Work in the laboratory of organic chemistry is associated with the presence of dozens of different solvents, as well as hundreds and hundreds of different intermediate products and end products. How do you make sure that in your hands (or rather, in a bottle, tube, flask or bubble) you have exactly what you need, not something else?
I will start with the simplest laboratory rules. If it's about solvents or relatively cheap common reagents, then it's only the label that's judged here. All these school tasks with "Vasya confused the labels, help him to identify the substances" - this is an exclusively speculative situation, in reality, it is easier to throw out both cans and not to guess where what. The majority of laboratory solvents (and there are usually about a dozen of them) are colorless and do not seem to differ much from each other. Some people manage to determine by smell, but you can never rely on it. This is especially complicated by the fact that when standing in the air (for example, poured into a glass or a flask), solvents can change their properties, for example, absorbing water or gases from the air - and sometimes it is important. Therefore, the best way to do this is to pour what you doubt and refill it by printing a new branded bottle. Although there are curiosities here - manufacturers can sometimes pour the wrong quality reagent, or your colleague could find an empty bottle and use it to drain dirty solvents without even bothering to sign the label with a marker...
Now for reagents and intermediates. Of course, all containers of substances must be signed, otherwise, the container will soon be in the trash. Remembering what's in there is fraught with the risk of losing a huge amount of time in case of an error, and it's often easier to re-synthesize.
But let's suppose that the reagent is valuable and there are doubts about its identity (and purity). Then the following methods are applied:
Determination of the melting point
If the substance is pure, solid and crystalline (non-crystalline is, for example, tar or resin), it must have a certain melting point. By placing the crystalline crystal on the thermometer and heating the latter above the electric tile, it is possible to catch the moment of meeting with an accuracy of several degrees. Derivatization has also been practiced - obtaining derivatives (salts, for example) and comparing their melting temperatures with tabular data.
The boiling point can also be used to determine the substance - but it is mainly applicable to solvents and other (few) easily boiling substances. It is not enough just to put a glass of liquid on the tile and measure it with a thermometer when the liquid starts boiling - it is necessary to build a distillation unit and distill everything, watching the thermometer readings. Hence, its limitations are time-consuming and require large quantities of the substance.
IR and UV spectroscopy
Basically, something about the substance can be said about its color. The reference books still write about the color of the substance in the description (although it is not customary to write about taste anymore, hehe). But the eye is not a precise enough instrument, moreover, most organic substances in the visible range are colorless (white or transparent, often with a yellowish hue). However, in other ranges, almost all substances have absorption bands (see for water), and when using the spectrometer you can get IR or UV spectrum of the substance.
Infrared spectra are more informative and, in principle, can be used to establish the structure, but in practice, this method is almost completely superseded by NMR spectroscopy (see below).
NMR
Opened just 60 years ago, nuclear magnetic resonance is the most common method available today. Consideration of the principles of work and other details will require a separate article, so in brief: the substance dissolves in a special solvent, the ampoule is placed in a powerful magnet and irradiated by radio waves of different frequencies (sounds cool, right?). The information about absorption, depending on the frequency, allows establishing the structure of such giant molecules as taxol or various proteins (Recently, the spectrum of a protein with a mass of 900 KDa was decoded, if you understand what I mean)
Requires no more than 50mg of substance, but the substance must be very pure - the presence of impurities greatly complicates the decoding of spectra.
Mass spectrometry
Also a very powerful method. The trajectory of the ion movement in the magnetic field allows establishing the molecular mass of the substance and fragments to which it decays during ionization. Again, the general rule is simple: the same substances must have the same mass spectra.
The method is very sensitive (micrograms of the substance are used) and in principle gives very important information about the structure, but is usually used to compare the sample of a substance with the standards - such problems usually arise in forensics. And if there are assumptions about what kind of substance it is, it is enough to make sure that the molecular mass coincides with the expected one.