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Purification by Fractional distillation/crystallisation




Fractional distillation:


  • Take the mixture to be separated in an R.B flask.
  • Add some porcelain pieces in the flask to avoid bumping.
  • Fit the R.B flask with an adapter & attach a fractionating column.
  • Introduce a thermometer. (The top of the thermometer bulb should be just below the side arm of the distilling flask, ensuring that the entire thermometer bulb will be bathed in the rising vapour as the liquid mixture is heated).
  • Connect a Liebig's condenser to the side tube of the adapter.
  • Circulate water from the lower end of the condenser, prior to heating the R.B flask.
  • Gently heat the flask until the liquid begins to boil.
  • Lower the heating rate and heat slowly at a constant rate to permit a good separation.
  • Continue boiling the liquid until the temperature remains the same for about 3-4 minutes & then rises quickly.
  • Collect the distillate in a receiving flask.



Points to Remember while Performing the Experiment in a Real Laboratory:


  1. Always wear lab coat and gloves when you are in the lab. When you enter the lab, switch on the exhaust fan and make sure that all the chemicals and reagents required for the experiment are available. If it is not available, prepare the reagents using the components for reagent preparation.
  2. Properly adjust the flame of the Bunsen burner. The proper flame is a small blue cone; it is not a large plume, nor is it orange.
  3. Make sure to clean all your working apparatus with chromic acid and distilled water and ensure that all the apparatus are free from water droplets while performing the experiment.
  4. Make sure to calibrate the electronic weigh balance before taking the measurements.
  5. Ensure that the desiccator  has sufficient amount of desiccant; Silica gel
  6. Use chromic acid to clean the crucible, then heat it and make sure to cool it and before placing in the desiccators. Ensure that you are handling the crucible, with cleaned tongs or with tissue paper .Never touch it with your hand.
  7. Switch on the oven and adjust the temperature to 1300 C. Make sure to use a cotton glove while working with a hot air oven.
  8. Make sure to clean the Kipp's apparatus tube with water and ensure that it has sufficient solid material; iron sulfinide and acid, H2SO4 for producing H2S gas.
  9. Clean all glass wares with soap and distilled water. Once the experiment completed recap the reagent bottles. Switch off the light, exhaust fan, hot air oven and Gas cylinder before leaving the lab.
  10. Discard the used gloves in a waste bin.


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