1. Purification of Substances

2. Know their principles, procedures and application
Objectives:
The need for pure substance
Criteria for purity
Methods of Purification:
Filtration
Crystallization
Know their principles, procedures and application

3. The NEED for pure substance
Importance of purity of substances used in everyday life:
Chemist need pure substances to study their properties.
Pure substances are used in industry to make useful products such as food and drugs.

4. The NEED for pure substance
In pharmaceutical industry, medicines must be tested for purity before they are sold. Impurities in drugs and food may produce undesirable side effects.

5. The NEED for pure substance
In highly precision engineering e.g. Production of silicon chips, even small amount of impurities can greatly reduce the effectiveness of a component in an electronic device.

6. The NEED for pure substance
In food and beverage industry, the types of chemicals which can be added into our food are controlled by certain governmental guidelines to ensure that our food and drinks contain chemicals that are safe for consumption.

7. Criteria for purity
A pure substance is made up of only one substance and not mixed with anything else.
In nature, many substances are not pure. Most are found impure as mixtures.

8. The need for pure substance
A mixture is a substance that contains two or more substances.
The substances do not react with one another chemically.
They can be easily separated into pure substances by purification techniques.
(Physical method)

9. Criteria for purity Pure Substances Mixtures Not Definite Properties
Boiling Point
Exact and Fixed
Not Fixed (Higher)
Exact and Fixed
Not Fixed (Lower)
Melting Point

10. Criteria for purity
Impurities affect the melting and boiling point in the following ways:
1) Decrease the melting point;
E.g. Frozen sea water melts below 0°C
at -2.5°C
2) Increase the boiling point
E.g. Seawater boils at about 102°C

11. Criteria for purity 3) Increase the range at which melting
and boiling occurs;
E.g. Coconut oil melts over a range
of temperature. Starts melting at
14°C and complete melting at 22°C.
Petrol fuel for motorcars boils over a
range of 35°C to 75°C.

12. Some important terms Some terms that you should know:
A solute is a dissolved substance (e.g. salt in salt solution).
A solvent is a substance which dissolve a solute (e.g. water in salt solution.
Miscible liquids are liquids that are completely soluble in each other.

13. Mixtures Solutions and suspensions
What is a solution?
A solution is a mixture that appears to be made of one substance.
copper (II) sulphate solution, fizzy drinks
What is a suspension?
A suspension is a mixture in which two or more parts can be seen with our eyes
oil + water, muddy water, chalk water

14. How do we obtain Pure substances from a mixture?
We have to first consider what are characteristics of the substances involved before we decide on the suitable method of purification.

15. Possible scenarios of mixtures!
To separate a soluble and insoluble solid (E.g. Salt and sand)/ To separate insoluble solids from a solution (E.g. Sand from water)
To separate pure solid from a solution (E.g sugar from sugar solution)

16. Possible scenarios of mixtures!
To separate or identify colors, dyes and pigment.
To obtain a pure liquid from a solution (E.g. water from salt solution).
To separate miscible liquids with different boiling points.

17. Types of separation techniques
Aim: To separate a soluble and insoluble solid (E.g. Salt and sand)/ To separate insoluble solids from a solution (E.g. Sand from water)
Suitable method of purification:
Filtration

18. Solid-Solid mixture
Solid-Liquid mixture
Filtration
1. Glass rod
3. Mixture
2. Retort stand
4. Filter paper
5. Filter funnel
Filtration is usual method of separating solid from a liquid.
6. Beaker
7. Filtrate

19. Filtration – How it works
Solid-Solid mixture
Solid-Liquid mixture
Filtration – How it works
Mixture poured through filter paper with tiny
holes (“pores”).
Filter paper with small holes
Small particles of liquid
Large particles of solid
Large particles of the solid gets
trapped.
- Residue
Small particles of liquid passes through the filter.
- Filtrate

20. Filtration – How it works
Solid-Solid mixture
Solid-Liquid mixture
Filtration – How it works
Applications:

21. Solid-Solid mixture
Solid-Liquid mixture
Filtration
What happens to the filtrate? Can filtration be done to separate solids dissolved in solution?

22. Notes Can Dyes be separated by filtration?
Can salt be obtained from a mixture of salt solution by filtration?
Dyes in ink go through the filter As dye molecules are smaller than the pores in the filter paper and go straight through them.
Salt in sea water cannot be separated from water by filtration, as the sodium chloride molecules are much smaller than the pores in the paper.

