the spots travel at different distances 
    because the dyes have different solubilities in the solvent (mobile phase) and they bond/stick to the paper (stationary phase) at varying degrees .
  19. a very common instrumental method is 
    gas chromatography 
  20. gas chromatography can 
    seperate out a mixture of compounds and help you identify the substances present 
  21. what is Rf value
    • Distance moved by spot
    • --------------------------------

    Distance moved by solvent
  22. label the diagram
    • a = cylinder of carrier gas with valve to control the flow 
    • b = sample injector
    • c = collector in which components are condensed 
    • d = detector 
    • e = chromoatogram
    • f = column in temperature controlled oven 
  23. what is the purpose of A 
    it contians inert gas that is the mobile phase 
  24. what is the purpose of f
    it is the column paccked with solid material , it is the stationary phase 
  25. what is the purpose of D 
    it is called a chromatogram . The number of peaks tells us the number of compounds present in the mixture . The retention time varies with the different components and is used to identify the substance . The relative are under each peak tells us the relative quantity of each component 
  26. how can substances be seperated by gas chromatography
    the sample is injected into the machine and is vapourised . The sample passes through a long column packed with a solid that is wound into a coil (stationary phase) . An inert gas (unreactive) (mobile phase) such as nitrogen , is passed through the column to move the sample through . Different substances in the mixture travel through the column at different speeds as they have different affinities for the mobile and stationary phase . This means that they reach the end of the column at different times and so are sepearted . 
  27. the greater the affinity the substance has for the mobile phase the 
    quicker the retention time 
  28. the greater the affinity the substance has for the stationary phase the
    slower the retention time 
  29. a mass spectroscopy can 
    identify tiny amounts of a substance quickly and accurately it does this by measuring the mass of the particles in the substance 
  30. in mass spec the molecules
    lose an electron to form a molecular ion . the Mr of the substance = the mass of the molecular ion . often the molecular ion breaks apart and other lighter fragments are detected aswell .
  31. a mass spec is often attatched 
    to a gas chromatography machine . after a smaple has been seperated by chromatography , the mass spectrum of each compound can be recorded allowing it to be identified 
  32. which peak gives the Mr and what is the substance 
    the peak furthest to the right is the molecular ion peak and gives the Mr of the substance - this means the Mr of this substance is 204 , this substance is mercury