By what means is composition of totally unknown liquid determined?
In: Chemistry
Usually by spectroscopy. There are different types of spectroscopy, each revealing a different aspect of the compound. NMR Spectroscopy works by putting a sample in a very strong magnetic field and irradiating it with radio waves. At some frequencies, the atoms absorb the radio waves, and from these frequencies you can work out the structure of the molecule (the same principle is used in MRI). Other types of spectroscopy, like IR and UV/VIS spectroscopy, measure the absorbtion of infrared/UV/visible light by the compound, which can act as a “fingerprint” of the molecule. Mass spectrometry determines the mass of the compound by ionizing it (charging the molecule) and shooting a beam of the ionized sample along a magnet. The magnet repels the charged molecules, and the degree of repulsion can be measured, which depends on the mass of the molecule.
This sort of ID is a chemistry task, and they have multiple tools at their disposal, from simple things like checking pH (acidity), osmolality (salt concentration), density, reactivity (whether it reacts with various known chemicals), to more complex stuff like [infrared analysis](https://en.wikipedia.org/wiki/Infrared_spectroscopy) and [mass spectrometry](https://en.wikipedia.org/wiki/Mass_spectrometry).
Spectroscopy in general is very revealing, because various elements and chemicals glow with different colors when burned or subjected to various frequencies of light, and a very sensitive instrument can detect these different colors. It’s the same method for figuring out what materials are in stars and distant planets out there.
I’ve actually just started learning this in biology lately. Chemists can test the composition of a substance through an ‘Indicator’, they’re substances or chemical compounds that, when added to another fluid will change the color depending on what ingredients are present. Some basic ingredients I’ve learned can be detected through this easier method are Protein and Starch. Starch hinting the existence of plant origin because it’s made up of concentrated glucose, and protein indicating that it’s part of an animal or came from one.
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