23. Solid-Liquid mixture
Evaporation
Aim:
To obtain a solute from its solution by vaporising ALL the solvent. (E.g. To obtain salt from salt solution.)
How it works?
When a solution is heated, only the solvent boils away while the solute remains. Evaporation is a process where ALL the liquid has been vaporised.

24. Solid-Liquid mixture
Evaporation
When a solution is heated, the liquid (solvent) evaporates, leaving behind the solid (solute) as residue.
You will have only the solid component, as the liquid component is lost as vapour to the surrounding.
salt solution
evaporating dish
Salt solution
heat

25. Evaporation 2 types of evaporation technique: Boiling to dryness
Solid-Liquid mixture
Evaporation
2 types of evaporation technique:
Boiling to dryness
Evaporation using water bath (slower)

26. There is Always conditions involved!!!
Solid-Liquid mixture
Evaporation
There is Always conditions involved!!!
Conditions!!!!!
This method cannot be used for substance which will decompose upon strong heating.
E.g. Sugar, potassium nitrate etc cannot be obtained from their solution by evaporation them to dryness. They will decompose.

27. Solid-Liquid mixture
Crystallisation
Aim:
To obtain PURE solids from a solution (solid will decompose upon strong heating).
Do you know others?
E.g. To obtain sugar from sugar solution, Copper (II) Sulphate from Copper (II) Sulphate solution.
solution

28. CRYSTALLISATION

29. Crystallisation Often after filtration. The solution must be
Solid-Liquid mixture
Crystallisation
Often after filtration.
The solution must be
saturated.
Pure crystals appear when the hot solution cools.
Crystals are then collected and dried between filter papers. 1. 2. 3. 4.
Glass Rod
Evaporating dish
Solution
Crystals
Solvent
Impure Solid
Crystals
Filter paper
Heat the solution until MOST of the solvent evaporate off. Allow it to cool. (Steps to make solution saturated)
Dissolved solid appears as pure crystals as the solution cools. Impurities remain dissolved in the solution.

30. Crystals Solid-Liquid mixture Sugar crystals Salt crystals
Copper(II) sulphate crystals

31. Have you noticed the difference of evaporation from Crystallisation?
In a Solid-Liquid mixture, both Evaporation and Crystallisation is used to obtained solid from its solution.
Evaporation
Crystallisation
Does solid decompose upon strong heating? No
Yes
How much solvent have to be vaporised?
All
Most

32. Summary: Evaporation & Crystallization
To obtain a soluble solid from a liquid in solid-liquid mixture
Evaporation
Crystallization
If solid decomposes on heating
If solid does not decompose on heating

33. Questions 1. Which of these is a pure substance?
Salt, Cooking oil, Tea leaves, Milk
A liquid is likely to be pure if
A. it is colourless.
B. it boils at an exact temperature.
C. it dissolves in water.
D. it is neutral.

34. Questions
Which of these techniques would produce the largest crystals from an aqueous solution of copper (II) sulphate?
A) Allowing the solution to evaporate naturally.
B) Boiling off the water from the aqueous solution.
C) Boiling off the water until crystals just start to appear.
D) Heating the solution strongly then cooling it using ice.

35. Questions A) dissolve – filter – evaporate -crystallise
4. Which of the following is the correct order for purifying potassium nitrate from impure saltpetre (naturally occuring potassium nitrate) using water as a solvent?
A) dissolve – filter – evaporate -crystallise
B) dissolve – evaporate – crystallise - filter
C) filter – dissolve – evaporate - crystallise
D) evaporate – crystallise – dissolve - filter

36. Paper Chromatography Aim: To separate and identify mixtures of dyes.
Definition:
Paper chromatography is used to
separate and identify small amount of
the substances that are soluble in the
same solvent.

37. Paper Chromatography
At the Start
At the End
Chromatogram

38. Paper Chromatography Solubility in solvent
Based on differences in relative SOLUBILITIES of the mixtures in a particular solvent.
The more soluble the dye in the mobile solvent, the further the dye travels.
The less soluble the dye in the mobile solvent, the less the dye travels.
Solubility in solvent

39. Interpreting Chromatograms
P Q R S X Y
soft drinks dyes
Which of the soft drinks contain a harmful dye?
A) P and Q B) R and S
C) Q and S D) P and R