[SciShow intro plays]
[Text: QQs: Why is my whiteboard so dirty?]

Hank: If you have an old, well-used whiteboard in your classroom, you might see something a little strange -- ghosts! Not the spooky kind If you are seeing those, that’s up to you. I'm talking about the ghosts of lectures past -- faded words and numbers, or a smudgy stain. Dry erase markers are designed to write on smooth, hard surfaces and erase without a trace. So how come they’re sometimes so hard to erase? And how can you get rid of the leftover marks?

The ingredients of a dry erase marker actually aren’t all that different from a permanent marker. Both have three main components: solvents, pigments, and polymers. The solvent is the liquid that carries all of the other ingredients inside the marker. And after you write something, it evaporates. Usually, markers use ethanol, which is the kind of alcohol people drink, or isopropanol -- aka rubbing alcohol -- because they evaporate quickly. Which means it’s harder to accidentally smudge whatever you’ve just written. The pigment gives the ink its color. For black markers, for example, manufacturers can use a fine powder called carbon black.

The key difference between permanent and dry erase markers is the third thing: the polymers. Polymers are compounds that are basically chains of repeating sections. Permanent markers normally have acrylic polymers, which have chains based on acrylic acid. These polymers are similar to the ones you would find in house paint or nail polish, and they help the colors stick. Dry-erase markers, on the other hand, have oily silicone polymers, also called release agents. They have chains made up of silicon and oxygen, and they form a slippery barrier between the pigment and the whiteboard. The teacher can take an eraser and wipe that dried layer of pigment right off the top.

The release agents mostly stick around and leave a transparent oily residue on the board. But here’s the thing: dry erase markers only work if you’re drawing on the right kind of surface. Specifically, it has to be a nonporous surface like glass, without any tiny holes that the fluid can slip into. With a porous surface, like a piece of paper, the silicone polymers won’t be able to form a barrier, and the dry erase ink will seep in, like any other marker. So, the best whiteboards are extremely smooth, with no pores at all.

After lots of use, though, whiteboards start to wear down and cracks and pores might open up, which can let the ink flow down below the surface and dry there. These stains -- or “ghosting” -- can also appear if you smudge around fresh markings, which shifts around the layers of polymers and pigments, so that the polymers don’t form that nice barrier between the pigment and the board. They’ll also usually show up if the ink has been left there for a really long time.

The trick to getting rid of this dried pigment is to re-dissolve it so it can be wiped away. In fact, most of those whiteboard cleaning sprays you can buy are just different kinds of solvents, some generic rubbing alcohol will do the exact same thing . And there you have it: your ghostly whiteboard problems solved with chemistry.

Thanks to all our Patreon patrons, who keep these answers coming. If you’d like to submit a question to be answered, or get these Quick Questions a few days before everyone else, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!