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Chemical Tests
Depending on its structure, a compound will generally react with some reagents but not with others. A particular type of compound called an aldehyde, for example, will react with a chemical called Jones reagent whereas another kind of compound called a ketone will not. Consequently, you can use chemical tests, such as the addition of Jones reagent or solubility tests, to help you figure out what kind of compound you have. Chemical tests are also very useful for detecting the presence or absence of specific compounds in a mixture.
Melting Points
Testing melting points is straightforward, and because the melting point is a characteristic of a given compound, it can serve as a means of identification. Unfortunately, many compounds have similar melting points. If a chemist knows what kind of compound is present based on chemical tests, he can prepare a derivative by combining the compound with another chemical to make a new compound. If you combine an aldehyde with 2,4-dinitrophenylhydrazine, for example, you get a certain kind of derivative, and you could compare its melting point with melting points for known aldehyde and 2,4-DNP derivatives to help identify your original compound.
IR and Mass Spectroscopy
Organic compounds absorb infrared light at wavelengths determined by their structures. By passing IR light through a sample to see which wavelengths it absorbs, a chemist can determine what kind of compound she has without using chemical tests. Mass spectroscopy is another powerful technique that determines the molecular weight of a compound by ionizing it then testing how it behaves in an electric and magnetic field. High-resolution mass spectrometers can determine the chemical formula for a compound without any other tests required. Combustion analysis is another older way to determine chemical formula. In combustion analysis, a compound is burned, and the mass of the combustion products is measured. Using this data, you can determine how many carbon, oxygen, hydrogen and nitrogen atoms were in the original compound.
Nuclear Magnetic Resonance Imaging
NMR is the youngest and most valuable of these techniques. An NMR spectrometer measures which frequencies of radio waves are absorbed by a sample in the presence of an external magnetic field. From this information, a chemist can determine the structure of a compound without the need for chemical tests and preparation of derivatives. Mass spectrometry is especially useful as an adjunct to NMR because it yields the molecular weight and chemical formula for the compound, and this information is helpful when interpreting NMR data.