L13 - 15 - Separation Techniques

objectives

Simple Distillation

—This separates a liquid from a solution e.g. separates water from salty water

—This works because water evaporates from the solutions, but is then cooled and condensed in a separate container

—Salt does not evaporate so it is left behind

The reason they separate is because the Salt has a very high boiling point due to the fact it is an ionically bonded compound - the water is a covalent compound and thus only the weak intermolecular forces need to be broken to change it's state

As different compounds in mixtures retain their properties even when mixed this still means that water has a boiling point of 100oC where as the Salt has a boiling point of many 1000's of degrees!

The apparatus above can be used to separate a liquid solvent from it's solution:

• 1 = Bunsen Burner (or heat source)
• 2 = Round Bottomed Flask (or Buchner Flask) - allows even heating across the bottom of the glass
• 3 = Delivery Tube
• 4 = Thermometer - used to tell the boiling point of the liquid
• 5 = Condenser - cools down the gaseous vapors so they condense into a liquid
• 6 + 7 = show how the cold water flows into the condenser
• 8 = collection vessel - usually a conical flask but in this case another round bottomed flask
• 9 + 10 = An open vacuum adaptor used to allow air out of the system

Fractional Distillation

—This can be used to separate two or more liquids e.g. ethanol and water

—This works because the different liquids all have different boiling points

—Fractional distillation is used to separate crude oil into useful liquids such as petrol

When a mixture contains many different fractions (i.e. has more than two compounds) then we use fractional distillation.

This is a more precise version of distillation where we separate the compounds in the mixture by their boiling points - similarly to above but not as simple as heating until a gas forms as many gases will form if you heat it too quickly.

We would heat the mixture until one gas comes off at a time - the collection vessel would then be replaced before we continue to heat to the nxt gas comes off the mixture.

Filtration

—This is used to separate an insoluble solid from a liquid

—It can remove any unreacted chemicals, by-products or solvents

—Another way to separate a solution is to make crystals

—This involves evaporating the solution to a much smaller volume and then leaving it to cool

—Crystals then form

Magnetism

Chromatography

—Can be used to separate mixtures of soluble compounds

—Mixtures suitable for separation by chromatography include inks, dyes and colouring agents in food

Method:

1.Simple chromatography is carried out on paper

2.A spot of the mixture is placed near the bottom of a piece of chromatography paper

3.The paper is then placed upright in a suitable solvent, such as water

4.As the solvent soaks up the paper, it carries the mixtures with it

5.Different components of the mixture will move at different rates - this separates the mixture out

6. As different compounds travel different distances they separate

7. When this distance is compared to the distance the solvent travels it is called the Rf value

Chromatography and Rf Values

—The Rf value of a particular compound is always the same if the chromatography has been carried out in the same way

—This allows industry to use chromatography to identify compounds in mixtures

—Chromatography can also be done when the different substances in the mixture are colourless

—The chromatogram can be exposed to a locating agent, which reacts with the invisible chemicals so that they can be seen.

R_f = distance moved by the compound ÷ distance moved by the solvent