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If a compound is placed before you and you are asked to determine what it is and what it's structure is, how do you actually do that?

This might be a baby question for most of you but I am not from chemistry background and so this amazes me!

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closed as too broad by Buttonwood, pentavalentcarbon, Todd Minehardt, Wildcat, bon Jun 11 '17 at 12:33

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ This is a very broad question and it would help to focus it a bit. Are you assuming that the compound in front of you is pure? Are you assuming it is a solid or can it be a liquid or gas? Are we assuming we know nothing about the compound? $\endgroup$ – bobthechemist Aug 21 '13 at 14:09
  • $\begingroup$ yes , let's say we know nothing about the compound. $\endgroup$ – biogirl Aug 21 '13 at 16:35
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    $\begingroup$ A full answer to this question could fill several volumes. $\endgroup$ – Aesin Sep 21 '13 at 10:11
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    $\begingroup$ There are textbooks devoted to this topic, even when we assume things like compound is 1) pure, 2) organic, and 3) not a polymer. $\endgroup$ – Ben Norris Sep 22 '13 at 16:05
  • $\begingroup$ For a true unknown a typical lab would use a combination of instrumental techniques to identify it. GC/MS, IR and NMR for instance for an organic compound. If it works, GC/MS is particularly powerful since it purifies the sample and identifies it at the same time. // An inorganic compound or mixture would use XRF & XRD. // There are massive databases of compounds to augment identification using each of these techniques as well. $\endgroup$ – MaxW Apr 24 '16 at 5:21
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Actually, it's quite simple for most substances. Use a gas chromatograph. If the substance can be vaporized at reasonable temperatures, this will give you enough information to pin down the chemical compounds, or at least rule out a wide range of compounds. We can tell because every compound has a unique retention time based on its mass and attraction to the medium coating the column. With enough reference data and good precision, it's easy to figure out firstly how many separate compounds are in the mixture and secondly what their chemical formulae are.

Failing that, there are other kinds of chromatography, not to mention mass spectrometry.

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    $\begingroup$ Gas chromatography is a separation technique, not a characterization technique. Identification using GC requires that the known compound has already been isolated. It is not likely true that "every compound has a unique retention time". I agree that you can estimate the number of compounds in a mixture but you cannot determine their chemical formula with GC alone. $\endgroup$ – bobthechemist Sep 22 '13 at 1:21
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    $\begingroup$ For example, GC does a very poor job with polymers, ionic compounds, minerals, etc. $\endgroup$ – Ben Norris Sep 22 '13 at 16:03
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The easiest way is to inquire where the compound originated, observe the compound with generic senses (color, texture, phase-gas/liquid/solid) and then make educated guesses based on that! If there remains any doubt as to the chemical or if that info is not available then a systematic process of ruling out the most likely possibilities is conducted. The single largest category in chemistry would be organic or carbon containing compounds. They may be analyzed using NMR to determine their structure with good accuracy. Any decent sized chemistry department will have an NMR available. For inorganic compounds, some simple acid-base tests could narrow the possibilities.

With the most coveted and expensive tools, it is easy to tell what elements compose a sample. I once visited a forensic laboratory where they had a scanning electron microscope that could display individual grains of gunshot residue (invisible to the naked eye)for which police swab the hands of suspects. The particular SEM I saw in action had an x-ray source connected to it, which allowed any point including an individual microscopic dot of suspected gunshot residue to be analyzed for its elemental composition. The guy there clicked on any part of the SEM image to display what element it was made of... completely automated!

One of the longest laboratory experiments I did in school was determining the percentage of iron in a mixture of iron and clay. It took 12 hrs and used only wet chemistry from 100 yrs ago.

There are many techniques that could be used but it really comes down to which one is most promising or available.

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The easiest and most precise method: grow single crystals and do x-ray diffraction experiment. as long the question is not which lanthanide I have, some chemical "common sense" + x-ray structure give u full answer. Of course, you still need to do other characterisations as NMR and IR spectroscopy.

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  • $\begingroup$ X-ray is great for crystalline materials. If you have a cross-linked polymer or a low molecular weight liquid or gas, you need something else. $\endgroup$ – Ben Norris Sep 22 '13 at 16:04
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Use a sample of the material and perform proton NMR, IR Spectrography, Gas Chromatography/Mass Spectrometry, then cross reference the information using COSY, DEPT and HETCOR analysis.

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