 In this next set of videos, we're going to be talking about chromatography, a technique that can be used to separate a mixture into its components based on the different ways those components interact with other materials. By the end of these videos, you should be able to answer the following four questions. 1. What do you need in order to use chromatography? 2. How does chromatography work? 3. How can chromatography be used to determine the composition and purity of substances? And 4. What are some different kinds of chromatography? Now, let's get started with our first question. What do you need in order to use chromatography? Every chromatography experiment requires three things. A substance that you wish to analyze. This is called the analyte. In our paper chromatography experiment, this was our coloured marker or our plant pigment. 2. We need a solvent for our analyte. This is called the mobile phase. For paper chromatography, this was our ethanol or water. And thirdly, we need a surface for our analyzing solution to move across. This is called the stationary phase. This was, of course, our paper. Different kinds of chromatography will use different forms of mobile and stationary phases. But when it gets down to it, you need at least these three components to be present in every type of chromatography experiment. How does chromatography work? In short, chromatography relies on how the components of the mixture we wish to analyze interact with the two phases of the chromatography setup. In particular, the interaction with the mobile phase is determined by how soluble that component is in the fluid of the mobile phase. The interaction with the stationary phase is determined by the adhesion of the component in the mixture to the surface of the stationary phase, that is, how well they stick together. The mobile phase and the analyte are able to travel along the stationary phase by capillary action. You can learn more about adhesion and capillary action in the video Surface Tension and Capillary Action. In our paper chromatography experiment, the substance that travelled up the paper the quickest has a much greater interaction with the mobile phase than with the stationary phase, while the slowest moving component has stronger interactions with the stationary phase than the mobile phase. In general, the higher the adhesion of the component to the stationary phase, the slower it will move. While the higher the solubility of the component in the mobile phase, the faster it will move. This is the basic physical principle of chromatography. The differential interaction of the components of a mixture with the mobile phase and stationary phases of our setup. Now, an additional question arises. What determines the solubility and adhesion of our component substances? As we covered in our video on intermolecular forces, dipole-dipole interactions which arise from molecules being polar is often the strongest intermolecular force that we consider. This means that the polarity of a substance will often play the leading role in determining its solubility in and adhesion to other substances. This is definitely the case for our paper chromatography experiment, where the cellulose in the paper and the water or ethanol in the solution have very different polarities. Hopefully, this information gives you an idea of how to answer our third question. How can chromatography be used to determine the composition and purity of substances? I'm going to finish this video here so that you guys can have a think about this question before moving on to the next video, which contains the answer.