Column chromatography | Intermolecular forces and properties | AP Chemistry | Khan Academy

• In our previous video, we talked about Thin Layer Chromatography. It was this technique used to figure out how many things you have in a sample or maybe say the relative properties, say the relative polarity of the things that you have in the sample.

And so what you do is you put a sample on typically a silica gel, which is the stationary phase, and then you put a mobile phase down here, which in this case might be less polar. It's going to move its way up the silica gel, and as it does, you can imagine it's going to interact with your original sample.

Parts of the original sample are going to move up with your mobile phase, and different parts are going to be attracted to that mobile phase and not attracted to the silica gel to different degrees, you can imagine. If there's a part of your original sample that is more polar, then it's going to be harder to move, because it's going to be attracted to the stationary phase and less attracted to the mobile phase, and so it's going to move less.

This might be the more polar part of your sample as it moves from its original location. The part that's less polar? Well, it's not going to be as attracted to the stationary phase, and so it's going to be more dissolvable in the less polar mobile phase, and so it will go further.

This will be the less polar part of your original sample, and so you can see that separation, and you could come up with other insights that we talk about in that other video. In this video, I'm gonna introduce you to a very, very, very similar idea. It's just that things are moving in a different direction, and that is the notion of Column Chromatography.

What you do in Column Chromatography, just like Thin Layer Chromatography, is let's say you have some type of vile, some type of a column. I guess I could say you'll typically see it with a little tap drawn down here because you might wanna see what comes out through the bottom. But what you do is you fill it with the stationary phase, which once again is oftentimes silica gel.

So this is stationary phase, stationary phase, and if it's silica gel in this case, silica gel, it doesn't have to be silica gel, but that's pretty common. This is going to be very polar, so let me write this, very polar. Then, you put some of your sample at the top here. So that's your original sample; you put it at the top, and then you have some mobile phase, the solvent that you're going to put on top of that.

So you're gonna do something like that, and what do you think is going to happen? Well, your mobile phase is going to start moving its way down through your stationary phase, and it's going to interact with your sample. Now, what's going to happen to the more polar components of your sample?

The more polar components of your sample are going to be more attracted to the stationary phase, less attracted to the mobile phase, so they're gonna move less. The more polar parts of your sample might only go, maybe that far, while the less polar parts of your sample, they'll move. There'll be less attracted to your stationary phase, and they'll move with your mobile phase more, and so you might have they might move down, down over there.

The big takeaway, the difference between the two, is just the direction you're moving in. In Thin Layer Chromatography, your more polar thing is the lower dot, while in Column Chromatography, your more polar thing is the upper layer right over here, this purple area right over here.

And of course, all of that depends on the polarity of your stationary phase and the relative polarity of your mobile phase, but what I just showed you is a typical situation. So keep in mind whether you're looking at Thin Layer Chromatography or Column Chromatography, which to pay attention to the direction.