Sciocrat
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Compound identification
Recently I found a water-well which had some kind of crystalline compound on its wall. I'm interested if there is a simple method for identification
of an compound, that could be done in a school lab? (could measuring some basic chemical/physical properties give accurate results?) Im planing to
gather a sample of the compound and find out what it is, if it is possible to determine that in a school lab.
I've done some search on google and wikipedia, and found only relatively complicated methods (for a high school student)...
Tnx in advance (and sorry for my bad english)
[Edited on 5-4-2007 by Sciocrat]
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12AX7
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Basic inorganic salt identification-
Collect a sample and dissolve in water. A reasonably strong solution (10g/100ml, say) will help. How much is soluble in a minimum amount of water?
Roughly how much does solublility change with temperature? Heat it until it stops losing weight -- how much volatile stuff (presumably, just water,
but watch out for smells as well!) does it contain?
First of all, it does help to recrystallize the material. It may be pure, or it may be a combination of salts. Observing the crystallization
carefully will show different crystal types, which can be seperated from each other and further purified, for later identification.
Take a small amount of the sample solution and test it with:
Strong acid (sulfuric acid, H2SO4) -- does it react, does it produce any odor? (HCl and HNO3 odors would be expected here.)
Strong base (NaOH solution) -- does it react to produce a precipitate? Does the precipitate dissolve in excess NaOH?
Ions (you're looking for a precipitate) -- anions:
Sodium chloride (source of Cl-)? Lead and silver ions react with chloride.
Sodium sulfate? (Allow some time for calcium sulfate to precipitate, it can go slowly. Sulfuric acid may also show a precipiate.) Barium, strontium,
calcium and lead (and other less common ions) react notably.
Sodium carbonate/bicarbonate? (May produce bubbles, especially the latter.)
Hydroxide? (Already done above.) Hydroxide and carbonate cause a precipitate with most ions except the alkalais (lithium, sodium, potassium,
rubidium, cesium). Ammonium will give off ammonia gas with a strong odor, and give a strong blue color with copper. Aluminum ions, among few others,
will readily dissolve in excess hydroxide to give a colorless solution.
(Note these are all sodium salts. Sodium salts are usually very soluble, so aren't likely to produce a misleading reaction.)
Cations:
Potassium chloride (If sodium chloride does not react.) Most potassium compounds are soluble, but several are only mildly soluble. That's why you
need a strong solution. Potassium nitrate (when ice cold), chlorate, perchlorate, and several esoteric cations, are low on solubility.
Barium (or strontium or calcium, in reducing order of effectiveness) chloride. Makes strong precipitate with sulfate and carbonate.
Nitrate is always soluble, but there is a way to detect it specifically. Add a little acid (HCl, for instance) and add a chunk of aluminum foil.
Hydrogen bubbles will be released, reducing nitrate ions to ammonium ions. Add NaOH and check for an ammonia odor, or add some copper sulfate and see
if it becomes deeply blue colored.
In short, the most common ions you are likely to find are sodium, potassium, calcium, magnesium, aluminum, chloride, sulfate and nitrate. The above
methods will allow you to establish which are present, partly by logical elimination and partly by direct identification.
For more information, look up a solubility chart. Knowing about the ions' behavior will make this clear.
Personally, I have idenfitied magnesium sulfate crusts in my basement. Presumably, mildly acid rainwater was flowing through the dolomite foundation,
releasing magnesium sulfate, which formed a crust on the indoors side. Calcium or potassium nitrate is sometimes common in nature, as a result of
decomposition (found on a cave or wall or basement near decomposing matter, including manure and other fertilizers).
Tim
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Sciocrat
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Thank you for your help, 12AX7, but unfortunately I didn't even had to carry the sample in the lab do identify it. At first sight, from 2-3 meter
distance, it looks like a crystalline compound (my friends confirmed that too), but when i scraped some and took a closer look, i realised that it was
actually candle wax)...
Take a look - http://www.imagesforme.com/images/5374lol.jpg
... and i thought i'm going to have some fun identifying the compound 
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