The art of separation
Yasmin Jayathirtha
In the last column, we had looked at chemical reactions. Under the same conditions, some substances will react, some will just change state, some may remain unchanged. To study reactions, we have to be sure that the chemicals we are studying are pure, as we need to know that the reactions are because of a particular substance, and not because of anything else in the mixture. There are standard methods of separation, some used in everyday life, some specialized. To demonstrate the various methods, let’s consider the kind of mixtures that can be present:
| Type of mixture | Method of separation | Examples |
|---|---|---|
| solid/solid | any difference in properties like solubility, magnetic behaviour, | sea salt*, iron and aluminium, copper sulfate commercial grade* |
| solid/liquid insoluble | muddy water, petrol and salt | filteration |
| solid/liquid soluble | sea water*, sugar solution | distillation, fractional crystallization, chromatography* |
| liquid/liquid immiscible | oil and water,* petrol and water | using a separating funnel |
| liquid/ liquid miscible | alcohol and water, crude oil, felt pens* | fractional distillation, gas chromatography# |
# very specialized technique
* will be discussed
Sea water is an interesting mixture, it has many salts dissolved in it and will usually contain some sand, i.e., it is an example of both soluble and insoluble solid/liquid mixtures. You can make your own sea water by dissolving sea salt (available in the market) in water. About 7g in 200 cm3 will make an acceptable replacement to sea water. The solution will be cloudy because of the sand and mud in the sea salt. Filteration will give a clear solution, containing many salts. These can be separated by fractional crystallization.
Take the clear solution of sea water and heat it till the volume reduces to about 60-70cm3. Some crystals will form by this time. Filter out the crystals and heat the remaining solution till it is reduced to ~30cm3. Filter and continue heating till all the water goes.
This shows fractional recrystallization, i.e., you can separate soluble solids using their differing solubilities in water. Do keep the samples after drying them; they can be used for analysis at a later stage.
I had advised you that copper sulfate can be bought cheaply at hardware stores. This will not be very pure, and will need to be purified by crystallization. Dissolve the sample of copper sulfate in a minimum amount of hot water (heat water to about 60-70°c and add slowly to the sample while stirring till it all dissolves) and filter. Allow to cool slowly. Filter and dry the crystals.1
Distillation is hard to show without equipment like condensers, however a simple demonstration can be to take salt water and put it to boil on the stove. Hold a lid over the vessel and show that the water condensing on the lid is not salty.2
Separation of two immiscible liquids needs a separating funnel, since trying to pour out the top layer invariably means that some of the bottom layer will come too. A simple way of illustrating the principle is to use a plastic syringe. Take some oil in a test tube and add some water to it. Agitate the mixture and suck it up in the syringe. Allow it to settle and then take out the two different layers by pressing the barrel gently.
Two miscible liquids are usually separated by fractional distillation. I know of no simple way of showing fractional distillation without condensers; will keep trying to work out something and will let you know if and when I figure it out.
Two liquids that are in a solution together can be separated by chromatography. Filter paper or blotting paper works very well and felt pens are an excellent source of solutions – they consist of water soluble inks mixed together. Take a filter paper circle or a piece of blotting paper. Make a dot with a black felt pen. Add water drop by drop on the dot, using a dropper. The black dot will spread out and show two/three bands of different colours. The principle of separation can be introduced to the students as – the inks are soluble but by different amounts, they get absorbed by the paper and redissolve in the water as they spread across the paper. The colour that dissolves more in water will move farther. If acetone is available in the lab, repeat the experiment using acetone, the colour positions are reversed. Very nice extension work will be to determine which of the colours are single colours and which are mixtures. Are the blacks from different makes of felt pens the same mixture or different? What about the other colours?
Having done these experiments, the idea of detection of drugs in the urine samples can be explained.
These droppers are available in chemical supply stores and are fairly cheap (~ Rs 1 per piece). One piece ink droppers will also work, but these have volume markings and are very versatile. I’ll be using them a lot.
References
- https://medium.com/write-outside-the-box/crystal-gazingliterally-42c77a56a82e#.eafpzbi8f. This is a blog by a young friend who got copper sulfate and re crystallized it.
- Solar stills; if you google this term, you will see some nice designs of solar stills and it looks possible to rig up one.
The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.
