Laboratory courses are required for medical school, and if you are a future physician, you will take two semesters of chemistry laboratory, as well as laboratory courses in other sciences. It's also worth noting that the MCAT will also ask questions about laboratory science, so begin paying attention to your labs and understanding what you are doing in them, and how it applies to what you learn in your lectures. Thus, it is with all of this information in mind that I say it is of use to understand different methods of separating mixtures. For a review on mixtures and the difference between heterogenous and homogenous mixtures, return to the previous post.
One method that is used to separate heterogenous mixtures that are solid-liquid is filtration. In filtration, the mixture in question is sent through a barrier of some sort, such as filter paper, that has fine pores (Masterton and Hurley 2015). In chemistry laboratory courses, you will become familiar with filter paper as it is used countless times in both inorganic and organic chem labs. In my experience with chemistry laboratories performing gravity filtration, a glass funnel with a stem is used to hold the filter paper that is folded up for placement inside the conical structure. This apparatus is attached to a support ring, which also holds a collection beaker at bottom of it to catch the liquid. The solid will remain on the filter paper inside of the funnel. There is also vacuum filtration, where a Buchner funnel and a water aspiration assembly are used to separate solid from liquid. In this set up, there is a side arm filter flask with a Buchner funnel on top (sort of a short, cylindrical funnel) that is attached via hose attachment to a vacuum aspirator. In the side arm filter flask will be your liquid and in the Buchner funnel will be your solid. (Side note: I sometimes found the filter paper to be in a pain in the butt to remove from the Buchner funnel; I would sometimes lose some product when removing my filter paper.) All of this being said, separation of mixtures is crucial to the study of chemistry and it's something you should definitely pay attention to when you practice it in the laboratory. Some of you may not agree with me on this, but I think that chemistry laboratory is extremely fun and a great way to obtain a deeper understanding of the concepts taught in lectures.
Another technique you will become familiar with in chemistry laboratory is distillation. Distillation is for use on homogenous mixtures. It utilizes the process of vaporization of the liquid part of the mixture to leave behind the solid residue within the distilling flask. Then, you can obtain the liquid again by condensing the vapor that you originally formed (Masterton and Hurley 2015). (Side note: This was not one of my favorite techniques that I used in the laboratory. Somehow, it always feels like a greater reward when you can see your solid forming crystals immediately as you go, like in filtration.)
And then, another method you will get to know for the separation of mixtures is chromatography. Like filtration, there are actually also a few different types of chromatography and you'll use them all before your undergraduate years are over. However, the different types of chromatography all have one thing in common: they are based on how fast a particular substance is able to move past a stationary phase that has the property of slightly attracting the substance, in a stream of liquid or of glass. One of my personal favorites, because of the visibility of the results (at least in the experiment that I used it in) is paper chromatography. In this type of chromatography, you begin by drawing a line near one edge of a sheet of paper in ink, chlorophyll, carotene, or whatever other pigment you would like to use. The paper is then placed inside of a solution, upright with "inked" side down, that will separate different "dyes" or pigments from each other and from your original pigment, as the ink moves upward. You will see bands that are different colors and will correspond to different dyes or pigments found in your original pigment. What makes this happen is the strength of attraction the different dyes/pigments have to the wet paper fibers; the stronger the attraction of the dye or pigment, the slower it will move (Ebbing and Gammon 2009).
Another type of chromatography is column chromatography, wherein a column in secured in place, and powdered chalk (or other powder) is placed inside of it, for the purpose of separating a solution containing different substances. The substances begin to separate into bands, similar to paper chromatography, after a pure liquid is placed into the column. The "bands" of substances are in different places along the column, and each substance is collected in a separate flask as it comes off the column (Ebbing and Gammon 2009).
More technically, you will also need to know a few more advanced terms to understand and to reiterate this concept in your own mind and to others (as well as on tests!). A slurry of an eluent, (the portion of the mobile phase whose role is to be the carrier) is prepared along with the stationary phase powder. This is what is placed inside of the column, more precisely. And after that, a solution to be separated is placed into the column, on top of the stationary phase, which is also known as the adsorbent and which is a solid, usually using a pipette. On top of that, sand, cotton, or another substance is placed. The eluent is then poured into column slowly, helping the solution to be separated to advance. At the end of the column, the different components that are being separated "elute" one by one. Fractions of the eluent are collected during the process of column chromatography.
The last type of chromatography that I will discuss in this post is gas-liquid chromatography. In this type of chromatography, also known as gas chromatography, the mobile phase or carrier is a gas, often an inert or non-reactive gas such as helium or nitrogen, respectively. In this form of chromatography, the stationary phase consists of either a solid or a liquid that adheres to a solid through the process. The station phase is packed into a column, and then gas is passed through the column. Substances are separated in the column based on their attraction to the stationary phase. A gas chromatograph is used in this process, as an instrument of chemical analysis for the separation of substances in a sample. The chromatography will show different peaks, which correspond individually to a different substance within the mixture. While the mixture passed the detector, these peaks were recorded automatically by the gas chromatograph (Ebbing and Gammon 2009).
If these concepts seem a little unclear right now and you haven't taken any laboratory classes yet, don't be worried. They will become much clearer after you've practiced them, which, like I said, you'll do countless times throughout your undergraduate years in inorganic and organic chemistry. Get familiar with them. Good luck and happy studying!
A prospective medical student, looking to help others succeed.