How to identify unknown compounds

[Entropee]

Our chem project is to identify 5 unknown compounds, just wondering if anyone here had any good ways to go about doing this besides the usual melting / boiling / ph tests etc.

 

[Borek]

Various kinds of spectroscopy, but even then limiting your possibilities with the cheapest and simplest methods is a good idea.

 

[espen180]

If they are unorganic compounds or salts, a series of pretests and test reactions should reveal the constituents.

If they are organic compounds, there are many test you can do to identify fuctional groups, but if you need the full structure of the molecules, you need spectroscopy.

 

[J Ratko]

Have you attempted the tests you listed with poor results? Or just looking for more effective, efficient, simple, or accurate tests?

Sorry if I seem confused, but "good" doesn't give me much to go on. What's good to me may not be good to you, whether something is good varies even with the individual according to task, for example: one may search for the most accurate way or the most expedient way, the two may, or may not, be the same, When one says "good" it can probably be assumed the one they are seeking to them is "good," but if what they seek is unknown to you how can you tell which is good? Without being more specific about what you're looking for, the best I can do is guess, and that's probably not much help to you.

Are the five compounds in a mixture requiring separation? I'm guessing you were given five individual samples. Yes, there are other good ways, quite a few of them, but to my knowledge all have downsides; higher cost, greater risk, more complexity, so on, and so forth. For example: chromatography, or use of an analytical reagent. So it's most sensible, often necessary, to narrow down the possibilities first with the simple tests.

It just occurred to me that I asked your same question myself back sometime in the 1950s, it was a little hard to believe those primitive tests were it, there had to be better more advanced modern jet-age ones. There were not and to my knowledge, for the most part, and although a couple tests are no longer part of the analytical process such as tasting unknown substances, there still isn't. These simple tests have been in use for quite some time and I expect they will continue to be. All are inexpensive, readily available, require little training or skill, and to a point they do work surprisingly well.

 

[ChmDudeCB]

Is this for high school/AP chemistry?

Place some in an evap dish and warm it up to see if it is a hydrate salt. If it melts or burns it is likely organic.

Mix up a bit of solution and do a flame test with a burner to check for metals.

Past this what you should do depends on the results. The idea behind the unknown is to get you to use your chemistry knowledge to know what sorts of tests are needed to find out information about a compound. A Merck Index will be your best friend. If you research the literature cited in the index on a compound you suspect, you can often find tests for the compound that have already been ran. Replicate them and cite your results.

Its better to show you had direction and didnt get the correct compound than to run a bunch of useless tests and give a BS answer.

 

[Entropee]

Thanks guys, just to clear a few things up, yes it is high school honors chemistry, they are not organic compounds, and basically we have a whole week of just doing various tests to them. I just have a feeling we will get all the common tests out of the way within a couple of days. If you guys want I can make a list of the compounds that our unknowns could be, maybe there are special tests for certain compounds that are unique to only that compound, I wouldn't know though.

 

[Borek]

Solubility, pH and substance color will most likely limit list of the possible substances - then some more specific tests can come handy. But don't expect anything much more sophisticated than using Cl^- to detect Ag⁺ by precipitation.