Purification and Separation Techniques

Purification:
Process of making something pure / removing foreign elements.
Separation:
Process of transforming a mixture into two or more products

Extracted from wikipedia

Thus, separating a mixture is not the same as purifying it. The mixture can be separated into 2 or more components but it may not be pure (it does not consist of that and only THAT component)

But why purify?

This is because most naturally-occuring substance in nature are mixtures. We require purity in things that we use or eat, like medicine.

Recall the topic on Elements, Compounds and Mixtures:

Compounds [E.g: Sodium chloride] can only be separated chemically but mixtures [rojak, sugar solution, Milo] can be separated easily by physical means. Thus separation techniques are ONLY used to separate mixtures.

There are many separation techniques. Some are listed below (click on them to link to wikipedia to find out more):

These are the important techniques that YOU need to really know:

1. Filtration
2. Evaporation to dryness
3. Crystallization
4. Distillation / Fractional distillation
5. Chromatography
   
   These are covered in class:
   
   
6. Sublimation
7. Oil-water separation using a separating funnel
   (the separation explained in Wikipedia is not needed for your level)
   
   Others:
   
   
8. Adsorption
9. Centrifugation and Cyclones - density differences
10. Decantation
11. Demister (Vapor) - removing liquid droplets from gas streams
12. Dissolved air flotation
13. Electrophoresis - Organic molecules, such as protein are placed in a gel. A voltage is applied and the molecules move through the gel because they are charged. The gel restricts the motion so that different proteins will make different amounts of progress in any given time.
14. Elutriation
15. Extraction
    - Leaching
    - Liquid-liquid extraction
    - Solid phase extraction
16. Flocculation - density differences utilization a flocculant such as soap or detergent
17. Fractional freezing
18. Oil-water separation - gravimetric separator used to remove suspended oil droplets from wastewaters in oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industries.
19. Precipitation
20. Recrystallization
21. Sedimentation
    - Gravity separation
22. Sieving
23. Stripping
24. Vapor-liquid separation - designed by using the Souders-Brown equation
25. Winnowing
26. Zone refining

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1. Filtration

It is a solid-liquid separation. A mixture of solid and liquid [E.g: sand + water] is poured through the filter paper into a beaker.

Residue: The solid that remains on the filter paper
Filtrate: The liquid that is collected

Filtration is not limited to solid-liquid separations such as sand and water. There is also a filtration system present in air-conditioning systems, where dust and dirt particles are cleansed from the air by the air filter.

Can you think of other examples that also uses filtration as a separation technique to separate an impure mixture?

2. Evaporation to dryness

After a solid-liquid filtration [E.g: sand + water], evaporation to dryness is sometimes used. The removal of water can be done by heating the substance. However this method can only be used if the solid or substance is non-volatile and stable. If otherwise, evaporation to dryness cannot be used.

This is due to 2 main reasons:

• The substance contains w_ _ _ _ of crystallisation.
  [E.g: hydrated copper(II) sulphate ]
  
• The substance decomposes / breaks down easily
  [E.g: sugar]

Thus other than sand and salt (unless otherwise specified that the substance is stable), all other substances are assumed to be not stable enough to undergo evaporation to dryness.

3. Crystallisation

Just like evaporation to dryness, crystallisation is also sometimes used together with filtration. Unlike evaporation to dryness which can be used for insoluble (sand) or soluble (salt) substances, depending on how fragile the substances are, crystallisation consist of a soluble substance dissolved in a liquid to form a solution.

Solute + solvent -----> Solution

[ E.g: Sugar + water -----> Sugar solution ]

One of the pre-requisite for crystallisation to occur is that the substance has to be soluble in the solvent. It is thus not possible for sand to crystallise. Do you see huge lumps of sand crystals on the beach?

Perhaps you can think of crystallisation as a 'growing' process. However, even though they do 'grow', they are not alive ! There are other factors before something can be considered a living thing. Recall your primary school science, do sugar crystals breathe or take in food or respond to changes around them?

For crystallisation to occur or for crystals to appear, you must first obtain a saturated solution.

(to be continued...)