What do scientists use to decide whether a substance is an element or compound?

Answer 1

Depends on what compound you are referring to. Organic or Inorganic. Basically, you can use EDX to identify the elements within the compound, work better for inorganic compound. As for organic compound, FTIR can be used.

Answer 2

Microscopes and other instruments and in conclusion: NMR, infrared spectroscopy, UV/visible spectroscopy, mass spectroscopy, atomic absorption spectroscopy, column chromatography, melting point - it depends on what you're trying to identify, whether you have a short list of what you expect to find or not, and how rich your lab is (some analytical instruments are REALLY expensive). Nuclear magnetic resonance (NMR) is a favorite of organic chemists because it provides a lot of information about how carbon and hydrogen are connected to each other, and indirectly how they are connected to oxygen, nitrogen, and other elements. Infrared spectroscopy can provide information about which kinds of atoms are bonded together. It doesn't provide the detail of who's connected to what that NMR does, but it provides more information about elements other than H and C in the molecule, which NMR doesn't. UV/visible spectroscopy provides information about double- and triple-bonds, and particularly about conjugated bonds (alternating double- and single-bonds). Atomic absorption spectroscopy identifies metals by the colors of light they emit when they're excited, though it's not much use in determining what the metal is bonded to in the complete compound. Mass spectrometry ionizes a molecule, which then often breaks apart into ionized fragments, and separates the ions and fragments by the ratio of mass to charge. The complete molecular structure can often be reconstructed by identifying the composition of fragments (only some combinations of atoms can yield a fragment with a particular molecular weight), then deducing where the fragment may have come from in the original molecule.

Column chromatography covers a lot of different techniques, but they all work basically the same way: Different molecules have different affinity (stickiness) for the material of the column. If the unknown is dissolved in a solvent and flows through the column, the molecules of the unknown will take more or less time to flow to the end of the column, depending on how easily they stick to and unstick from the column material. Because different substances leave the column at different times, column chromatography can also be used to collect different unknowns separately, very convenient for further analysis. Column chromatography isn't so good for identifying a completely unknown substance, but it's really good for identifying which substances are in an unknown, and in what proportion, if you know what things could be in the unknown, but don't know for sure which ones are actually there. If one of the unknowns stays in the column for the same amount of time as a known substance, you've a pretty good idea that the unknown and the known are the same molecule.

Melting point measurements are really simple but really powerful for identifying a pure substance. Melting points are well-defined, and they're different enough for a lot of compounds that you can often identify a compound from its melting point. More useful when you're synthesizing compounds is that melting point isn't so much a point as a range, and the wider the range of temperatures over which a substance melts, the less pure that substance is. So, melting point can both confirm that you have the product that you intended to synthesize, and tell you how pure your product is.

For mixtures of ionic compounds, qualitative analysis (which consists of mixing stuff into the solution and looking for precipitates and color changes) can identify the ions that are in solution. However, sensor technologies are today faster than a qualitative analysis and more accurate for a lot of ions, so qual isn't done so much today.

Answer 3

The recommended procedures are chemical/physical analysis to determine the composition and the properties.

Answer 4

by looking at the Periodic Table :)

Answer 5

Methods