Separation of liquids by distillation and by paper chromatography

There are a number of ways that you could separate liquids in Chemistry. This is an outline of the required steps to undertake one of these methods.

Aims

To separate liquids using fractional distillation and chromatography.

Fractional distillation

Method

In the first stage of ethanol distillation, water and ethanol solution is heated.Separating water and ethanol
  1. Place some anti-bumping granules in the bottom of the flask.
  2. Add a mixture of water and ethanol to the flask. Connect the flask to the fractionating column and secure it with a stand, boss and clamp.
  3. Attach a condenser to the top of the fractionating column, connect it to a cold water tap and sink, and secure it over a beaker.
  4. Heat the flask using an electric heater or a boiling water bath.
  5. Collect a small sample of the distilled liquid, then turn the heater off.

Results

  1. describe the appearance of the distilled liquid
  2. if the apparatus included a thermometer at the top of the column, record the maximum temperature reached as the liquid was collected

Analysis

  1. Place a 2 cm3 sample of the distilled liquid on a heatproof mat and apply a lighted splint.
  2. If the temperature was measured during the process, compare this to the boiling points of possible liquids in the mixture. These would be water or ethanol.

Evaluation

Discuss why fractional distillation is used to separate this sort of mixture.

Paper chromatography

Method

  1. draw a pencil line across the chromatography paper, 1 to 2 cm from the bottom
  2. use a pipette or capillary tube to add small spots of each ink to the line on the paper
  3. place the paper into a container with a suitable solvent in the bottom
  4. allow the solvent to move through the paper but remove the chromatogram before it reaches the top
  5. allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
A pencil line is drawn across a sheet of chromatography paper and spots of ink or plant dye are placed along it. The paper is held abovea basin containing solvent.

Results

Record the results in a suitable table. For example:

InkSpot colourDistance travelled by spot (mm)

Analysis

  1. Calculate the Rf value of each spot: Rf = \frac{\textup{distance~travelled~by~substance}}{\textup{distance~travelled~by~solvent}}
  2. Compare the Rf values and colours of each spot in the inks. Describe their similarities and differences.

Evaluation

Question

A student measured the distance moved by a spot to its lowest part instead of to its centre. How does this mistake affect the Rf value calculated for the spot?

It is smaller. The top line of the Rf expression is less, but the bottom line is the same.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardHarmPrecaution
Hot apparatusSkin burnsAllow apparatus to cool before touching it
EthanolFlammableOnly heat the ethanol-water mixture with an electric heater or in a water bath
Broken glassCutsBe careful when placing the fractional distillation apparatus in the clamp and do not overtighten the clamp
Harmful solventSkin irritationAvoid skin contact, eg wear gloves
Harmful solventBreathing difficultiesEnsure adequate ventilation or use a fume cupboard
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