Topic 3.9: Separation of Solutions and Mixtures

Separating solutions: the gold in the green

This is one tree. It looks like a forest, but an estimated forty-seven thousand trunks grow from one root system, so the whole hillside is a single aspen called Pando. Every fall it turns gold, and that gold was in the leaves all summer long, hidden under the green. So a leaf is really a mixture of colors. I’ve ground some leaves into a green soup with a little solvent, and we’ll pour it through filter paper.

Before I pour it, what do you think will drip into the flask? The torn-up leaf stays on the paper, but the liquid coming through is just as green, so the filter caught the pieces and let all the color through.

Filter paper is a mat of fibers with gaps a few micrometers wide, small enough to catch a piece of leaf, but a pigment molecule is a couple of nanometers across, about a thousand times smaller. So you can’t separate the parts of a solution with a filter, and Hercules’s brine would pour straight through too, salt and all. Instead we sort by attraction, with chromatography and distillation.

Inside the column

For chromatography, we pour the green liquid onto a glass tube packed with white powder and let a clear solvent trickle down through it. And look at that, the gold pulls away from the green and runs on ahead, all the way out into its own vial. A botanist named Mikhail Tsvet did this in 1903, and a few years later he named it chromatography, color writing.

So why does the gold win? Let’s go inside the column, into the white powder, which is silica, the same compound as sand. Its surface is covered in O–H groups, so it’s very polar, and since it stays put, it’s called the stationary phase. The solvent flowing past, petroleum ether, is a nearly nonpolar mix of hydrocarbons, and since it moves, it’s the mobile phase.

When chlorophyll touches the surface, its oxygen atoms form hydrogen bonds with those O–H groups, so it sticks for a while, going nowhere. Carotene, the gold pigment that ran ahead, is all carbon and hydrogen, so it feels only a weak attraction, lets go almost right away, and rides on with the solvent.

Both of them stick and both of them ride, and while they’re riding they move at exactly the same speed, so what separates them is how much of the time each one spends stuck. It’s like two buses with the same speed limit, where one stops at every stop and the other hardly ever stops, and the one that keeps stopping falls further and further behind.

Back outside, the gold is out. But the green is still near the top, holding on to the silica. See if you can get the green off the column. Pick a solvent and run it. Now the leaf’s two colors sit in two vials, the same molecules, just sorted.

And the sentence chromatography questions are looking for is that carotene traveled farther because it’s attracted less to the stationary phase, relative to the mobile phase, so it spends more of its time moving. Carotene is also lighter, but mass isn’t what decides it, and acetone just proved that by moving the green without changing its mass at all.

Reading a chromatogram

Turn the column on its side and you get a TLC plate, the same silica on a sheet of glass, with the solvent soaking upward and carrying each spot only as far as its attractions let it. Paper chromatography works the same way, with the paper as the polar stationary phase. When you set one up, the start line goes in pencil so it won’t run, and the spot sits above the solvent so it doesn’t dissolve into the pool.

Then you mark the solvent front as soon as the strip comes out. To compare spots, we divide how far the spot traveled by how far the solvent front traveled, both measured from the start line. That’s the Rf value, the centimeters cancel, and it’s always between zero and one. Here’s a classic AP question. A dye ended up close to the solvent front, so which conclusion does the evidence support?

The 2017 AP exam added a trap, so here’s that question with our own numbers. Dyes A, B and C ran on polar paper with a nonpolar solvent, and the front went eight point oh oh centimeters, with A, B and C at four, two and seven. An unknown ran on a second strip whose front only reached five point oh oh centimeters, and the unknown went two point five oh.

Two and a half is closest to B, but the strips ran different distances, so we can’t compare raw distances, and that’s exactly what Rf fixes. A is four over eight, which is zero point five zero zero, B is two over eight, zero point two five zero, and C is seven over eight, zero point eight seven five. Now you find the unknown’s Rf, to three significant figures.

So the sentence that earns the point is that the unknown is dye A, because its Rf matches A’s under the same conditions, the same fraction of the way, though not the same distance. And C, with the biggest Rf, is the least polar, because it’s the least attracted to the polar paper relative to the nonpolar solvent, which we can only say because we know the paper is the polar phase.

Distillation

Distillation can do what the filter couldn’t, and get the water back out of salt water like Hercules’s brine. When we heat it, water molecules escape into the vapor, but the ions stay behind, because the water around them and their pull on each other hold them tight. The vapor turns back into a liquid in a condenser, cooled by water on its own separate path, and fresh water drips into the receiving flask.

When both liquids can escape, the one whose molecules hold each other less tightly sends more of them into the vapor at the same temperature, which gives it the higher vapor pressure and the lower boiling point, so the first liquid you collect is enriched in it, which means richer, though not pure. Take acetone, the nail polish remover, and ethanol, the alcohol in hand sanitizer. Ethanol has an O–H group, so its molecules hydrogen bond to each other, while acetone’s oxygen has no hydrogen on it.

If we distill a mixture of the two, which one is enriched in the first drops, and why? Every separation we just did depended on which liquid we used, which raises a deeper question, why something dissolves in one liquid and not another.