This presentation is based on the Cambridge IGCSE Chemistry syllabus but is helpful for all. It covers various separation techniques such as filtration, crystallization, chromatography and many more. It explains all concepts. Advanced topics are also covered such as separation of colorless substances and using retention factor.

1. Separation and
purification,
Chromatography
Chapter 21: experimental design and separation techniques

2. Learning
Objectives

3. Mixtures

4. What are
mixtures?
● Contains more than one substance
● Not chemically combined
● Can be separated by physical methods
● Mixtures in which solute is uniformly
distributed over the liquid solvent are
called solutions.
● Solute is dissolved in a solvent
● Solvent is liquid to which solute is added

5. Solutions
● solute + solvent = solution
● Particles of solute fit between particles of solvent
● Solubility of substances depends on their particles
● Increasing temperature or stirring = increasing solubility
● Solutions with water as solvent are aqueous.

6. Separation of
mixtures

7. Why are
mixtures
separated?
● It is the first step to remove impurities
● Helps us get pure substances
● To obtain products after a reaction
● It is crucial for our health

8. Filtration ● To separate insoluble solid and liquid mixtures
● Solid is trapped in filter paper
● Liquid passes (filtrate)

9. Crystallization ● To separate soluble solid liquid mixtures
or solutions
● Works because soluble solids tend to be
less soluble at lower temperatures.
● Solid is removed from saturated solutions
● Solute forms crystals at lower
temperatures
● Very useful in obtaining salts

11. Evaporation
● Separate solid liquid mixtures (solution)
● Used when the solubility of substance does not decrease at lower temperature
● The solvent evaporates leaving solute

12. Separating a mixture of
two solids
● Forming a solution that dissolves only one solid
● Example: sand and salt
○ Add water to the mixture, and stir. The salt dissolves
○ Filter the mixture. Sand is trapped in filter paper, but the
salt solution passes
○ Rinse the sand with water, and dry it in an oven
○ Evaporate the water from the salt solution, to give dry
salt.
● For sugar and salt ethanol is used which dissolves
sugar only

13. Simple distillation
● To obtain the solvent from a solution
● It could be used to obtain water from
salt water, for example
● Many countries in the Middle East
obtain drinking water by distilling
seawater in giant distillation plants

14. 1
2
3
Method
Heat the solution in the flask. As it boils,
water vapour rises into the condenser,
leaving salt behind.
The condenser is cold, so the vapour
condenses to water in it.
The water drips into the beaker. It is
called distilled water. It is almost pure.

15. Apparatus

16. Fractional distillation
separate a mixture of liquids makes use of their different boiling
points
Used in petroleum industry to refine
crude oil
Used to separate gases in a liquified air

17. Method 1. Heat the mixture in the flask. At about 78 °C, the
ethanol begins to boil. Some water evaporates too. So
a mixture of ethanol and water vapours rises up the
column.
2. The vapours condense on the glass beads in the
column, making them hot.
3. When the beads reach about 78 °C, ethanol vapour
no longer condenses on them. Only the water vapour
does. So water drips back into the flask. The ethanol
vapour goes into the condenser.
4. There it condenses. Pure liquid ethanol drips into the
beaker.
5. Eventually, the thermometer reading rises above 78
°C – a sign that all the ethanol has gone. So you can
stop heating.

18. Paper
chromatography
● To separate two or more solids that are
soluble
● Makes use of different solubilities
● Black ink is a mixture of different colors which
could be separated by chromatography

19. How it works
Depends on how substances interact with
the solvent and chromatography paper.
The more soluble a substance is in the
solvent, the further it will travel up the
chromatography paper

20. Paper chromatography to
identify substances
Mixture X is thought to contain substances
A, B, C, and D
We can check it using chromatography

21. Chromatography to identify colorless substances
1. 5 solutions are given of amino acids
2. Identify the amino acids
3. Place spots of the 5 solutions on pencil line on chromatography paper
4. Roll chromatography paper and place it in a beaker with the solvent
5. Solvent rises up the paper. When it has almost reached the top, remove the paper
6. Mark a line in pencil on it, to show where the solvent reached.
7. Put the paper in an oven to dry out
8. Next spray it with a locating agent to make the amino acids show up. Ninhydrin is a
good choice.
9. After spraying, heat the paper in the oven for 10 minutes. The spots turn purple.

22. continued...
10. Mark a pencil dot at centre of spots of amino acids
11. Find Rf value = distance moved by amino acid / distance moved by solvent
12. Finally, look up Rf tables to identify the amino acids

23. Separation Techniques covered:

24. Thanks a lot!